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Demographic observations (1963–1973) on the chimpanzees of Gombe National Park, Tanzania
G. Teleki E. E. Hunt, Jr. J. H. Pfifferling
Demographic Observations (1963-1973) on the Chimpanzees of Gombe National Park, Tanzania
Department of Anthropology, 409 C. R. Carpenter Building, Pennsylvania State University, University Park, Pa. 16802, U.S.A.
This report provides a longitudinal demographic profile for a small study population of chimpanzees living in the Gombe National Park of Tanzania, East Africa. The sizeand composition of this study population, and the trends in mortality, natal&y and migratory patterns it experienced over a span of ten years, are described and analysed within various ethological and ecological frameworks. The validity of the demographic profile is examined in relation to field methodology, as the study population was intensively provisioned with bananas for several years. The results yield some guidelines for managing and conserving wild chimpanzee populations, and special emphasis is placed upon protection from diseases that involve human vectors. The data presented here may also offer some novel insights to the evolution of pongid and hominid populations, so tentative steps are taken to place the Gombe profile into a broader demographic perspective.
Received and accepted 1 April 1976
1. Introduction Longitudinal demographic information on wild non-human primate populations remains sparse even though numerous species have been extensively studied in their natural habitats (see Baldwin & Teleki, 1972, 1973, 1974, in press; Baldwin et al., 1975,1976 and in press for reviews of field studies). The few reports that do include demographic data tend to cover brief observation spans (e.g. Berger, 1972) or to derive from colonies transplanted into artificial settings (e.g. Burton & Sawchuk, 1974; Drickamer, 1974). Only preliminary demographic data are currently availabIe for the African and Asian apes: Pan troglodytes (Nishida, 1968, in press; Lawick-Goodall, 1968, 19756; Kortlandt, no date; Kuijsten, 1972), Pongo pygmaeus (Horr, 1975, in press; Rodman, 1973; Cohen, 1975), Symphalangus syndactylus (Chivers, 1974). These and ofher field studies of living pongids have yielded a wide selection of demographic material, but data on the natality, mortality and migratory patterns of groups or populations cannot be reliably compared because temporal spans and methodological treatments vary greatly. The present report focuses on a continuous IO-year span of demographic events and trends within a small population of long-haired chimpanzees (Pan troglodytes schweinfurthii) inhabiting the Gombe National Park (4’40’ S, 29”38’ E) of western Tanzania. Our prime concern, in view of the current paucity of published data, is of course the presentation of descriptive field observations. But the relevance of the chimpanzee demographic model to other areas of anthropoIogica1 research will aIso be explored, especially along the following lines. (a) To the paleoanthropologist, demographic studies of living non-human primates may yield models which can then be applied to refine speculations about patterns and processes once exhibited by early hominid populations, such as australopithecines. Although modern human populations have experienced major demographic changes in recent centuries, which are reflected in a steep global growth rate, hominid populations are estimated to have grown only incrementally during the preceding million or more years (e.g. Coale, 1974). The demographic profiles of non-human primate populations, Journal of HumanEvolurion(1976) 5,559-598
560
G. TELEKI
ET AL.
on the other hand, have probably maintained greater stability throughout the Pleistocene, so the patterns exhibited by living monkey and ape populations may provide an accurate mirror for visualizing demographic traits and trends in the early stages of hominid evolution. In addition to serving as a prehistoric template, such demographic models-and especially those derived from African pongids, in view of their biological, behavioral and psychological proximity to hominids-could help resolve some of the dilemmas encountered in paleodemographic research (Mann, 1975; McKinley, 1971). (b) To the primatologist, demographic data provide a key to understanding the structure and organization of social units, and to the adaptive role these have played for various species in an array of habitats. Much attention has recently been given to the ways that primate adaptations are governed by relationships between organizational and ecological variables (Aldrich-Blake, 1970 ; Struhsaker, 1969 ; Gartlan, 1973 ; Clutton-Brock, 1974; Eisenberg, Muckenhirn & Rudran, 1972). Yet no cogent, comprehensive theory of adaptation has emerged from these numerous schemes-perhaps because none rests firmly upon a foundation of demographic profiles. (c) To the cultural or medical anthropologist, demographic studies of non-human primates may, by virtue of the rather simple and elegant models obtainable therefrom, provide insights about the manner in which biological, epidemiological, sociological and other adaptive mechanisms regulate human populations. The progress of recent investigations along these lines (Fox, 1972; Dumond, 1975; Lancaster, in press) has again been hampered by lack of demographic information suitable for interspecific comparison. (d) To the wildlife biologist and game manager, accurate demographic data on living non-human primates are essential for devising suitable policies and programs of conservation. Such information is particularly vital to the task of protecting African and Asian apes, which are today threatened or endangered by human activities in many regions (Harrisson, 197 1; Bermant & Lindburg, 1975). Moreover, breeders of nonhuman primates could, given accurate numerical information, evaluate the success of their custodianship by comparing fertility and survivorship rates in captive and wilderness settings (Teleki & Baldwin, 1975). At present both conservation and reproduction programs suffer from lack of demographic information. 2. The Gombe Setting Research on the Gombe chimpanzees was initiated in 1960, and has been sustained for nearly 15 years. Since 1967, when the Gombe Stream Research Centre was officially founded, the project has developed into a multifaceted, multidisciplinary study of nonhuman primate ethology and ecology. Numerous reports on chimpanzees have been completed by members of the research center, and some reports on other indigenous species are also available (Gombe bibliographic entries are marked with asterisks). Reports describing aspects of the local climate, terrain, flora and fauna include those by Lawick-Goodall (1968), Teleki (1973a), Glutton-Brock (1972) and Wrangham (1975). No detailed, systematic studies of the Gombe ecosystem are as yet available. For the purposes of this report, it should suffice to note that the terrain of this small park (approximate map dimensions : 2.3 x 13.9 kilometers) is rugged, cut into many steep-sided ridges and valleys by a dendritic stream system that flows an horizontal distance of about 1500-3000 metres and drops a vertical distance of about 750 metres
GOMBE CHIMPANZEE
between
the escarpment
Tanganyika;
crest of the Great Rift Valley
that the climate is tropical,
rainfall of 75-125
centimeters,
characterized
land, woodland
and the eastern shore of Lake by a lack of frost, a mean annual
and a diurnal temperature
15°C at night to as much as 37°C at midday;
561
DEMOGRAPHY
range of 28°C which rises from
that the Aora comprises a mosaic of grass-
and rain forest whose distribution
is closely related to topography;
and
that the fauna1 community
is rich in primates (prosimian, monkey, ape and human) and Contrary to the 80 km2 estimate given small vertebrates but sparse in large mammals. in previous reports for the size of the park, new calculations, based on Quarter Degree Sheet #92 (published in 1961 by Tanzania’s Geological Survey Division at Dodoma), yield a map area of 32 km2 for the entire park. However, the true surface area may be as much as 20 % larger when the rugged terrain is taken into account. All density measures given in this report are based on the new area estimate.
Some
pertinent
in a later section.
to evaluating
demographic
trends are presented
In the early stages of the field study, chimpanzees in the rugged National
terrain
Park.
and dense vegetation
meteorological
data
were elusive and difficult to observe
characterizing
many
portions
of Gombe
A banana provisioning
system was therefore launched in 1962 to hasten Continued throughthe apes to the presence ofhumanobservers.
the process of habituating
out most of the decade sampled here, artificial feeding reached a peak in frequency sessions) and volume the middle
1973a;
(about 68 kg bananas per session containing
and late 1960s but was then gradually
Wrangham,
bulk provisioning
1974).
Recent
evaluations
suggest that the procedure
reduced
4400 calories)
in the early 1970s (Teleki,
of the consequences modified
(daily during
behavior
of long-term,
large-
patterns, including
the
frequency of aggression and the kinds of mammals taken in predation, and organizational
patterns.
trends during the 1963-1973
The effects of this procedure
as well as nutritional upon demographic events and
study period will be explored in a later section of this report.
3. Demographic The raw census data collected between
Baselines
1 July, 1963 and 30 June, 1973 appear in Table
1.
During this decade, the records included 38 named males (plus 1 unnamed male birth) and 49 named females (plus 2 unnamed female births), as well as 6 cases of fetal death. All census data are tabulated
between
deaths logged for 1964 actually include June, 1964. Birth dates are known
or estimated
consecutive
mid-year
points.
all cases observed between for every individual
For example, 1 July,
the
1963 and 30
in the census records.
For
infants born to named females during the sampled decade, the events could be timed to the day in many cases and to the month for all others. As chimpanzee births occur throughout considered
the annual
cycle,
unobserved
births-i.e.
to have taken place at the mid-year
estimates become increasingly
those prior to 1 July,
1963-are
point of the estimated birth year.
prone to error as age rises, and are probably
These
more accurate
for females than for males because the ages of the former can often be tied to reproductive events. That is, the average age of females at birth of the first infant (1 l-13 years) can be added to the average pregnancy interval (4-6 years) between each known offspring. Flo’s minimum age, for example, woul,d thus be 12 + 5+ 5 + 5 + 5 + 5, or 37 years. Some additional time (3-5 years) could be factored into age estimates for old females on the premise that some fetal wastage and infant mortality occurred, for the
562 Table
G. TELEKI
1964 1965 1966 1967 1968 1969 1970 1971 1972 1973
Baseline census population
1
Years
ET AL.
Births*
Census
data
(1 July,
Deaths?
23 29 30 24 24 21 21 22 24 16
18 22 32 29 22 22 24 28 27 23
41 51 62 53 46 43 45 50 51 39
2 2 1 1 0 0 1 3 2 0
2 3 4 0 1 2 2 1 1 2
4 5 5 1 1 2 3 4 3 2
0 1 6 1 3 1 1 0 0 2
2 2 0 3 7
Tally 234 Annual x23.4
247
481
12
18
30
15
24.7
48.1
1.2
18
3.0
196380
Fetal deaths:
June,
the stud
Migration Y_ Imnugr.
Emigr.
$6
$3
??
0 1 1 0
2 3 6 4 10 3 1 1 1 2
0 3400+7 0 37 1 0 2 2 0000 10003-3 10101 101 0 0 2 0 0000 0 00
18
33
6
+6
1.5 1.8
3.3
0.6
0.6
2
1973) for
+17
Net
??
0 0
0+10 0 +2 0
0 0
0 0
8
6-14
0.8
4 5 6 3 2
03 +1 +2 0
-8-10
1.7
Total pregnancies§
4 4 3 2
+5
36
0.5
3.6
I.0
* Figures include unnamed births. t Figures include unnamed deaths. 5 Observed fetal deaths are probably below actual occurrence level, especially in the early years. $ Combined list of live births and fetal deaths.
average birth interval may in such cases be reduced by 2-4 years. mum age would then become
40-45
Flo’s estimated maxi-
years.
As such events may not be evenly distributed in the lifespan of all females, minimum age estimates are used in all demographic measures presented pearance
here.
The ages of males are based on more arbitrary
and behaviour,
with opinions
obtained
evaluations
of size, ap-
from several investigators.
Death dates are inherently more difficult to establish, especially for mature individuals, because emigration
and mortality are not always distinguishable
in field conditions.
Thus
the death category here includes only those cases which were directly observed and those for which 1973,
strong circumstantial
19756;
appearances servers.
Teleki,
1973c).
evidence
and all new chimpanzees
Both categories
was available
The migration
remain
who
somewhat
category
(Lawick-Goodall, includes
were previously flexible
because
1968,
1971,
all unaccountable
dis-
unknown
to human
some emigrants
ob-
may have
died and some immigrants may have been familiar to named chimpanzees. Only those individuals who entered into or departed from the main study area for a minimum of 12 months are considered
migrants.
Most named chimpanzees were observed within the central study area (see maps in Teleki, 1973a, 19756) at least once per month, and for many there are daily or weekly attendance records. But few individuals were present for the entire 1963-1973 decade, and there are gaps of as much as 6-12 months in the attendance profiles of some chimpanzees. To cope with these discrepancies, ape-fears were calculated for all named individuals in order to generate life tables, fertility schedules and reproductive rates with For example, female Winkle was thought to have been born in maximum accuracy. 1960 (at the mid-year
point, as she was unknown
then) and was observed
in the study
GOMBE CHIMPANZEE
area from 21 August, accordingly
563
DEMOGRAPHY
1968 to the end of the sampled decade
on 30 June,
1973.
She is
defined as contributing
0.8 ape-year for her age at entry (estimated at 8 years), and 1.O ape-year for each of the following 4 years (ages 9, 10, 11, 12). Any individual present for the entire study period
is of course allotted
IO.0 ape-years.
The tally for the
decade yields 237.2 ape-years for males and 246.0 ape-years for females. This technique is designed to cope with two problems specific to data collected field primatologists. beyond
First, some individuals
a home range normally
precludes
the assumption
occupied
temporarily
travel
measure
a prescribed zone. years”-is essential in demographic Even among baboon
alone
by
within or
by a given social unit, and this phenomenon
that anyone not observed by investigators
A standard
membership.
may
of observed
is no longer within
attendance-perhaps
“primate
studies of all species exhibiting
flexible social unit
troops, which are often considered
stable in member-
ship, there are temporary and permanent migrants (Ransom, 1971; Harding, 1973). Second, field data on primate survivorship is less likely to be complete than the mortality data available matologists
to demographers
studying
human
populations.
An assumption
by pri-
that individuals
must be present even when they are not observed for long The ape-year measure periods may artificially inflate various demographic calculations. eliminates such assumptions. Many of the Gombe chimpanzees
grades-infant,
juvenile,
adolescent
and in many cases adulthood and old.
But when physiological
Goodall,
1967, 1968, 1973).
therefore 1975b).
contains Further
and behavioral
discrepancies inconsistencies
among
(We
mature
of 2-3
(e.g. Nishida,
several experienced
interval
1968; Sugiyama,
and if a consensus opinion
field investigators
a note of caution
here about
(Lawick-Goodall, when reports from
1968; Lawick-Goodall,
about the probable
during the course of a study, then the above problems
must introduce
rates (see Lawick-
data on age-class membership
are introduced
are available
(and mixed
the resultant intervals
at variable
years in any given
in class assignment
1975b). If birth dates for many individuals reached
criteria are thus combined
The most recently published
grounds,
into two stages-prime
to assess rates of development,
because individuals
different field sites are compared
individuals
into one of four specific age
physical and/or behavioral
can in turn be readily segmented
with an added measure of intuition) are likely to be inconsistent
can be readily placed and adult-on
ages of older
are largely resolvable.
using growth
captive studies of the same primate species because captivity
can be
rates obtained
can dramatically
from
accelerate
both physical and behavioral development.) But for purposes of demographic analysis, standardized age intervals are essential. We have accordingly chosen to use an interval of 4 years for each of the three lowest age grades listed above, an interval of 14 years for the prime adult grade and an interval of 10+ years for the old adult grade. Thus, infant covers a span of O-4 years, juvenile 5-8 years, adolescent 9-12 years, prime adult 13-26 years? and old adult 27-38 + years. The upper terminus on this scale is close to the estimated age of Flo, the oldest Gombe chimpanzee. As the sample is very small for wild chimpanzees, this upper figure remains flexible. The 12-year marker for achieving adulthood represents an average for males and females. When the two sexes are treated separately, female adulthood is reached at about 11 years, the mean age at first pregnancy, and male adulthood at about 13 years. In order to maintain
consistency
in all analytical
procedures,
all named chimpanzees
have been assigned to age classes on the basis of the number of years elapsed since their
564
G. TELEKI ET AL.
known or estimated dates of birth.
This is in some cases an over-simplification
able data, but will have to serve until field studies covering
of avail-
several generation
spans are
completed. 4. Chimpanzee The chimpanzees
inhabiting
Gombe
National
Population
Units
Park can be viewed in terms of three basic
types of numerical units : a park population, a study population, and a social community. TWO of these units-the park and study populations-are probably somewhat artificial in their construct, the first because park boundaries were drawn long before the ecological and ethological needs of resident chimpanzees were known and the second because long-term
banana provisioning
probably
altered the structure
of natural
units
within
the
park system. The three types of units must therefore be clearly defined in relation to one another and to other units described in the literature before the structural (size, composition) and dynamic (mortality, natality, chimpanzee society are examined in detail. All chimpanzees Some chimpanzees
living
within
occasionally
Gombe
migration)
National
range beyond
characteristics
the park pop_dution.
Park constitute
the park borders,
of Gombe
sometimes
for several
days at a time, but today this unit is probably a near or complete reproductive isolate surrounded on all sides by physical barriers: the rift escarpment and lake shore to the east and west, and human to the east (Teleki, completed,
settlements to the north and south as well as beyond
1973a).*
but it probably
A detailed
census of this park population
consists of about
100-150
chimpanzees
the rift
has not yet been (Lawick-Goodall,
1968). The park population some 20-50
members
These communities productively
contains (Teleki,
at least three social communities, each of which includes 19753),
can be spatially
nor behaviorally
and perhaps
and socially
isolated
because
one or two smaller units as well.
distinguished,
but they are neither re-
some migration
occurs
among
them at
Gombe and elsewhere (Lawick-Goodall, 19756; Nishida & Kawanaka, 1972). Due to the interdisciplinary and international nature of field research on chimpanzees, the literature abounds with synonymous
labels referring to a fundamental
organizational
unit of chimpanzee society that contains some 30-80 members and occupies a given home Thus, range. Such ranges may be discrete or overlapping, stable or seasonally mobile. the natural
“social
called a group a unit group Goodall, “troop”
unit”
(Reynolds (Nishida,
19758).
originally
described
& Reynolds, 1968),
and a social community
In a primatological
level of organization,
by Itani & Suzuki (1967) has also been 1965), a regional population (Sugiyama, 1969))
framework
(Teleki,
1974, 19756;
this chimpanzee
whereas in ethnological
Lawick-
unit is comparable
to
terms it is perhaps most closely The matrifocal unit, which has
allied to a “band” level of human social organization. been structurally and functionally described for several non-human primate societies (Teleki, 1975a), is therefore not, as Lawick-Goodall (1968) once postulated, the only lasting social nexus among wild chimpanzees.
* The eastern range of Pan troglodytes today runs parallel to the western arm of the Great Rift Valley, with several narrow belt that extends from Lake Albert to the latter, the chimpanzee range foilows the to Mt. Lubangagulu (6’35’s) in Tanzania, inland (Kane, 1972).
populations inhabiting a Lake Tanganyika. Along eastern shoreline at least no more than 30-40 km
GOMBE
The Gombe
Stream
Research
larger social communities
within
CHIMPANZEE
Centre
565
DEMOGRAPHY
is located
within
the home range
the park, one that is hereafter
referred
of one of the
to as the “Ka-
kombe
Community” because a valley having that name lies near the center of this home range (see map in Teleki, 19753). B ecause most or all of the Kakombe community presumably became habituated to humans via banana provisioning, and also because most observations between 1963 and 1973 occurred within a small central study zone that covers a map area of about 8 km2, and perhaps as much as 10 km2 surface area, this community is the core of the demographic survey reported here. As will be described in a later section, however, the membership of Kakombe com-
munity was probably augmented by several pseudo-migrants who were attracted to the main study zone by the availability of bananas, and who therefore adopted temporary or permanent
residence
in this area.
This larger demographic
constellation,
which con-
records, constitutes a study pojulation whose If all chimpanzees suspected of having mean size during the decade was 48.1 individuals. tains all named
chimpanzees
pseudo-migrant
status are subtracted
the Kakombe
community
Population opportunity rugged
from the annual
densities cannot The
census records,
the mean
size of
would be about 40 individuals. be precisely
for error is further increased
terrain.
the actual
in the center’s
calculated
from these census figures,
by a problem
in measuring
and the
surface area in such
estimated
map area of the park (32 km2) may be much less than surface area (32 + 6 km2), and a similar discrepancy may exist (8 km2) and the surface area (8 + 2 km2) of the central study zone.
topographic
in the map area
Moreover, estimates of crude and specific densities should reflect differences in patterns of habitat utilization, and preliminary data on ranging habits at Gombe suggest that several km3 of the steep, rocky terrain near the escarpment
crest are not routinely
Given this variability only be roughly 2.2-3.1
in census and area measurements, Starting with 100 chimpanzees estimated.
and the s.d. about 2.94.0;
and the s.d. to 4.3-6.1. community
exhibit
for the entire
with 150 chimpanzees,
utilized.
crude and specific densities can in the park, the cd. is about the c.d. would rise to 3-334.6,
Within the central study zone, the study population
a c.d. range of 4.8-6.0,
park region.
which falls within
and Kakombe
the 2.2-6~1
As most or all of this zone is heavily
range
utilized,
given
the specific
density should be similar to the above figure.
Pojwlaiion size : the annual census The
annual
size of the Gombe
high of 62 members records
include
study population
to a 1973 low of 39 members
96 chimpanzees
: 87 named
fluctuated (Figure
considerably-from
a 1966
1).
census
The
1963-1973
individuals, 9 unnam-ed births and fetal a very high turnover during these years:
deaths. But the study population experienced only 4 females and 9 males (13.5 % of 9’)o were present for the entire decade. births, 33 postnatal plus 6 fetal deaths, and a net gain of 2 immigrants between
With 30 1966 and
1972 (when migration was probably least affected by provisioning), the study population’s composition differed considerably from year to year. Indeed, given these gross internal changes it is remarkable that the population maintained as much numerical and compositional stability as shown in the accompanying graph. In order to estimate the probable size of the Kakombe community,
the average number
566
G. TELEKI
ET AL.
Figure 1. Temporal fluctuations in the Gombe study population (solid line) census data, and in the estimated size of the Kakombe community (broken line), shown in relation to various banana provisioning schedules (A-E).
PROVISIONING
::I, , . , , , , , , . ,964
,965
,966
1907 1968 ,969
1970 197,
1972 ,973
YEARS
of suspected
pseudo-immigrants-i.e.
nanas-can
be subtracted
corrections
those entering
are made only between
was most consistent.
the local home
from the annual census totals.
these trends and provisioning
in Figure
1, these
1966 and 1972, the period when banana provisioning
This provides a crude comparative
within both natural and artificial
range to seek ba-
As depicted
measure of demographic
units,. The coincidental
schedules
(see stages A-E
and/or in Figure
trends
causal links between 1 and Table
9) are
discussed in later sections. Pojudation When
composition : age and sex classes
the mean annual membership
line (48.1 =
of the study population
13.1 ‘A juveniles,
age classes break down as follows: 17.0 ‘A adolescents, 54.7 y0 adults. The adult/subadult
entire study population is 1.21. Sex ratios within the study population differences
among
100.0 females,
is used as a numerical
100 %), the four major
the four age grades.
while the age-specific
infants, 85.3 for juveniles,
base
15.2 % infants, ratio for the
vary little on an annual basis but reflect major The 1963-1973 mean sex ratio is 94.7 males to
sex ratios in the four main classes are:
141.2 for adolescents,
and 94.8 for adults.
breaks down into 83.6 for prime adults and 200.0 for old adults.
65.9 for
The latter class
Thus, while the sexes
seem to be at or near parity within the study population
as a whole, and also within the adult class as a whole, there are marked differences in the remaining classes. For example, males are greatly exceeded by females in the infant and juvenile classes while the reverse is true in the adolescent and old adult classes. These discrepancies suggest that sexspecific factors regulate survivorship within age-classes, all the more so in regard to excess adolescent and old adult males because female births exceeded male births by 50.0 % during the 1963-1973 period (see Table 1). Two population pyramids have been constructed from the baseline census data to further illustrate these features (Figure 2). Pyramid A depicts only percentage membership in various age classes, and shows that a fairly even distribution exists in all but the two youngest classes. Pyramid B adds a sex-class dimension to this age distribution, and thereby shows that females decrease rather evenly during the middle ages (9-24 years) and more sharply at later ages (25-40 years), while males decrease sharply during early
GOMBE
CHIMPANZEE
Figure 2. Percent distribution of age-classes (pyramid A, top) and of age- and sex-classes (pyramid B, bottom) in relation to total population size. Both pyramids reflect small percentages of young individuals (infants and juveniles) in the Gombe study population.
567
DEMOGRAPHY
A
Tzi -
33-36
1
I
29-32 25-28
I
y21-24
z ;;r;-, -
I
13- 16 9-12
1
7 o-4 -
I IO
5
0
15
1 20
PERCENT
ZG -33-36
d
6
2
29-32
25=18 21-24
y
1
z17-20 13-16 9-12 5-8 O-4 -
I I I I f
1
0
5
IO
5
IO
PERCENT
(13-20 years) and more gradually at later ages (2 I-40 years). A preponderance of old adult males is clearly illustrated in pyramid B. There is unfortunately little merit in speculating about the natural causes behind these phenomena because differences
adulthood
in male and female survivorship selective
effects of provisioning
in the 21-32 upon
year brackets may simply be related
the mortality
and migratory
to
patterns of the two
sexes. When compared to standard pyramids for wildlife populations (e.g., Qdum, 1971), the chimpanzee study population seems to reflect neither marked stability nor potential for rapid expansion.
In fact, when the topheavy appearance
of the chimpanzee
pyramids
is viewed in the context of the epidemics recorded at Gombe, the indication is that considerable time is needed to achieve reproductive recovery from such major crises. The slow
maturation
contribute
and
reproduction
to a recovery
chimpanzees,
rates described
in the
following
lag which may span an entire generation,
section
could
which, for the Gombe
is an average of 19.6 years.
be typical for wild chimpanzees,
A topheavy structure may, on the other hand, for the K-group pyramid at the Mahali site also leans in
a topheavy
in pressj.
Although
direction
(Nishida,
there is still no precise scale for gauging
in wild ape populations,
normality
of age-sex distributions
an urgent need exists for field techniques
whereby
the repro-
ductive status of a potentially threatened primate population can be monitored. Data on the composition of the Kasakati (Izawa, 1970) and Mahali (Nishida, in press) study populations supplement those available from Gombe, and the combined figures can be used to develop an “optimal” matrix of age/sex composition panzee unit. These three sites are selected for comparison proximity and ecological similarity.
for an hypothetical chimdue to their geographical
As shown in Table 2, the three study populations exhibit some variation in percentages of immature and mature males and females, but the variation is well within what would
568
G. TELEKI
Table
2
ET AL.
Percentage distribution of individuals, by age and sex, for three chimpanzee population units in western Tanzania, combined to depict an hypothetical population with an optimal composition
Gombe (study population) Males Females Mature Immature Samples :
Kasakati
Mahali
(Z group)
(I( group)
Males
0.27 0.28 0.23 0.22 N = 4% 1 mean over 10 years
B Males
Females
0.20 0.30 0.25 0.25 N = 36 members over 2 years
Average Unit
Females
0.21 0.34 0.09 0.36 N = 26.3 mean over 8 years
Males
Females
0.23 0.20
0.30 0.27
Note: the marker age separating immaturity years in each study population.
be expected
if age and sex distributions
varied randomly
(x2 =
and maturity is about 12
1.61, p = O-90 at 5 degrees
of freedom). Also shown in the table is an average, or optimal, matrix which reflects a preponderance of both immature and mature females. Comparison of these figures with the Gombe
matrix
suggests that the Gombe
study population
is somewhat
deficient
in
both categories. Even if the Gombe study population continuing
percentages
reproduction
additional
measure
to counteract
populations
deviations
managers
could
monitored
and to ensure
if and when necessary, from
need cause no immediate
should be carefully
from
then
regional
determine
long-term
development
become
alarm,
in the coming
it is evident
that this
years in order to evaluate
survival.
Steps could then be taken,
of further
discrepancies.
When
data
available, or local
this tentative model can be refined to Field primatologists and wildlife norms.
the need for protective
action
on a more
pragmatic
basis than is now common. 6. Chimpanzee The analytical disciplines Gombe
techniques
of human
data-small
death dates, etc.-are
Population
Dynamics
applied in this section have been selectively
demography numerical
and wildlife
samples,
in someways
biology.
continuous
borrowed
However,
census records,
imprecise
unique to the field study ofnon-human
tions, and do not always allow for application primatologists
have not yet developed
our approach
is often experimental.
of standard
the analytical Demographic
from the
the nature
demographic
of the
birth
and
primate populatechniques.
As
tool kit needed for this special task, research on non-human primates is
clearly in need of development and refinement, and this is but a first step in that direction. The raw data upon which analyses of mortality, natality and migratory patterns are based
appear
the 1963-1973 Mortality
in Figure
3, which
is a graphic
summary
of all events recorded
during
study period.
patterns
During that decade 33 postnatal deaths were known or estimated to have occurred within the study population, the causes running a gamut from contagious disease to fatal accidents. There were 6 additional cases of fetal death, representingl5.4 ity recorded, but these will be discussed with natality patterns.
% of the total mortal-
569
GOMBE CHIMPANZEE DEMOGRAPHY
Figure 3. Raw data on mortality (top), natality (middle) and migratory (bottom) patterns in the Gombe study population during 1963-1973.
0 0
The
distribution
adolescents, chimpanzees,
48.5 o/oadults). with mortality
the susceptibility wastage.
of postnatal
deaths
but highly variable
of infants
is close to parity
Deaths (ii=331
n
Fetal Deaths
0
Live Births
Emigration
12
(n=6)
Deaths
Postnatal
Immigration
14
54.5 % females)
Fetal
(n=6)
(8 =30)
(n=23) In= 18,
along sex lines (45.5 % males,
along age lines (33.3 % infants, 9.1 % juveniles,
Mortality
in the first four years of life probably to both
No proof has been obtained
9.1 %
is heaviest in the infant and adult classes of Gombe direct
(e.g. disease)
from East African
being exacerbated
and indirect
by
(e.g. orphanage)
studies that carnivore
predation
contributes to chimpanzee mortality (Teleki, 1973a,d), although West African populations are heavily preyed upon by humans (Harrisson, 1971; Sabater Pi & Groves,1972; Gartlan,
1975).
Diseases,
parasites,
causes of nearly all wild chimpanzee
genetic
abnormalities
and injuries
are the likely
deaths.
An epidemic of some paralytic disease (probably poliomyelitis) and two severe bouts of respiratory illness (probably influenza) struck the study population in 1966, 1968 and 1969. Directly or indirectly-that is, via infection or orphanage-these events accounted for most of the 23 postnatal deaths logged over this 4-year span. In fact, some 30 ‘A to 40 % of the mortality recorded at Gombe during the 1963-1973 period can probably be attributed to pathogenic the estimate underscores
causes. Even if some cases have been incorrectly diagnosed, the high susceptibility of this population to contagious diseases.
570
G. TELEKI
Control
of epidemics
conservation,
ET AL.
should thus be a key issue in future programs
at Gombe
and probably
elsewhere
of management
and
as well.
Given this insight, it i.s especially unfortunate that so little is known about the epidemiology of wild chimpanzee populations. Some general information on disease symptoms and internal parasites is provided by Lawick-Goodall (1968,1975), who refers to undiagnosed (one
cases of skin ailments
resembling
(perhaps
goitre),
influenza
and peritonitis. Attempts
Ciliates,
nasal
hookworms
congestion
growths
often
as we11 as confirmed and roundworms
to treat the paralytic
on the face and neck
accompanied
by coughing
cases of pneumonia,
have been recovered
disease
(Lawick-Goodall,
enteritis
from feces. 1968)
and a
as Rhinophycomycosis enteromorphae (Roy & Cameron, 1972 ; Roy Given the duration of the Gombe project, the epidemiological in field conditions.
1974), comm.)
noted
Budongo
sparse.
graphic
Forest.
Behavioral
descriptions
1968,
Lawick-Goodall
1971,
of chimpanzee
1975a-c;
& Hamburg,
deaths are somewhat
Suzuki,
1971a;
1974), but such information
Bygott,
more com-
1972;
Teleki,
is of little use in demo-
analysis of the causes of mortality.
No conclusive traction
Other field projects have fared no better: Kortlandt (pers. cases at Beni, and Suzuki (1971 b) observed an amputee at
7 “paresis”
(Lawick-Goodall,
19736;
evidence
or dissemination
distribution
of crude
that an historical mains somewhat middle
and colds),
mange),
diagnosed
data are extremely
mon
lung
or common
have been made
facial fungus,
(one resembling
years,
of a direct
death
linkage
link between
banana
provisioning
of disease has yet been obtained rates so clearly
cannot
circumstantial.
parallels
at Gombe.
the various
and the conBut the annual
provisioning
schedules
be discounted even though the evidence at present reMortality rates were low in the early years, high in the
in the final years of the decade (Figure 4). The crude death rate ranged from a maximum of 2 17*4/ 1000 in 1969 to a minimum or 19.6/1000 in 1972. Computed from the 33 cases backed by conclusive or strongly circumstantial evidence, the mean mortality rate for the decade was 6%0/l 000. If all losses from the study population
and low again
were considered
as deaths
Figure 4. Temporal distribution of mortality and natality rates per 1000 individuals for the Gombe study population. Fetal wastage is excluded from both samples.
(i.e. no migration),
the crude
250
death
rate would
-
Mortality
--__
Natality
200-
:_
ISO-
L B w 2 az
IOO-
SO-
I 1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
GOMBE
reach
11 l-7/1000
The
individuals.
CHIMPANZEE
true rate, which probably
these two extremes, seems high for such a small population. raised mortality
571
DEMOGRAPHY
is further strengthened
by a concentration
lies somewhere
between
The premise that provisioning of all known prenatal deaths
in the same period that postnatal mortality reached a peak (see Table 6 for a non-parametric runs test of fetal wastage). Abnormally high and frequent aggregations of chimpanzees within a small area, accompanied by increased aggressive limited food resource, may well have raised prenatal and postnatal exceptional
levels between
1967 and 1970.
Various
introduced
and transmitted
quite effectively
in these conditions
(see summary and/or
contagious
competition for a mortality rates to
diseases may have been of high density and stress
of similar cases in other species by Stott, 1969), with imported
bananas
human suppliers perhaps serving as vectors.
Survivorship. Postnatal Gombe
mortality
study population
cases-the published
(Table
can be transformed 3). Despite
into
a current
life table
for the
the structures of a small sample-only
33
computations are enlightening because no survivorship scale has ever been for wild or captive chimpanzee groups from which individuals were not arti-
ficially
culled. Demographic data obtained from captive colonies are neither complete nor reliable (Bourne, 1972; Butler, 1972), and the unpublished demographic report on Kortlandt’s Beni study population (Ku&ten, 1972) attempts to reconstruct chimpanzee
demography Table
3
Age intervals
by mathematically
reshaping human mortality tables and then extrapolating
Current life table based on known and estimated ages at death of 33 Gombe chimpanzees, with both sexes combined. Mean length of life is 13.5 years
Number dead
Apeyears
(years),
xtox+n o-2 1 3AA 5-6 1 7-8 B 9-10 11-12 I c 13-14 15-16 17-18 19-20 D 21-22 23-24 I 25-26
8 3 2 1 S 1 1 1 0 : 2 3 0 2 1 0 1
42.6 34.8 32.4 35.3 40.6 48.4 44.2 39.7 27.4 24.3 26.0 24.8 19.5 12.8 11.0 10.5 5.4 2.0 1.5
Probability of dying in each age interval,
&
Number entering each age interval out of 1000 born, L!
Average years of life remaining to survivors in each age interval %
0.188 0.086 0.062 0.028 0.049 0.041 0.023 0.025 0.036 0.000 0.038 0.081 0.103 0.234 0.000 0.190 0.185 0.000 0.667
1000~0 659.3 550.8 484.7 457.9 414.2 380.9 363.5 345.5 321.1 321.1 297.2 251.0 202.0 118.5 118.5 77.8 51.7 51.7
11.9 15,6 16.5 16.6 15.5 15.0 14.2 12.9 11.5 10.3 8.3 6,9 6.0 5.2 6.1 4.1 3.7 3.1 1.1
Note: to facilitate comparison with other demographic features, the age intervals are also represented in terms of age grades: A, infant; B, juvenile; C, adolescent; D, prime adult; E, old adult.
572
G. TELEKI
to obtain
a fit with wild chimpanzee
ET AL.
population
curves.
Similar
limitations
occur
at
other field sites, such as Mahali, where Nishida (in press) has collected data on natality and infant mortality but has not yet obtained a general picture of postnatal mortality. In contrast ape-years
to the life tables commonly
have
(the d, column) other components
been
used to compute
among
the Gombe
generated
for wildlife and human
the probability
chimpanzees.
in the life table are standard
of dying
The procedures (see, for example,
populations,
for each
age interval
used to generate Deevey,
all
1970).
Abridged life tables for each sex, based on 15 male deaths and 18 female deaths, were then compiled to illustrate differences in male and female life expectancies at various Table
4
Abridged for each 15 males
life table results showiqg average sex, based on kmnwn and estimated and 18 females
Age intervals (years)
O-2 3-s 9-14 15-20 21-26 27-32 33-38
life expectancies ages at death of
Average years of life remaining to survivors in each age interval, e, / Males
\ Females
(years)
(years)
14.3 15.9 16.6 13.3 El.9 6.7 3.0
IO.9 16.2 15.3 13.2 8.4 5.5 3.0
Figure 5. Actual survivorship in the Gombe study population (solid line) during 1963-1973, and estimated survivorship in a hypothetical wildchimpanzee population (dotted line) based partly on Kortlandt’s data from the Beni site. The latter has been adapted from Kuijsten (1972). Both curves are on a logarithmic scale.
20
AGEIN
3”
YEARS
3”
GOMBE CHIMPANZEE
ages (Table
4).
The results indicate
573
DEMOGRAPHY
of life
that the expectation
at birth (e,,) is greater (er5). The dif-
for males than for females, but that parity is achieved by early adulthood
ference arises mainly from a less favorable set of death rates for females between infancy The causes remain unidentified, and may involve physiological, ecoand adolescence. logical and/or
behavioral
factors.
When plotted on a logarithmic
scale, from the I, column
in Table
3, the survivorship
curve shows a rapid drop during the initial 4 years of life and a steady decline over the following age intervals (Figure 5). The curve in general exhibits a remarkably even convex shape that is characteristic the end of the life span (Odum, (1972) is also provided
of populations in which most mortality occurs toward 1971). The survivorship curve obtained by Ku&ten
for comparative
purposes. although
what skewed because fragments of several populations The life expectancy that management particularly
and survivorship
rates exhibited
policies should include contingency
among females, when the population
his rates are proba’bly
some-
were combined. by Gombe
chimpanzees
plans for reducing
suggest
infant mortality
is struck by unforeseen calamities
such
as epidemics. Swift recovery by such a small and isolated population is essential if gaps in the age pyramid are to be averted, and may warrant artificial stimulation (e.g. medication) by wildlife managers. Orphan mortality. At Gombe,
chimpanzee
(Clark, in press), and are physically, mothers
infants nurse
psychologically
for
an
for at least that many
tapers of& offspring
years. Even after the nutritional may hinge mainly upon continuing
survival
mother for several additional patterns within the younger behavioral
of
years (Lawick-Goodall,
1975u-c).
5.2 years upon their
basis of dependence association with the
Mortality
and survival
age classes may therefore be strongly affected by social and
factors (relationships
factors (weather,
average
and socially dependent
within matrifocal
units, etc.) as well as various extrinsic
disease, etc.).
The deaths of G named deaths, are listed in Table
orphans,
all of whom
died within 3.5 years of their mothers’
5. All but one of these (Pepe) died well before the expected
time shown for their respective
ages in the life table.
male whose death was probably
unrelated to being orphaned.)
died within 0.1-2.5
(Pepe was a paraplegic The remaining
adolescent 5 orphans
years of their mothers, and only one (Flint) was older than 4.5 years.
In at least 3 cases (Cindy,
Merlin,
Flint) the symptoms
prior to death included
general
listlessness and lethargy, with severely reduced maintenance activity and social inter-. action. And in the particular case of Flint, who was intensively observed during the 2 5 days after his mother’s death, psychological
distress and depression may well have contributed
to or even caused death, although he also acquired an intestinal infection (Lawick-Goodall, 197%; La-wick-Goodall & Hamburg, 1974), These and other forms of “grieving” behavior reported elsewhere (Teleki, 19736) suggest that wild chimpanzees perceive death among close kin as an emotional events.
issue, and may possess a generalized
concept
of such
Precise prediction of the age at which orphans are likely to survive parental mortality is prohibited by the small size of the available sample. The data merely suggest that a. death/survival transition occurs for most orphans around 5 years of age, but may stretch another 3-4 years in special circumstances. Prior to the 5-year limit, nutritional deficiencies, coupled mortality.
with social and other deprivations,
are the most likely causes of orphan
30 24 24 30 27 26 37
24 26 24 37 27 37 37
December 1964 October 1968 January 1968 December 1964 January 1969 March 1965 December 1972
October 1968 March 1965 January 1968 December 1972
January 1969 December 1972 December 1972
Bessie Vodka Sophie Bessie Oily Marina Flo
Vodka Marina Sophie Flo
Oily Flo Flo
*d = d = -t = ? = ?A = C = ? =
16 26
January 1968 March 1965
F F
F F F
F
M M M
M M
M
M M
M
13.6 16.5 19.5
11.3 11.8 12.6 13.5
4.5 6.3 6.6 7.5 7.6 7.8 8.9
3.2 3.7
14.2 12.1 10.3
4.3 + 0.5 + 0.5 +
16.5 16.5 16.5 16.6 16.6 16.6 16.6
15.0 15.0 15.6 14.2
d + + ? + + d
15.6 15.6
11.9
4.7 + 3.5 d -? 0.5 +
2.5 4.7 4.4 3.5 4.4 8.2 0.1
0.2 d 1.6d
0.1 d
Faben -
Faben Figan -
-
Evered Pepe Faben Figan Fifi -
Pepe Miff Bumble Jomeo Sally -
Sally Sniff -
M 19.5 - --
19.5 16.5
13.6 11.8 19.5 16.5 13.5
7.8 7.5 11.3 12.6
C -
A A A ? A ? ? C A C C -
F F M F -M M M M F ---M M --
M 11.8
? A A
F 12.6 M 6.6 --
Known death, probably related to being orphaned. Known death, probably unrelated to being orphaned. Surviving at end of study period on June 30, 1973. Disappeared, possibly dead. Adoption behavior (described in Lawick-Goodall, 1975c) observed between older sibling and orphan. Contact and interaction, but no overt adoption behavior, observed between older sibling and orphan. Orphan not observed in association with older sibling.
Evered Figan Faben
Jomeo Pepe Sally Fifi
Beattle Sherry Sniff Bumble Gilka Miff Flint
F
Cindy Merlin
1.3
Behavior of older sibling toward orphan?
Circe Marina
F
Sorema
Name of Sex and known or age of suspected older older sibling (years) at age when sibling mother died
Orphan’s expectation of life
24
(years) when mother died
Subsequent survival span (years) of orphaned offspring*
January 1968
spring
spring
Known or estimated age of offspring
Sophie
Off-
Off-
Name of Sex of
Date of mother’s death
Name of mother
Mother’s estimated age at death
Mortality and survival patterns among 17 named offspring who were orphaned (at 1.3-19.5 years of age) when 7 mothers died during the 1963-1973 study period
Table 5
% b t*
!I.+
2
P
UI 2
GOMBE CHIMPANZEE
Whether
adoption
of orphans
by older siblings
uncertain.
The final column in Table
commonly
exhibited
575
DEMOGRAPHY
has a positive influence
5 suggests that adoptive
behavior
also remains
tends to be most
toward the youngest
orphans, but sibling adoption does not seem to promote survival below the 5-year age limit. Nor is there any indication that the sex of In terms of wildlife the older sibling regulates adoptive behavior or influences survival. management, it would be advisable to monitor the welfare reproductive females who
of
head matrifocal units, having large matrifocal
and to investigate the potential units within a social community.
survival
benefits
provided
by
Natality patterns During
the 1963-1973
study period,
36 pregnancies
terminating
in 30 live births and 6
The disparity previously fetal deaths were recorded within the study population. for the sex ratio of postnatal deaths among infants (3 males/S females = 0.375) matched
by the disparity
with a low reproductive
present
in births
(12 males/l8
rate, a slow maturation
females = O-667).
rate, and a low incidence
noted is not
Together
of twin pro-
duction (observed occasionally in captivity but not yet in the wild), the above discrepancies in female natality and mortality rates point to a low growth rate for chimpanzee
populations.
The
picture
is somewhat
incomplete,
however,
because
the sex
of stillbirths and miscarriages was not determined. The annual distribution of crude birth rates is a mirror image of the crude death rates described
previously,
and were therefore
as likely
affected
by provisioning
schedules.
Natality rates were high in the early years, low in the middle years, and high again in the final years of the decade (see Figure 4). The crude birth rate ranged from a maximum of 97.6/1000 the entire
in 1964 to a minimum
IO-year period.
the study population Measures
of infant
of 18~8/1000 in 1967, with a mean of 62*0/1000
As the mean mortality
strayed from equilibrium death
rate
(233*3/1000),
and natality
largely as a result of migratory combined
(466*7/1000), infant mortality risk rate (68*3/1000), 1000) can also be obtained from the census records.
for
rates are close to equality,
infant
plus fetal
and fetal mortality
variations. death
risk rate
rate (43.11
These measures are all based on a reduced infancy span of O-l year, rather than on the usual 4-year interval, to match demographic standards established for human populations. A test was undertaken
to determine
year could be assumed for live births, the Gombe
study population.
whether
a constant
fetal deaths,
probability
and total pregnancy
of occurrence terminations
* These data, when fitted to Poisson distributions
per in
(Cooper,
* The fit of an observed set of a events, classified by their frequency of occurrence (f), can be examined by the chi-square method (Bliss, 1953). This procedure is one-tailed, however, in the sense that it merely gives an accurate probability that the observed unbiased variance (se) is greater than the observed mean (a) of the distribution. In a perfect Poisson distribution, s2 = 2. Another approach was therefore adopted, using this equation as a null hypothesis, in order to obtain a two-tailed test of the conformity of each empirical distribution to the expected distributions. The observed mean (2) was taken as an expected variance with infinite degrees of freedom. (This technique increases the conservativeness of the test.) The observed variance (s”) was then considered in terms of its own degrees of freedom (n - 1). The variance ratio (F) was finally computed, using appropriate degrees of freedom in the numerator and denominator, and the two-tailed probability obtained from a table of F (Lindquist, 1953).
576 Table
G. TELEKI 6
Number of events,
f
Types of events
Live births Fetal deaths Total pregnancies
ET
AL.
Annual frequencies of live births, fetal deaths and total pregnancies in relation to expected Poisson distributions, with a non-parametric runs test of annual fetal death frequencies (see census data in Table 1)
Live births obtained b obt
Number of events,
Live births expected, b=a
Fetal deaths obtained, dObt
Fetal Total deaths pregnancies expected, obtained, dem Pobt
Total pregnancies expected, P exn
5 4
6 3
0 0
0
1
1
2 3 3 1 1 0
: 2 2 1 1 1
Obtained Obtained unbiased mean and variance, expected variance,
?I
s2
30 6 36
2.22 0.27 1.60
i 3.00 0.60 3.60
Fmln_l 1.35 2.22 2.25
P (two-tailed) 0.20 0.19 0.19
Non-parametric runs test when a = number of years without fetal deaths, b = number of years with fetal deaths and u = number of runs yields R, = 5 yrs, nl, = 5 yrs and u,,.,,s = 3.
1972), show that the observed or obtained numbers of pregnancies are all close to the expected numbers required by the Poisson hypothesis (Table 6). It should be noted, however, that the empirical variances are consistently smaller than the empirical means. These results suggest that an underlying constant probability of conception seems to occur within the study population on an annual basis. A non-parametric runs test on fetal deaths between 1966 and 1970 shows that such wastage is significant at the 0.05 level (Table 6). As mentioned previously, most fetal deaths were probably associated with the stresses accompanying banana provisioning. Fertility schedules. Male chimpanzees at Gombe reach puberty at 9-10 years of age, but may not contribute significantly to the reproductive pool of the population for another 3-4 years as a result of social peripheralization until full maturity is achieved (LawickGoodall, 19756). Females typically begin to exhibit sexual swellings of the perineum at 9-10 years of age, and then experience an additional 1-2 years of adolescent sterility; these swellings become larger and more regularly paced during this period, and menarche probably occurs at about II-12 years of age (Lawick-Goodall, 19753). First pregnancies are likely to occur between 11 and 14 years of age (Clark, in press).* Most females are * Most of the dates marking developmental stages differ in wild and captive chimpanzees. Rates of physical, behavioral and probably psychological maturation can be 2 or more years slower in the wild (compare this report to Riopelle, 1963). Body sizes and weights are significantly smaller at Gombe (Teleki, 1973c; Lawick-Coodall, 19756).
GOMBE CHIMPANZEE
DEMOGRAPHY
577
likely to remain reproductively viable until death, although Flo, the oldest member of the study population, ceased to show swellings 2.8 years prior to her death. No verified cases of menopause have to date been recorded among wild chimpanzees. The mean generation span, or elapsed time between birth of a female and birth of her median offspring, is about 19.6 years for a sample of 10 Gombe females with 3 or more recorded births. In order to establish a standard measure of reproductive performance in the following calculations, the onset of a female’s reproductive span (wr) is here assumed to occur at age 10 years because a gestation period of about 8 lunar months can be subtracted from the minimum age at which Gombe females produce their first offspring. Termination of the reproductive span is assumed to occur at 35 years of age because the oldest Gombe female gave birth to her last infant when she was about 33 years of age. As the ages of initial and final parturition may be highly variable in a small population, however, these marker dates should be viewed as conservative estimates. It is possible that the 25-year average reproductive span used here may in some cases extend another IO-15 years. Among the 49 named females present in the Gombe census data, a sample of 27 females experienced ape-years in the lo-35 years age bracket. The surveillance period for this cohort encompassed a total of 137.8 ape-years. Of these 27 females, 20 exhibited either observed pregnancies or associated progeny during the 1963-1973 study period. Of the remaining 7 females, 6 were at or beyond the lo-year age level for only 6 months to 2 years of reproductive life. Only one female (Gigi) fell within the lo-35 age bracket and did not become pregnant at all during the decade. The reproductive performance of Gombe chimpanzees can be estimated from the above observations (Table 7). Given a potential reproductive span (w$ - wi) of 25 years, the data presented in the table yield the total pregnancy experience (O-235 x 25 years = 5.9 pregnancies), the total fertility (0.188 x 25 years = 4.7 live births), and the total lifetime fetal death experience (5.9 - 4-7 = I.2 fetal deaths) per female in the study population. Over the entire 25-year fertility span, the average probability of a live birth is 0.19 per annum for each female. When only the first 5 and last 5 reproductive years are compared, however, the figures show a 33 % drop in fertility, largely because most pregnancies occurred among those females who were IO-20 years of age. Nonetheless, the number of live births expected (be& is in fact close to the actual distribution observed (bobcj) among Gombe females. A chi-square test, measuring chance divergence of obtained and expected live births when the probability per year is 0.19, shows that the divergence is indeed small (x2 = 1*17,p = 0.9 1 at 4 degrees of freedom). Hence, these chimpanzees show essentially no statistical loss of reproductive capability with advancing age. According to the data presented in Table 7, the net reproductive rate (0.130 x 5 = 0.65 female offspring) is considerably lower than the gross reproductive rate (0.577 x 5 = 2.90 female offspring) in the Gombe study population. This discrepancy results from severe mortality in the young female cohort. The neonatal sex ratio in the small Gombe sample contains a scarcity of males, but the life table indicates that the prereproductive mortality of males is more favorable than that of females. Even so, this compensation is not enough to yield as many male offspring as female offspring a generation later. Coupled with the mortality and migratory patterns described in other sections, the above results allow for a guardedly positive prognosis regarding the future survival of the Gombe study population. But the survival balance may well shift into a negative *
O-228 0.193 0.238 0.128 0.154 z= 0.188
27
Live
z= 0.235
0.285 0.242 0.238 0.256 0.154
Live+ dead
Annual probability of birth
10 8 6 2 1
bexe
Live births expected if probability of birth is 0.2 years,
11*
7 0 3 1 0
Male live births observed
0.363
0.133 o*ooo 0.102 0.128 0.000
fi 0.478 0.404 0.301 0.184 0.071
P.
Males living at midpoint of each age interval,
0.119
0.064 0.000 0.031 0.024 o*ooo
Pafs
Agespecific fertility for surviving male offspring,
18
5 a 4 0 1
’
0.577
0.095 0.193 0.136 0.000 0.153
fs
Observed annual probability of live female births,
chimpanzees
Female live births observed
rates of male and female
Observed annual probability of live male births,
and reproductive
0.367 0.299 0.258 0.125 0.011
Pz?
Females living at midpoint of each age interval,
study
0.130
0.035 0.058 0.035 0.000 0*002
Pzf.
Agespecific fertility for surviving female offspring,
in the Gombe
* Note: these figures differ from the 30 births shown in the main census records because the mother of one male infant was not identified and aged in the initial study period.
29*
1376
Totals
6
12 a 7 1 1
3 2 0 1 0
52.7 41.4 29.4 7.8 6.5
Live births,
10 15 20 25 30
Fetal deaths
b obt
Apeyears
Reproductive performances population, 1963-1973
WI= 10 ws = 35
Age of mother,
Table 7
GOMBE
direction
if novel causes
CHIMPANZEE
of mortality-such
be held at a minimum .*
mans-cannot
struck the study population
as diseases The
paralytic
performance
of Gombe
ities is likely to be a long one. to virulent
Birth interuals.
Among
females,
by human vectors,
park boundaries
wild chimpanzees,
from
transient
hu-
epidemics
that
several females of breeding
only briefly
as orphans.
Given
the
the road to recovery from such calam-
If other chimpanzee
diseases transmitted
tions within fixed, narrow
contracted
and respiratory
in 1966, 1968 and 1969 eliminated
age, as well as several young females who survived low reproductive
579
DEMOGRAPHY
populations
are as highly susceptible
the enclosure of small, isolated popula-
may produce the capacity
a major
threat
to compensate
to survival. for high rates of
loss in the lower age classes is related to a shift in birth spacing when dependent offspring die. This mechanism warrants special attention because it carries important implications It has, in fact, been routinely for management of wild populations. ‘%-aise” reproductive rates in captive colonies (Butler, 1972; Bourne, Of the 20 females who produced females experienced various
used to artificially 1972).?
live or dead offspring during the 1963-1973
successive pregnancies.
As the number
period, 11
of successive pregnancies
for
females ranged from 1 to 4 cases, the total sample actually includes 17 paired In 7 cases (41%) the first offspring remained alive when a subsequent one was
events. born, and in 10 cases (59 %) the first died prior to the birth of the second. The average
interval
between
the birth dates of two consecutive
offspring
when each
survives is 5.6 years. According to Clark (in press), Gombe mothers lactate in the absence of perineal swellings for 3-4 years after parturition, then resume their cycles while still lactating
for about 2 more years, and finally wean their offspring at an average
of 5.2 years.
But if this routine
offspring, average
perineal
cycling
sequence
resumes much
of 1.6 years after the first dies.
70 % reproductive compensation (5.6 normal time interval between live births. related
to offspring
sooner and a second infant
survivorship
between Gombe
(Table
mortality
experienced
an
the birth dates of successive
females can therefore
achieve
off-
about
8). to have given birth to their first pro-
period, the mean age at the time of this event was 11.8 years.
As 3 of the 9 recorded cases resulted in prenatal or pregnancies
is born within
is high during the first 2 years
l-7/5*6 = O-696) when offspring die within the The data clearly indicate that offspring spacing
For the 9 females who were known or suspected geny during the 1963-1973
age
by the death of one dependent
As infant mortality
of life, the overall birth interval-measured spring-drops to an average of I.7 years.
is directly
is interrupted
deaths, it is probable
in the early years of the reproductive
risks for the offspring
that first pregnancies, span, involve greater
(x2 = 2.4, p = O-09, at 1 degree of freedom).
* Chimpanzees are especially prized as models in biomedical research, partly because they are susceptible to so many diseases contracted by humans (Harrisson, 1971). There is consequently some risk that contact between humans and wild chimpanzees will follow the pattern of past contact with North American Indian peoples, or present contact with such groups as the Kreen-Akarore of South America. T Despite the extensive medical and nutritional care given to captive chimpanzees, as well as having birth intervals greatly shortened by separation of infants and mothers, the reproductive rate of the Gombe study population compares favorably to breeding rates in captive colonies (Teleki & Baldwin, 1975). Considering the expense of domestic breedina programs for primates (Hobbs, 1975), Their ov&all productivity and effiy ciency should be questioned via comparison with wild populations,
580
G. TELEKI
Table
8
AL.
Birth intervals among successive offspring produced by chimpanzee mothers in the Gombe study population, 1963-1973
Viability of initial offspring when next progeny born
Number of cases, N Range
Dead Alive
10 7
Based
on the fertility
chimpanzees during
ET
a lifespan
however,
schedule
can theoretically
1.0-2.8 3.0-7.6
and birth
be expected
of 40 years.
1.6 5.6
interval
0.6 1.5
data
estimate
the balancing
effect of a reduction
Furthermore,
since old females
achieve
reduced ductive
record
is known
represents
mechanism
exhibited
adaptive
adaptations
6 live offspring,
extreme
this conservative
Given
(Lawick-Goodall,
Birth
or groups
interval
that serve to compensate units,
in the birth
interval
the above
estimate
raised to sexual maturity. female in the Gombe
with 5 surving
of reproduction
by individuals
significance.
beyond
when
their last is further The repro-
census records,
the age of 4 years,
adoption,
compensatory
any compensatory
a chimpanzee
population
has
may be one of many physiological
for a low reproduction
Orphan
serving a similar
and survivorship, within
reduction
at least in those cases observed
social adaptations
habitat.
success among wild chimpanzees.
picture
rate among wild chimpanzees. matrifocal
reproduction,
in a natural
may well die before
1975),
that are successfully
of Flo, who, as the most long-lived
to have produced
probably
major
independence
to about 3-4 offspring
of 6 pregnancies
female are likely to live beyond
dependent progeny
7 and 8), female
of achieved
operate to reduce survivorship
an age of 4 years despite die.
(0.001
a maximum
no more than 5 offspring per reproductive
offspring
6.7
(see Tables
to experience
Th is is not a realistic
because many contingencies
Consequently,
Birth interval in years Mean Standard-----T deviation
rate and a slow maturation
which seems to operate largely within to date,
may in turn be one of many
role.
Migratory patterns Field
investigators
entire chimpanzee the transfer 1970;
of individuals
Nishida
“migration” migratory members
have described
as migration both the seasonal
social communities
from one portion
nomadic
movement
of a home range to another
or small clusters from one community
to another
of and
(e.g., Izawa,
& Kawanaka, 1972 ; Nishida, in press). As used in this report, the term Furthermore, inter-community refers only to the latter phenomenon.
patterns
at Gombe
of one community
seem to include
two patterns:
a temporary form where
members of another for some hours, days, and a more permanent form where various perhaps even weeks before returning home; social ties and eventually full residence are established in a new community, perhaps for a lifetime in some cases. As used here, the term “migration” again refers to only the latter form. An arbitrary
but convenient
meet and join
span of one year of presence
in or absence
from the study
population was used as the minimum duration for assigning migrant status. If all observed departures and arrivals defined in this manner are classified as inter-community migration, then 41 cases of transfer into or out of the study population occurred the 1963-1973 period (see Figure 3). This sample yields an annual immigration
during rate of
GOMBE
2.3 and an emigration
CHIMPANZEE
rate of 1.8 individuals,
581
DEMOGRAPHY
with a net migration
rate of $0.5
chim-
But the validity of these figures is highly suspect because the introduc-
panzees per year.
tion and subsequent reduction
of banana provisioning was probably an integral regulator of migration during certain years. Of the total number of migrants, fully 17 cases (41%) of immigration occurred in 1964 and 1965 while 14 cases (34 %) of emigration occurred in 1973.
Thus, 75 ‘A of the observed
cases occurred
within a mere 30 ‘A of the time en-
compassed by the census records. When the composition of these cohorts is more closely examined, 8 of the 17 “pseudo-immigrants”
the data show that
added at the start of the decade and 8 of the 14 “pseudo-
emigrants” subtracted at the end of the decade were the same individuals. With the exception of 1 female, who was a recent immigrant with only 3.3 ape-years of residence, and of the 5 offspring
born to 3 migrant females who arrived early and then departed
again late in the decade, the final “mass emigration” of 1973 consisted of those who appeared before 1965. It seems noteworthy that staff members of the research center called these 8 individuals
“southern
they were frequently kilometers
chimpanzees”
seen leaving
toward,
to the south of the Kakombe
never seemed to achieve
until the early 197Os, on the grounds
that
arriving from, or travelling in an area some station. Indeed, several of these individuals
full social membership
in the resident community
and often
traveled as a cluster in the main study zone. Coupled
with these qualitative
of the migrants observed another social community. “southern
chimpanzees’
impressions,
If this is corroborated
do not eventually
unit but simply blend into a neighboring
community,
data suggest that many may have belonged
by further field research-i.e.
wherein kinship, consort, coalition
social
intensity of the social bonds that deter-
will be amply demonstrated.
we can at best cautiously
to
if the
establish themselves as an independent
one-the
mine cohesiveness in a natural community now stands, however,
the demographic
at the start and the end of the decade
speculate
As the evidence
that the social nexus within a
and other special relationships
are known
to comprise a matrix of strong bonds, is sufficiently long-lasting and exclusive to withstand years of regular interaction with members of another community. Until 1968 no ‘rcommunity” Ieve 1 o f organization was recognized by observers of the Gombe
chimpanzees.
sistent (Lawick-Goodall,
Only the matrifocal unit had been recognized 1968).
as prevalent and per-
Then, largely due to the flurry of reports produced
the topic of larger social units by Japanese
primatologists
working
on
at other study sites,
Gombe investigators gradually began to view the study po@&on as a social community (Lawick-Goodall, 1973, 1975). Wh en this unit segmented into two unequal portions, while provisioning
was being reduced
fissioning of one Lrcommunity”
during the 197Os, the event was interpreted
into two (Bygott, 1974, in press;
nascent smaller
unit has now been accorded
label “Kahama
community”,
area to the nearby Kahama The ethological
evidence
seem strong, especially
full community
Wrangham,
status, identified
having apparently moved south from the Kakombe ValIey area (Lawick-Goodall, 19753). favoring
this interpretation
when investigators
are looking
of a fissioned community outward,
as the
1975).
This
by the Valley may
so to speak, from the
observation base of the study population; yet the demographic picture presented here suggests that the fissioning interpretation may have been premature. The segmentation of the study population was, in other words, perhaps no more than the departure of a transient group whose members originally and also finally belonged to a neighboring community. Before complete acceptance is given either interpretation, however, the
582
G. TELEKI
available subgroup
ET
evidence should be carefully should be obtained.
In order
to obtain
a more
AL.
reviewed
reasonable
and additional
approximation
data on the “fissioned”
of the normal
scale of inter-
community migration at Gombe, we will now focus on a much smaller sample of transfers into and out of the study population. The equivocal data gathered at the start and the end of the study period will be ignored, and attention given to the 10 remaining cases of permanent migration (possibly 14 cases, if 4 additional dissappearances are counted as transfers) that occurred between mid-1965 and mid-1972. Based on the conservative sample
of 10 cases, the net migration
with an annual
immigration
The discrepancy but, although
(Izawa,
Nishida,
1974;
incomplete
may be due to the attraction
inter-community
societies
rate is calculated
1970;
migration
Sugiyama,
Nishida,
at +0*3
rate of 0.9 and an annual emigration of banana
provisioning
1973;
Nishida
in Kakombe
Valley;
of wild chimpanzee
& Kawanaka,
in press; Pusey, in press), the available
per year
rate of 06 individuals.
appears to be characteristic
1972;
Kawanaka
&
data on rates are probably
and inconclusive.
Yet there are some basic themes that begin to emerge. tion and at least 4 cases-possibly There
chimpanzees
8 cases-of
At Gombe,
emigration
6 cases of immigra-
were logged in seven years.
were no male migrants observed.
scents, 2 adults) 3 adults),
Moreover, of the 6 female immigrants (4 adoleand the possible 8 female emigrants (2 infants, 1 juvenile, 2 adolescents,
7 individuals
were kin-related.
These
data lead to two tentative
generaliza-
tions ; one, that permanent migration among chimpanzee communities is, in contrast to what has been reported for other non-human primate societies (e.g. Rudran, 1973; Packer,
1975),
some migrant
a predominantly chimpanzees
or
even
merely accompany older kin. (Young males become migrants that follow matrifocal units.) have been noted
exclusively
in press).
were initially
and female There
introduced
migrants
were commonly
lone young
are at least 2 cases at Gombe
the tendency bability
When
for avoidance
of permanent
No comparative
(Nishida
two, that offspring who
adults
(11-14
& Kawanaka,
where unnamed
by local males to the study population
several such temporary appearances, estrous cycling probably contributes into new communities.
activity;
could, in the latter situation, sometimes At the Mahali site no adult male transfer
in age) who were often in estrus at the time of transfer Nishida,
female
(36 ‘A of 14 cases) are likely to be dependent
years 1972;
young females
while in estrus, and, after
eventually became permanent immigrants. Hence, to the probability that females transfer permanently
males of different
or conflict
intercommunity
data on frequencies
communities
seems to be higher, transfer
meet, on the other hand, thereby
lowering
the pro-
by males.
of temporary
uersas permanent
migration
have
been reported from any chimpanzee study site. Yet it seems likely that genetic exchange among neighboring communities would occur in both contexts. Indeed, gene flow may be higher in the case of temporary migration because male chimpanzees, who on occasion travel far into the home range of another community at Gombe (Lawick-Goodall, 1968, 1975b), are known to sometimes develop brief consort relationships with sexually receptive females during such sojourns. Paucity of observations on transferring, and on the temporary verms permanent aspects of migration, unfortunately prohibit further speculation about the biological phenomena involved in inter-community migration by wild chimpanzees. detailed theoretical
The biological mechanisms of intercommunity migration are in more need of field investigation if a chimpanzee model of social organization is to achieve significance
for ethnologists
and paleoanthropologists.
583
GOMBE CHIMPANZEE DEMOGUPHY
7. Ecological Regulation of Chimpanzee Populations Seasonal
variation
panzees
in the availability
is one of many factors
(Wrangham,
1975;
Teleki,
of floral and fauna1 resources
affecting
1976).
their ranging
Fluctuations
habits
utilized
in Gombe
in nutritional
by chim-
National
Park
factors are in turn likely
to affect stamina
and health. Individual body weights, for example, can vary as much as 10 ‘A during the annual cycle of seasons. Moreover, seasonal differences in the techniques of food collection and predation, which entail flexible grouping strategies that are presumably
geared
expenditure, resources
exploitation
are being heavily
probably
provides
exploited,
optimum
while minimizing
animal proteins McGrew,
(Suzuki, 1974),
1971~;
regular
1973; Teleki,
to some extent regulate
aggregation
for the transmission
in the consumption
Nishida,
energy Some
population.
at specific sites within the home range;
the resultant
conditions
It is even possible that seasonal variations 1974;
of key resources
are dispersed while others are clustered
when the latter panzees
to maximize
may in turn affect disease patterns within a chimpanzee
of specific
1973a, 1974;
susceptibility
of chim-
of fatal pathogens. foods, such as,
Hladik
& Viroben
to diseases.
Although nutritional and epidemiological regulators are not yet well documented, we can discuss meteorological regulators with somewhat greater precision. A representative S-year
sample
(1968-1970)
of precipitation
cycle of seasons at the Kakombe Gombe
climate
Wrangham,
Valley
have been reported
and temperature
station,
elsewhere
data, showing the annual appear in Figure 6. Many aspects of the
(Lawick-Goodall,
in the park:
4 months of dry conditions
tions (October-May). by 4 months
of long rains.
The monthly
(June-September)
The wet season includes
with less than a month
Seasonal
difference
distributions
tion during 1963-1973
transitions
about
Comparison
throughout
to the negative fetal wastage
However,
encompassed
season, there is some possibility wastage.
distributed
stable for all seasons, although
7 does not include
(July-December)
If live births
often
in the Gombe
study populaof months has
seasonal trends. The samples 1 because the specific dates of
are unknown.
rainy months may contribute in the field.
intervals,
7, in which the normal chronology
of these raw data suggests that deaths
is relatively
of wet condi-
of short rains followed
occur at fairly constant
of deaths and births recorded
while births are more randomly
Figure
1973a;
from year to year.
appear in Figure
3 cases in each category
observed
Teleki,
and 8 months 4 months
been rearranged (as in the previous figure) to emphasize of deaths and births used here differ from totals in Table
gradient
1968;
1975), so it should suffice to note here that two distinct seasons occur annually
are associated with precipitation the year. The mean temperature the slight temperature
because
most such events were not directly
since all 6 fetal deaths
occurred
in a 6-month
by the dry season and the short-rain that climatic
and fetal
deaths
drop in the
effect of high precipitation.
or other ecological
are added together,
portion
period
of the wet
factors contribute
to fetal
then 21 pregnancy
term-
inations were logged between July and December while only 12 pregancy terminations occurred between January and June. As the 21 pregnancies which terminated between July and December may signify a conception the initial months of the wet season, a relationship tion may lie hidden
behind
the relatively
peak some 8 months between conception
even distribution
earlier, during and precipita-
of live births.
Incomplete
584
G. TELEKI
ET AL.
Figure 6. Meterological data (1968-1970 averages) in Kakombe Valley, Gombe National Park. Note that the maximum monthly precipitation occurs during the short-rain period of the wet season, and that the minimum daily temperatures overlap that same period.
“C 35
30
2.5
20
as they are, these data already point toward new questions. Does copulation frequency exhibit seasonal fluctuations ? Do seasonal variations in grouping tendencies, determined perhaps by cyclic availability of food resources (e.g. fruiting trees, termite migrations), affect sexual behavior and, consequently, conception rates? When additional data become available, more refined questions about ecological regulators will undoubtedly emerge. Births are distributed in a relatively even pattern throughout the year. Deaths, on the other hand, seem to be closely associated with precipitation. Fully 87 % of 30 postnatal deaths occurred during the 8-month span of the wet season. More importantly, 57 % of the postnatal deaths occurred during the 4 months of short rains marking the onset of the main wet season. If the mean monthly rainfall is taken as the independent variable, the Pearson product moment correlation between precipitation and mortality is significantly positive (r = $0.74, j =
GOMBE CHIMPANZEE
Figure 7. Monthly and seasonal distribution of general mortality (top), sex-specifii mortality (middle) and live births (bottom) in the Gombe study population during 1963-1973.
585
DEMOGRAPHY
LongRains ..)....
Dry
Plus6 Prenatal
Deaths
A
S
0
N
D
J
F
M
A
M
June
July
A
S
0
N
D
J
F
M
A
M
June
July
A
S
0
N
0
J
F
M
A
M
June
July
N=30
dependent offspring additionally risk contracting diseases from mothers and/or siblings with whom they have regular contact. When monthly mortality is graphed separately for each sex, a slight tendency toward a male peak between October and January versusa female peak between December and March can be noted. It is possible that males are slightly more prone to die during the short rains while females are slightly more prone to die during the early portion of the long rains. Additional data are needed to determine whether this sex-specific difference is significant. 8. Methodological
Contamination
of
the
Gombe
data
As illustrated in Figure 1, the major fluctuations observed in the 1963-1973 census dataand presumably also in the estimated membership of the Kakombe community-can be chronologically matched to stages in the history of banana provisioning (Teleki, 1973a; Wrangham, 1974). When the demographic components underlying these trends are closely examined, the timing of peaks and slumps in mortality, natality and migratory patterns seem to coincide with feeding schedules (compare Figures 1, 3 and 4). Considered independently, annual variations in each demographic variable could have been
586
G. TELEKI
ET AL.
caused by natural factors-i.e. regulators not related to human activities. It is possible, for example, that the mortality rate rose sharply during 1966-1969 due to an exceptionally heavy influx of contagious diseases which spread from neighboring chimpanzee populations into the Gombe area: or that the drop in natality during the same period was merely part of a normal series of sigmoid oscillations that is perhaps common in wild chimpanzee populations. There is, in sum, no concrete evidence proving that banana provisioning was the direct cause of the major changes observed in the Gombe study populations. When all historical information is collated (Table 9), however, it seems plausible that banana feeding-together with its byproducts of heightened tension, frequent competition, and spatial compression of chimpanzees at the main Kakombe station-either produced or exacerbated several trends in the 1963-1973 census records. The extent to which provisioning affected various trends is difficult to determine in retrospect. Of the various consequences mentioned by Wrangham (1974), aggressive competition and contraction of diseases are most pertinent here. Given that correlation and causation are not always identical, it still seems noteworthy that the run of fetal deaths observed between 1966 and 1970 coincide with the most intense period of provisioning at which time inter- and intra-specific aggression were also high. More aggression around and against pregnant females may well have accelerated the prenatal mortality rate. Bananas are not normally available to the Gombe chimpanzees (except by possible raiding of nearby plantations), but this novel fruit became a major food resource for many regular visitors to the main research station. When massive amounts of bananas which contain only l-2-1.5 g protein per 100 g edible matter (Leung, 1968), are consistently fed to apes that are accustomed to an eclectric omnivorous diet (Teleki, 1974, 19753), negative effects upon natality and mortality, perhaps via lowered resistance to disease, might be expected. For the years 1964-1967, the annual records of poliomyelitis in the human population of Kigoma Region included 18, 143, 31 and 7 cases (D. E. Parmas, Regional Medical Officer, pers. comm. ). In August and September of 1966 there were 8 known cases within 15 km of Gombe National Park. As the epidemic of paralytic disease among the Gombe chimpanzees was recognized in September of the same year, it seems likely that human vectors were involved in the transmission of polio to the chimpanzees. If provisioning affected the Gombe study population along such lines, the magnitude of their demographic responses can be measured by calculating population growth during 1966-1972, a time span when the “pseudo-migration” phenomenon least influenced population stability. Applied to this census period, the growth formula* yields a net 10~sof 17.7 chimpanzees in 7 years, or a per annum loss of 2.5 individuals, from an initial total of 66 named chimpanzees. Given that immigration from regions outside the park boundaries is probably minimal these days, it is evident that the study population could not for long sustain such losses without draining neighboring communities. If the net migration rate of +0.5 chimpanzees per year recorded in 1963-1973 included some individuals who were actually residents of the Kakombe Valley area, and were erroneously “added” as they became habituated to human observers, and the +0*3 net annual migration rate recorded in 1966-1972 is a more realistic measure of transfer, the difference * (P* - P,)ZCIp, = populationgrowth between the first census (PI) and the secondcensus(Pa), with a constant(K) of 100.
GOMBE
Table
CHIMPANZEE
Chronology of provisioning and Gombe Stream Research Centre
9
587
DEMOGRAPHY
demographic
Demographic
Provisioning events
events
at
the
events
Stage
Period
A
August 1962March 1965
Introductory period when many Study population rapidly increases in new chimpanzees are habituated size, but trend is deceptive because most additions are not births but adult via irregular provisioning. “immigrants.” Numerous “southern chimpanzees”attracted to feeding area.
B
March 1965August 1967
Experimental and developmental stage when several distribution techniques are tried, with several increases in the frequency and the volume of banana provisioning.
Study population peaks in 1966, then proceeds to drop rapidly. Mortality rate rises sharply in 1966, due largely to epidemic of paralytic disease. Natality rate drops sharply in 1966 and 1967. Run of fetal deaths begins in 1966.
C
August 1967June 1968
Scheduling and volume of provisioning is repeatedly changed in attempt to cope with increasing difficulty of observing large aggregations of chimpanzees, and also with increasing intraand inter-specific competition for bananas.
Study population continues to shrink. Mortality rate rises sharply in 1968, due partly to an epidemic of respiratory illness. Natality rate bottoms out in 1967 and 1968, with nutritional (e.g. excess bananas) and/or behavioral (e.g. aggressive interaction) factors involved. Run of fetal deaths peaks in 1967.
D
July 196% June 1969
Major compensatory actions taken to reduce stressful situation at feeding station, where 80+ primates (baboons and chimpanzees) gather. often Plans made for new feeding systern.
Study population bottoms out in 1969, following a second epidemic of respiratory disease. Mortality rate peaks in 1968, then drops swiftly in 1969 when feeding is reduced. Natality rate still low, but begins to recover in 1969. Run of fetal deaths continues.
E
June 1969July 1973
Provisioning system completely redesigned to supply few bananas at irregular intervals to lone visitors or small groups.
Study population recovers gradually, due this time mainly to births rather than adult additions. Mortality rate bottoms out in 1971 and 1972, and no epidemics occur. Run of fetal deaths stops in 1970. Mass %migration” of “southern chimpanzees” occurs late in 1972.
between
these rates could be an approximate
or more communities
living near the Kakombe
measure of the drainage
experienced
by one
Community.
The Gombe research setting serves to illustrate how crucial a role can be played by procedures and methodologies applied to long-term study of wild primates. Even though habituation through provisioning may initially seem advantageous and harmless, it can lead to unexpected
dangers for the population
under study.
deemed necessary, we recommend that investigators in order to strike a balance between acceptable
If intensive provisioning is regularly monitor demographic trends and excessive disruption. Moreover,
having applied extensive provisioning, Gombe researchers must now evaluate all results and interpretations in terms of the effects wrought by this technique on a plethora of ethological and ecological factors (e.g., Teleki, 1973a). It remains likely, even then, that some authors will persist to ignore or reject Gombe observations (e.g. Reynolds, 19’75).
588
o.
TELEKI
ET
AL.
9. Tracing matology Modern
anthropologists
Paleodemographic
Trends
favor integration
of demographic
who in principle
Via Pri-
research
primatology, ethnography and paleoanthropology nonetheless acknowledge shortage of suitably complete and comparable data in various areas of research the review by Baker and human partly
primate
because
& Sanders,
1972).
populations
The gap between
studies of extant non-human
seems to have grown particularly
field primatologists
have not provided
That
and apes, should to some extent the Gombe
wide in recent
years,
and accurate
longi-
the detailed
tudinal data needed for drawing comparisons with demographic tions. The Gombe research program, together with other monkeys
studies of human populalong-term
close this gap in the coming
data base is in many
respects
in
a severe (see, e.g.,
severely
limited
field studies of years.
in scope,
possessing
neither the temporal does not completely
nor quantitative depth expected by students of human demography, impair their value. The data we have already presented amply deat one level, the necessity and feasibility of conducting demographic research
monstrate,
on wild non-human speculation-in relevant such
primates.
We now venture
order to explore the potential
to studies of modern
primatological
research.
models of chimpanzee
and early hominid
The
population
for assessing the plausibility If the broader
human
aim,
then,
structure
implications
step-to
populations
is to determine
and dynamics
and accuracy
theoretical
to take another
a level of open
utility of these data, to see whether
insights
can be gleaned from
whether
demographic
may provide a new framework
of some evolutionary
reconstructions.
of the Gombe results are to be fruitfully exploited
however, it is essential to stress (a) that the information base is small and is therefore prone to erroneous interpretation, and (b) the representativeness of the data must be evaluated.
Neither
completion
of additional
caveat
considered,
for some demographic
site listings in Baldwin tion has been met. The chimpanzee
can at present field research
& Teleki,
gests that the studypopulation
therefore
conclude
chimpanzee
that
population
albeit with caution,
data
in Table
in several basic parameters. and sex distributions-have
data obtained 19?3),
demographic
results are summarized
be satisfactorily in primate
10.
at other
but cannot
doesnotdiffer
The first requires The
second
can
be
sites have been reported
(see
be fully resolved until the first condi-
now available Despite
eliminated.
demography.
for comparison
obvious
radically
limitations,
with the Gombe
this information
sug-
from other chimpanzeepopulations
More detailed comparisons of two specific parameters-age lent further support to this assumption (see Table 2). We
the Gombe structure
to explore
data
are in many
and dynamics, the relevance
respects
representative
and that the Gombe
of non-human
primate
of wild
results can be used, demography
to re-
search in hominid paleodemography. Several authors have recently presented demographic analyses on extinct hominid and extant human populations, and some have also reviewed the interpretive problems encountered McKinley,
in paleodemographic 1971;
Mann,
studies
1975 ; Dumond,
framework for the Gombe results. As shown in Table 11, chimpanzee
(Acsadi 1975).
& NemeskCri, These
life expectancy,
sistently lower than similar parameters in early hominid Qnly the Choukoutien survivorship (O-320) is slightly
1970;
Weiss,
1973;
reports serve as a comparative
survivorship,
and fertility
and modern human lower than Gombe
are con-
populations. survivorship
Z-group
K-group
Band A
Kasakati
Mahali
Filabanga 43
28*
50
80
48*
49
85
< 1.0
1.3-2.6
1.6-2.4
2.4-4.7
29-4.8t
c. 5.0
5.0-7.0
1.5
1.1
0.8
I .4
1.2
1.5-1.8
1.8 ?
0.5
0.5
0.8
06
0.9
0.5
0.6
0.6
0.8
0.9
0.6-0.7
0.7-0.8
13
67”
20
50*
120
9*
23*
Continuous or intermittent* study period (months)
chimpanzees per kma.
0.5-0.8
at Gombe is 3.6-4.6
Mosaic grassland/ woodland forest
Mosaic grassland/ woodland Mosaic grassland/ woodland/forest
Mosaic grassland/ woodland/forest Mosaic grassland/ woodland/forest
Forest edge/plantation
Rain forest (with selective logging)
t Estimated density range for entire park population
Study population M-group
Gombe
Mahali
-
Beni
group
Socionomic sex ratio
Nishida & Kawanaka (1972), Nishida (1968, in press) Izawa & Itani (1966), Izawa (1970) Nishida & Kawanaka ( 1972)) Nishida (1968, in press), Sugiyama (1973) Itani & Suzuki (1967), Suzuki (1969), Kano (1971)
Reynolds & Reynolds (1965), Sugiyama (1968, 1973), Suzuki (1971) Kortlandt (nd., pers. comm.) Kuijsten (1972) This paper
Source
Picnic Site
age ratio
type
Total male/ female sex ratio
Budongo
Adult/ subadult
General habitat
Names of observed units
Sites Known* or Estimated estimated crude size of density units per kma
Comparative summary of basic demographic data obtained from field studies of chimpanzees in eastern Zaire (Beni site), western Uganda (Budongo Forest), and western Tanzania (Gombe N. P., Kasakati Basin, Mahali Mtns)
Table 10
590 Table
G. TELEKI
11
ET AL.
Comparison of selected demographic traits in Gombe panzees, extinct hominids, and extant human societies Location or organizational level
Populations
Pan troglodytes* Australopithecus Homo erectus
Expectation SurvivorBirth Age-specific of life at ship at interval fertility age x years age x years in years rate for female offspring
Gombe National Park South Africa Choukoutien Maghreb-type; Taforalt, Afalou Gatherer-hunters Proto-agriculturalists
Homo sapiens Homo sapiens* Homo sapiens*
cbii-
k,
e1.5
115
11.9 21.4
12.9 12.7 26.9
0.364 0.650 0.3200.463
5.6 5.8 4.0 3.5
0.128 (B,,) -
16.5 19.0
20.0 19.8
0.500 0.500
3.5 2.5
0.213 (&) 0.174 (&,)
* Extant populations.
(0*364),
and this difference
the column
showing
panzees exhibiting ingly anomalous
seems well within
estimated
birth
an interval
sample
has an opposite
error.
In contrast,
with living chimThe seemtwice as long as that of living humans.
about
australopithecine
an expected
intervals
survivorship
trend,
(O-650) will be discussed below.
These demographic measures have been selected for comparison largely because relevant data are available for a temporally and phylogenetically broad range of populations.
Consequently,
as more complete to accumulate,
there remains
demographic comparisons
of data listed in Table
of other
populations
wise be obtained
line diverged,
may extend knowledge
by paleodemographers.
are no more than rough estimates, hominid human The
and modern primate
comparison nonetheless
human
populations
survivorship
is in reasonable
measures
1 I is not that chimpanzees
stock from which the hominid primate
considerable
is of course limited
adults at an age of about
feasible.
The
are supposed to represent
but rather
significance a population
that profiles of living non-human
of demographic processes that cannot otherEven though some figures shown in the table
well.
in Figure
accord with survivorship
likely to survive beyond
will become
But
continue
the Gombe data are remarkably well aligned with early It remains to be seen whether other non-
fit equally
that
interpretation.
ape and human populations
populations.
curves depicted
interesting
room for erroneous
profiles of monkey,
8 also show that the chimpanzee
in the same set of hominid
by the small sample the
13 years,
life cycle
sizes underlying
of Gombe
profile Precise
these curves.
chimpanzees-who
enter old age by about
40 years-does
populations.
It is
are prime
27 years of age, and are not
not seem to differ greatly
from the life cycle of
australopithecines. It is thus tempting to assume that other demographic features shown by Gombe chimpanzees (e.g. mortality and natality patterns) are also in close accord with similar features once exhibited by australopithecine populations. Because the australopithecine sample has a shortage of fossil material in the lower age brackets and includes remains from a large time span rather than a single resident population, it is extremely difficult to conduct paleodemographic reconstructions (Mann, 1975). Such difficulties may account for the anomalies apparent in Table 11 where the australopithecine
sample
seems to have an exceptionally
high survivorship
rate while
GOMBE CHIMPANZEE
DEMOGRAPHY
591
Figure 8. Survivorship curves for: A, Australopithecusspp., based on a sampleof 120-155individuals mainlyfrom SouthAfricansites(McKinley, 1971; Mann, 1975); B, Homo erectus, based on a sample of 22 individuals from the Choukoutien site (Acsadi Nemesktri, 1970) ; C, Homo sapiens of the Maghreb-type, based on a sample of about 200 individuals from the Taforalt and Afalou sites (Acsadi & Nemeskeri, 1970); D, Pan trogZodytes,based on a sample of 33 individuals from Gombe National Park (this paper).
AGE
IN YEARS
being similar to Gombe chimpanzees in other respects. The Gombe data may here serve to supplement and refine some of the paleodemographic results obtained by McKinley (1971) and Mann (1975). In both Table 11 and Figure 8 certain australopithecine traits, such as the survivorship probability at age 15 years (0.650) and the entire subadult portion of the survivorship curve (A), seem to be out of phase with both the pongid and hominid data. Only when the raw data are statistically smoothed (Weiss, 1973) does the mean lifetime remaining to australopithecines at 15 years of age (eis = 12.7 years) approach the mean lifetime remaining to chimpanzees at the same age (eis = 12.9 years). Accordingly, it is possible that the mean ages at death proposed for australopithecines (18.0-22.9 years by McKinley and 17-2-22-2 years by Mann, the initial figure for the robust form and the latter for the gracile form) were closer to the mean age at death now exhibited by Gombe chimpanzees (13.5 years). The Gombe data may supplement paleodemographic results in other ways as well. The average interval between births at Gombe is 5.6 years, with a total lifetime productivity of 4.7 or fewer offspring. If the 5.8 year spacing proposed by McKinley (1971: 424) is accurate, then the lifetime fertility of australopithecines may also have been within a range or 4.0-5.0 live births per female. These are of course quite tentative estimates of early hominid survivorship and reproduction. They are, however, based on twu sources of data: living nonhuman primates and fossil hominid remains. If a more comprehensive model of chimpanzee population
G. TELEKI
592
ET AL.
structure and dynamics can eventually be generated from a larger sample of field studies, and if that model would prove to be as compatible with evolutionary trends as the current data seem to be, primatology would add a new dimension to demographic research in ethnology and paleoanthropology. Incomplete as it is now, the model already suggests that a gradual increase of life expectancy and survivorship, along with a gradual decrease in birth spacing, may have jointly contributed to a global expansion of hominid populations since their divergence from common primate stock some millions of years ago. If such evolutionary processes can in time be brought into sharper focus through comparative demographic studies on living monkeys, apes and humans, field primatology will have served to expand our theoretical horizons. A more pragmatic point has perhaps been better demonstrated by this report: namely, that much of the demographic data collected by field primatologists can be arranged, analyzed and presented in accordance with standard procedures used in wildlife biology and human demography. This, in our opinion, is an essential step in producing results that will have comparative value in research focusing on intertaxonomic and interdisciplinary problems. 10. Summary Vital demographic
data on Gombe chimpanzees
The following summary of key numerical data pertains, in the strictest sense, only to the Gombe study population. We believe, however, that this population can provisionally be considered as representative of an average chimpanzee social unit, or community, because its structural components fall within the natural range of size and composition exhibited by unprovisioned populations studied at other sites. 1. Population size and composition (10 year sample) Total size :
mean = 48-l range = 23 = 1.21 = 0.95 = 0.86
Adult/subadult ratio Male/female ratio Turnover rate 2. Mortality and natal@ patterns (10 year sample) Crude death rate Infant death rate Crude birth rate Male/female ratio at birth Male/female ratio for infant deaths 3. Migratory patterns (7 year sample) Annual immigration rate :
Annual emigration rate :
= o-07 = 0.23 = O-06 = O-67 = 0.37
mean standard deviation range mean standard deviation range
= = = = = =
0.86 0.90 2 0.57 1.28 3
GOMBE CHIMPANZEE
Net annual migration
= o-29
rate
Adult-male/adult-female Adultlsubadult
593
DEMOGRAPHY
= 0.00
migrant ratio
= 0.43
migrant ratio
4. Reproductivepatterm (10 year sample) Total pregnancies expected in lifetime of an average female Total live births expected Total fetal deaths expected Annual probability Average
= 4.7
birth interval
dies:
mean =
deviation=
1.70 0.60
(in years) when previous offspring lives:
mean= 560 standard deviation= 1.50 = 0.65
rate (female offspring)
Gross reproductive
I.2
= 0.19
of live births per female
birth interval (in years) when previous offspring
Net reproductive
=
in lifetime of an average female
standard Average
= 5.9
in lifetime of an average female
rate (female offspring)
= 2.90
5. Lijk history data (10 year sample) Male life expectancy
(in years) :
Female life expectancy Combined
at birth=
(in years) :
life expectancy
(in years) :
Age (in years) at death: Maximum
15.9
at birth=
10.9
at age 8=
16.2
at birth=
11.9
at age 8=
16.6
mean=
13.5
standard deviation=
12.5
age (in years) at death:
Generation
14*3
at age 8=
span (in years) :
males=
33
*
females=
37
*
mean=
19.6”
standard deviation = 2.12 range= Age (in years) of females at birth of first infant
mean= standard deviation=
Note:
asterisks(*)
mark figures that are probably
of many adult chimpanzees
are only approximately
conservative
8 11*8* 0.34
range= 1 estimates, as the ages
known.
Some hypotheses and conclusions As demographic
investigation
stage of development,
of non-human
the following
primate
populations
selection of comments
is at such a nascent
is offered mainly as a stimulant
to further research. (a) The membership
of a chimpanzee
population
unit may vary from year to year,
or even month to month, but age and sex ratios are likely to hold stable on a long term basis. Based on a sample of 3 discrete populations, stability is estimated to occur when there are about 75 males to 100 females and 90 immature members to 100 mature members. (b) Female births seem to occur somewhat more frequently than male births, but high infant mortality 6
among
females can compensate
to the extent that the sex
594
G. TELEKI
ET AL.
ratio in a given population
unit remains
population
within
unit
dispersed
at or near parity.
a region
can vary
The composition
considerably,
but
of a
a slight
preponderance of females can be expected in an average population sanple. The sex-specific factors involved in differential mortality patterns have not been identified. (c)
Migratorypatternsareprobablyinfluenced on a temporary
byage, sexandkinship.
Adultmalesmay,
basis, migrate as or even more often than adult females,
are less likely to permanently
transfer into other communities.
but males
Subadult
members
of matrifocal units seem to account for a high proportion of the more persistent form of inter-community migration. The dual roles played by “temporary” versus “persistent” migration in genetic exchange among communities require further
study.
(d) Mortality patterns.
patterns Illness
are more and
susceptible
death
season, when precipitation year,
although
dation where. (f)
The
upon
being strongly
survivorship
curve
as in many shared (g) When
human
account
influenced
of the wet and con-
factors.
negligible,
approaches
a markedly
convex
that most mortality This
of deaths per
The effect of pre-
at Gombe
shape
and else-
after the initial
4
occurs toward the end of the life span,
may be one of several
demographic
patterns
by the higher primates. multiple
likely to produce
factors more
are considered, than
3-4
commercial
exploitation
does not exhibit
average
chimpanzee
that will achieve
female
maturity.
to survive prime adulthood.
of wild populations
(h) With a low reproductive adoption of some orphans), and long birth intervals
the
offspring
no more than 2 offspring can be expected
tion
for a high percentage
by climatic
is probably
groups.
are natality
reach daily minima,
mortality
years of life, and indicates
than months
Live births occur throughout the that conception is seasonally influenced.
there is some possibility
chimpanzee
vagaries
the early
the wet season.
Diseases and to some extent accidents year, the former
during
is high and temperatures
tinue at a high rate throughout (e)
to seasonal
are common
is not Perhaps
Accordingly,
must be strictly regulated.
rate (notwithstanding rate, a high infant mortality a slow maturation rate, a limited fertility span in females
between potential
surviving
offspring,
for rapid
population
a wild
chimpanzee
growth.
A long
popularecovery
period can be expected after a population has been struck by calamitous events. Management and conservation programs should therefore focus on isolating populations (i)
from such events,
especially
and on protecting the lives of subadult If the age and sex ratios characteristic
contagious
diseases carried
and adult females. of wild chimpanzee
populations
by humans, maximize
reproductive potential, then similar ratios may be advantageous in captive breeding programs as well. Emulation of natural community size may also be advantageous. Other wild chimpanzee demographic traits should be examined for the purpose
(j)
of designing biologically efficient breeding programs, and demographic, as well as ethological and ecological, factors should be incorporated into such programs. As some demographic traits exhibited by wild chimpanzees seem to be in reasonable accord with human demographic patterns, especially at the early hominid level, the detailed population models that can be constructed by field primatologists may aid in reconstructing hominoid evolutionary processes.
GOMBE CHIMPANZEE
We are indebted
to the Gombe
and the Republic
of Tanzania
during
two years
Stream Research for providing
of field research
Centre,
financial
in Gombe
595
DEMOGRAPHY
the National
and logistical
National
Park;
Geographic
Society
support to G. Teleki
and to Jane
Goodall,
Director of GSRC, for granting unlimited access to the center’s data files during a visit to the station by J. H. Pfifferling. We are also grateful to the many field investigators who contributed
to those records
over more than a decade
of research
on chimpanzees;
and in particular vital
to David Bygott, Richard Wrangham and Patrick McGinnis for providing The advice and comments of Alan Mann, John supplementary information.
Pfeiffer,
Toshisada
Nishida,
Adriaan
Kortlandt,
and Paul Leslie, who read preliminary
Special thanks are due Lori Baldwin, drafts of the typescript, are also greatly appreciated. whose patient encouragement and assistance were invaluable. References
Gombe bibliographic
entries are marked with asterisks.
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Recent advances in the study of inter-unit-group relationships and social structure of wild chimpanzees of the Mahali Mountains, In (S. Kondo, M. Kawai, A. Ehara & S. Kawamura, eds) Proceedings from the Symposiaof the Ffth Congress of the International Primatological Society, Tokyo: Japan Science Press. Kortlandt, A. (no date). Computation of the longevity and mortality from the age composition of chimpanzees in the wild. Unpublished manuscript. University of Amsterdam, The Netherlands. Kuijsten, A. C. (1972). Een model voor de leeftijdsopbouw van het wild levende chimpanses. Unpublished manuscript. Univeristy of Amsterdam, The Netherlands. Lancaster, J. B. (in press). Sex roles in primate societies and the evolution of the division of labor in man. In (M. S. Teitelbaum, ed) Social and Biological Bases of Sex Di&rences. *Lawick-Goodall, J. van (1965). Chimpanzees of the Gombe Stream Reserve. In (I. Devore, ed) Primate Behavior: Field Studies of Monkeys and Apes. New York: Holt, Rinehart and Winston. *Lawick-Goodall, J. van (1968). The behaviour of free-living chimpanzees in the Gombe Stream Reserve. Animal Behaviour Monograph 1, 161-311. *Lawick-Goodall, J. van (1971). In the Shadow of Man. Boston: Houghton-Mifflin. *Lawick-Goodall, J. van (1975a). Cultural elements in a chimpanzee community. In (E. W. Mensel, ed.) Precultural Primate Behavior. Basel: S. Karger. *Lawick-Goodall, J. van (19756). The behaviour of the chimpanzee. In (G. Kurth & I. Eibl-Eibesfeldt, eds) Hominisation and Behavior. Stuttgart: Gustav Fischer Verlag. *L.awick-Goodall, J. van (1975c). The chimpanzee. In (V. Goodall, ed.) The Questfor Man. New York: Praeger. “Lawick-Goodall, J. van & Hamburg, D. A. (1974). Gombe East, Gombe West. Stanford Magazine 2, 66-70. *Leung, W. T. W. (1968). Food composition table for use in Africa. Manual published by U.S. Department of Health, Education and Welfare, Bethesda, Md. *Lindquist, E. F. (1953). 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*McGrew, W. C. (1974). Tool use by wild chimpanzees in feeding- upon driver ants. .lournal of Human _ Euoluti~n3, 5011508.. McKinley, K. R. (1971). Survivorship in gracile and robust australopithecines: a demographic comparison and a proposed birth model. American Journal of Physical AnthropoloQ 34,417-426. Nishida, T. (1968). The social group of wild chimpanzees in the Mahali Mtns. Primates 9, 167-224. Nishida; T. (1973). The ant-gathering behavior by-the use of tools among wild chimpanzees of the Mahali Mountains. .lburnal of Human Evolution 2, 357-370. Nishida, T. (in-press). Social structure of chimpanzees in the Mahali Mountains. In (J. Goodall & D. A. Hamburg, eds) The Great Apes. (Perspectives in Human Evolution, vol. IV.) Menlo Park: W. A. Benjamin. Nishida, T. & Kawanaka, K. (1972). Inter-unit-group relationships among wild chimpanzees of the Mahali Mountains. Kyoto University African Studies 7, 13 l-169. Odum, E. P. (1971). Fundamentals of_Ecalogy. Philadelphia: W. B. Sanders. *Packer. C. 11975). Male transfer in olive baboons. Nature 255.219-220. *Pusey, ‘A. (in press). Intercommunity transfer of chimpanzees in Gombe National Park. In (J. Goodall & D. A. Hamburg, eds) The Great Apes. (Perspectives in Human Evolution, vol. IV.) Menlo Park: W. A. Benjamin. Reynolds, V. (1975). How wild are the Gombe chimpanzees? Man 10,123-125. Reynolds, V. & Reynolds, F. (1965). Chimpanzees of the Budongo Forest. In (I. DeVore, ed) Primate Behavior : Field Studies of Monkeys and Apes. New York: Holt, Rinehart and Winston. *Ransom, T. W. (1971). Ecology and social behavior of baboons (Papio an&s) at the Gombe National Park, Unpublished Dissertation. Berkeley: University of California. Riopelle, A. J. (1963). Growth and behavioral changes in the chimpanzee. Zeitschrift fur Morphologic und Anthropologic 53, 53-6 1. Rodman, P. S. (1973). Population composition and adaptive organization among orangutans of the Kutai Reserve. In (R. P. Michael & J. H. Crook, eds) Comparative Ecology and Behaviour of Primates. London: Academic Press. *Roy, A. D. (1974). Rhinophycomycosis enteromorphae occurring in a chimpanzee in the wild in East Africa. American Journal of Tropical Medicine and Hygiene 23, 935. *Roy, A. D. & Cameron, H. M. (1972). Rhinophycomycosis enteromorphae occurring in a chimpanzee in the wild in East Africa. AmericanJournal of Tropical Medicine and Hygiene 21, 234-237. Rudran, R. (1973). Adult male replacement in one-male troops of purple-faced langurs (Presbytis senex senex) and its effect on population structure. Folia primatologica 19,166-192. Sabater Pi, J. & Groves, C. (1972). The importance of the higher primates in the diet of the Fang of Rio Muni. Man 7, 239-243. Stott, D. H. (1969). Cultural and natural checks on population growth. In (A. P. Vayda, ed) Enviroment and CutturaE Behavior. New York: Natural History Press. Struhsaker, T. T. (1969). Correlates of ecology and social organization among African cercopithecines. Folia primatologica 11,80-l 18. Sugiyama, Y. (1968). Social organization of chimpanzees in the Budongo Forest, Uganda. Primates 9, 225-258. Sugiyama, Y. (1969). Social behavior of chimpanzees in the Budongo Forest, Uganda. Primates 10, 197225. Sugiyama, Y. (1973). The social structure of wild chimpanzees: a review of field studies. In (R. P. Michael & J. H. Crook, eds) Comparative Ecology and Behaviour of Primates. London: Academic Press. Suzuki, A. (1969). An ecological study of chimpanzees in a savanna woodland. Primates 10, 103-148. Suzuki, A. (197la). Carnivority and cannibalism observed among forest-living chimpanzees. Journal of the Anthropological Society of Nippon 79, 30-48. Suzuki, A. (19716). On the problems of conservation of the chimpanzees in East Africa and the preservation of their environment. Primates 12,415418. *Teleki, G. (1973a). The Predatory Behavior of Wild Chimpanzees. Lewisburg: Bucknell University Press. *Teleki, 6. (19733). The omnivorous chimpanzee. Scientific American 228, 32-42. *Teleki, G. (1973~). Group responses to the accidental death of a chimpanzee in Gombe National Park, Tanzania. Folia primatologica 20, 81-94. “Teleki, G. (1973d). Notes on chimpanzee interactions with small carnivores in Gombe National Park, Tanzania. Primates 14, 407-411. *Teleki, G. (1974). Chimpanzee subsistence technology: materials and skills. Journal of i&man Evolution 3, 575-594. Teleki, G. (1975a). The evolution of primate subsistence patterns. Paper presented at 14lst Annual Meeting of the American Association for the advancement of Science. New York. “Teleki, G. (1975b). Primate subsistence patterns: collector-predators and gatherer-hunters. Journai of Human Evolution 4, 125-184.
598
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AL.
Teleki, G . (1976). Spatial and temporal dimensions of routine activities performed by chimpanzees of An ethological study of adaptive strategy. Dissertation. Gombe National Park. Tanzaniai -. lJn&ublished _ University Park: Pennsylvania State Univers&. . _ Teleki, G. & Baldwin, L. A. (1975). Breeding programs aim to keep this a planet of the apes. Smithsonian 5, 76-81. Weiss, K. M. (1973). Demographic models for anthropology. Memoirs of the Society of American Archeologists 27, 1-186. “Wrangham, R. W. (1974). Artificial feeding of chimpanzees and baboons in their natural habitat. Animal Behaviour 22,83-93. “Wrangham, R. W. (1975). The behavioural ecology of chimpanzees in Gombe National Park, Tanzania. Unpublished Dissertation. University of Cambridge, England.
Demographic Observations (1963-1973) on the Chimpanzees of Gombe National Park, Tanzania
Department of Anthropology, 409 C. R. Carpenter Building, Pennsylvania State University, University Park, Pa. 16802, U.S.A.
This report provides a longitudinal demographic profile for a small study population of chimpanzees living in the Gombe National Park of Tanzania, East Africa. The sizeand composition of this study population, and the trends in mortality, natal&y and migratory patterns it experienced over a span of ten years, are described and analysed within various ethological and ecological frameworks. The validity of the demographic profile is examined in relation to field methodology, as the study population was intensively provisioned with bananas for several years. The results yield some guidelines for managing and conserving wild chimpanzee populations, and special emphasis is placed upon protection from diseases that involve human vectors. The data presented here may also offer some novel insights to the evolution of pongid and hominid populations, so tentative steps are taken to place the Gombe profile into a broader demographic perspective.
Received and accepted 1 April 1976
1. Introduction Longitudinal demographic information on wild non-human primate populations remains sparse even though numerous species have been extensively studied in their natural habitats (see Baldwin & Teleki, 1972, 1973, 1974, in press; Baldwin et al., 1975,1976 and in press for reviews of field studies). The few reports that do include demographic data tend to cover brief observation spans (e.g. Berger, 1972) or to derive from colonies transplanted into artificial settings (e.g. Burton & Sawchuk, 1974; Drickamer, 1974). Only preliminary demographic data are currently availabIe for the African and Asian apes: Pan troglodytes (Nishida, 1968, in press; Lawick-Goodall, 1968, 19756; Kortlandt, no date; Kuijsten, 1972), Pongo pygmaeus (Horr, 1975, in press; Rodman, 1973; Cohen, 1975), Symphalangus syndactylus (Chivers, 1974). These and ofher field studies of living pongids have yielded a wide selection of demographic material, but data on the natality, mortality and migratory patterns of groups or populations cannot be reliably compared because temporal spans and methodological treatments vary greatly. The present report focuses on a continuous IO-year span of demographic events and trends within a small population of long-haired chimpanzees (Pan troglodytes schweinfurthii) inhabiting the Gombe National Park (4’40’ S, 29”38’ E) of western Tanzania. Our prime concern, in view of the current paucity of published data, is of course the presentation of descriptive field observations. But the relevance of the chimpanzee demographic model to other areas of anthropoIogica1 research will aIso be explored, especially along the following lines. (a) To the paleoanthropologist, demographic studies of living non-human primates may yield models which can then be applied to refine speculations about patterns and processes once exhibited by early hominid populations, such as australopithecines. Although modern human populations have experienced major demographic changes in recent centuries, which are reflected in a steep global growth rate, hominid populations are estimated to have grown only incrementally during the preceding million or more years (e.g. Coale, 1974). The demographic profiles of non-human primate populations, Journal of HumanEvolurion(1976) 5,559-598
560
G. TELEKI
ET AL.
on the other hand, have probably maintained greater stability throughout the Pleistocene, so the patterns exhibited by living monkey and ape populations may provide an accurate mirror for visualizing demographic traits and trends in the early stages of hominid evolution. In addition to serving as a prehistoric template, such demographic models-and especially those derived from African pongids, in view of their biological, behavioral and psychological proximity to hominids-could help resolve some of the dilemmas encountered in paleodemographic research (Mann, 1975; McKinley, 1971). (b) To the primatologist, demographic data provide a key to understanding the structure and organization of social units, and to the adaptive role these have played for various species in an array of habitats. Much attention has recently been given to the ways that primate adaptations are governed by relationships between organizational and ecological variables (Aldrich-Blake, 1970 ; Struhsaker, 1969 ; Gartlan, 1973 ; Clutton-Brock, 1974; Eisenberg, Muckenhirn & Rudran, 1972). Yet no cogent, comprehensive theory of adaptation has emerged from these numerous schemes-perhaps because none rests firmly upon a foundation of demographic profiles. (c) To the cultural or medical anthropologist, demographic studies of non-human primates may, by virtue of the rather simple and elegant models obtainable therefrom, provide insights about the manner in which biological, epidemiological, sociological and other adaptive mechanisms regulate human populations. The progress of recent investigations along these lines (Fox, 1972; Dumond, 1975; Lancaster, in press) has again been hampered by lack of demographic information suitable for interspecific comparison. (d) To the wildlife biologist and game manager, accurate demographic data on living non-human primates are essential for devising suitable policies and programs of conservation. Such information is particularly vital to the task of protecting African and Asian apes, which are today threatened or endangered by human activities in many regions (Harrisson, 197 1; Bermant & Lindburg, 1975). Moreover, breeders of nonhuman primates could, given accurate numerical information, evaluate the success of their custodianship by comparing fertility and survivorship rates in captive and wilderness settings (Teleki & Baldwin, 1975). At present both conservation and reproduction programs suffer from lack of demographic information. 2. The Gombe Setting Research on the Gombe chimpanzees was initiated in 1960, and has been sustained for nearly 15 years. Since 1967, when the Gombe Stream Research Centre was officially founded, the project has developed into a multifaceted, multidisciplinary study of nonhuman primate ethology and ecology. Numerous reports on chimpanzees have been completed by members of the research center, and some reports on other indigenous species are also available (Gombe bibliographic entries are marked with asterisks). Reports describing aspects of the local climate, terrain, flora and fauna include those by Lawick-Goodall (1968), Teleki (1973a), Glutton-Brock (1972) and Wrangham (1975). No detailed, systematic studies of the Gombe ecosystem are as yet available. For the purposes of this report, it should suffice to note that the terrain of this small park (approximate map dimensions : 2.3 x 13.9 kilometers) is rugged, cut into many steep-sided ridges and valleys by a dendritic stream system that flows an horizontal distance of about 1500-3000 metres and drops a vertical distance of about 750 metres
GOMBE CHIMPANZEE
between
the escarpment
Tanganyika;
crest of the Great Rift Valley
that the climate is tropical,
rainfall of 75-125
centimeters,
characterized
land, woodland
and the eastern shore of Lake by a lack of frost, a mean annual
and a diurnal temperature
15°C at night to as much as 37°C at midday;
561
DEMOGRAPHY
range of 28°C which rises from
that the Aora comprises a mosaic of grass-
and rain forest whose distribution
is closely related to topography;
and
that the fauna1 community
is rich in primates (prosimian, monkey, ape and human) and Contrary to the 80 km2 estimate given small vertebrates but sparse in large mammals. in previous reports for the size of the park, new calculations, based on Quarter Degree Sheet #92 (published in 1961 by Tanzania’s Geological Survey Division at Dodoma), yield a map area of 32 km2 for the entire park. However, the true surface area may be as much as 20 % larger when the rugged terrain is taken into account. All density measures given in this report are based on the new area estimate.
Some
pertinent
in a later section.
to evaluating
demographic
trends are presented
In the early stages of the field study, chimpanzees in the rugged National
terrain
Park.
and dense vegetation
meteorological
data
were elusive and difficult to observe
characterizing
many
portions
of Gombe
A banana provisioning
system was therefore launched in 1962 to hasten Continued throughthe apes to the presence ofhumanobservers.
the process of habituating
out most of the decade sampled here, artificial feeding reached a peak in frequency sessions) and volume the middle
1973a;
(about 68 kg bananas per session containing
and late 1960s but was then gradually
Wrangham,
bulk provisioning
1974).
Recent
evaluations
suggest that the procedure
reduced
4400 calories)
in the early 1970s (Teleki,
of the consequences modified
(daily during
behavior
of long-term,
large-
patterns, including
the
frequency of aggression and the kinds of mammals taken in predation, and organizational
patterns.
trends during the 1963-1973
The effects of this procedure
as well as nutritional upon demographic events and
study period will be explored in a later section of this report.
3. Demographic The raw census data collected between
Baselines
1 July, 1963 and 30 June, 1973 appear in Table
1.
During this decade, the records included 38 named males (plus 1 unnamed male birth) and 49 named females (plus 2 unnamed female births), as well as 6 cases of fetal death. All census data are tabulated
between
deaths logged for 1964 actually include June, 1964. Birth dates are known
or estimated
consecutive
mid-year
points.
all cases observed between for every individual
For example, 1 July,
the
1963 and 30
in the census records.
For
infants born to named females during the sampled decade, the events could be timed to the day in many cases and to the month for all others. As chimpanzee births occur throughout considered
the annual
cycle,
unobserved
births-i.e.
to have taken place at the mid-year
estimates become increasingly
those prior to 1 July,
1963-are
point of the estimated birth year.
prone to error as age rises, and are probably
These
more accurate
for females than for males because the ages of the former can often be tied to reproductive events. That is, the average age of females at birth of the first infant (1 l-13 years) can be added to the average pregnancy interval (4-6 years) between each known offspring. Flo’s minimum age, for example, woul,d thus be 12 + 5+ 5 + 5 + 5 + 5, or 37 years. Some additional time (3-5 years) could be factored into age estimates for old females on the premise that some fetal wastage and infant mortality occurred, for the
562 Table
G. TELEKI
1964 1965 1966 1967 1968 1969 1970 1971 1972 1973
Baseline census population
1
Years
ET AL.
Births*
Census
data
(1 July,
Deaths?
23 29 30 24 24 21 21 22 24 16
18 22 32 29 22 22 24 28 27 23
41 51 62 53 46 43 45 50 51 39
2 2 1 1 0 0 1 3 2 0
2 3 4 0 1 2 2 1 1 2
4 5 5 1 1 2 3 4 3 2
0 1 6 1 3 1 1 0 0 2
2 2 0 3 7
Tally 234 Annual x23.4
247
481
12
18
30
15
24.7
48.1
1.2
18
3.0
196380
Fetal deaths:
June,
the stud
Migration Y_ Imnugr.
Emigr.
$6
$3
??
0 1 1 0
2 3 6 4 10 3 1 1 1 2
0 3400+7 0 37 1 0 2 2 0000 10003-3 10101 101 0 0 2 0 0000 0 00
18
33
6
+6
1.5 1.8
3.3
0.6
0.6
2
1973) for
+17
Net
??
0 0
0+10 0 +2 0
0 0
0 0
8
6-14
0.8
4 5 6 3 2
03 +1 +2 0
-8-10
1.7
Total pregnancies§
4 4 3 2
+5
36
0.5
3.6
I.0
* Figures include unnamed births. t Figures include unnamed deaths. 5 Observed fetal deaths are probably below actual occurrence level, especially in the early years. $ Combined list of live births and fetal deaths.
average birth interval may in such cases be reduced by 2-4 years. mum age would then become
40-45
Flo’s estimated maxi-
years.
As such events may not be evenly distributed in the lifespan of all females, minimum age estimates are used in all demographic measures presented pearance
here.
The ages of males are based on more arbitrary
and behaviour,
with opinions
obtained
evaluations
of size, ap-
from several investigators.
Death dates are inherently more difficult to establish, especially for mature individuals, because emigration
and mortality are not always distinguishable
in field conditions.
Thus
the death category here includes only those cases which were directly observed and those for which 1973,
strong circumstantial
19756;
appearances servers.
Teleki,
1973c).
evidence
and all new chimpanzees
Both categories
was available
The migration
remain
who
somewhat
category
(Lawick-Goodall, includes
were previously flexible
because
1968,
1971,
all unaccountable
dis-
unknown
to human
some emigrants
ob-
may have
died and some immigrants may have been familiar to named chimpanzees. Only those individuals who entered into or departed from the main study area for a minimum of 12 months are considered
migrants.
Most named chimpanzees were observed within the central study area (see maps in Teleki, 1973a, 19756) at least once per month, and for many there are daily or weekly attendance records. But few individuals were present for the entire 1963-1973 decade, and there are gaps of as much as 6-12 months in the attendance profiles of some chimpanzees. To cope with these discrepancies, ape-fears were calculated for all named individuals in order to generate life tables, fertility schedules and reproductive rates with For example, female Winkle was thought to have been born in maximum accuracy. 1960 (at the mid-year
point, as she was unknown
then) and was observed
in the study
GOMBE CHIMPANZEE
area from 21 August, accordingly
563
DEMOGRAPHY
1968 to the end of the sampled decade
on 30 June,
1973.
She is
defined as contributing
0.8 ape-year for her age at entry (estimated at 8 years), and 1.O ape-year for each of the following 4 years (ages 9, 10, 11, 12). Any individual present for the entire study period
is of course allotted
IO.0 ape-years.
The tally for the
decade yields 237.2 ape-years for males and 246.0 ape-years for females. This technique is designed to cope with two problems specific to data collected field primatologists. beyond
First, some individuals
a home range normally
precludes
the assumption
occupied
temporarily
travel
measure
a prescribed zone. years”-is essential in demographic Even among baboon
alone
by
within or
by a given social unit, and this phenomenon
that anyone not observed by investigators
A standard
membership.
may
of observed
is no longer within
attendance-perhaps
“primate
studies of all species exhibiting
flexible social unit
troops, which are often considered
stable in member-
ship, there are temporary and permanent migrants (Ransom, 1971; Harding, 1973). Second, field data on primate survivorship is less likely to be complete than the mortality data available matologists
to demographers
studying
human
populations.
An assumption
by pri-
that individuals
must be present even when they are not observed for long The ape-year measure periods may artificially inflate various demographic calculations. eliminates such assumptions. Many of the Gombe chimpanzees
grades-infant,
juvenile,
adolescent
and in many cases adulthood and old.
But when physiological
Goodall,
1967, 1968, 1973).
therefore 1975b).
contains Further
and behavioral
discrepancies inconsistencies
among
(We
mature
of 2-3
(e.g. Nishida,
several experienced
interval
1968; Sugiyama,
and if a consensus opinion
field investigators
a note of caution
here about
(Lawick-Goodall, when reports from
1968; Lawick-Goodall,
about the probable
during the course of a study, then the above problems
must introduce
rates (see Lawick-
data on age-class membership
are introduced
are available
(and mixed
the resultant intervals
at variable
years in any given
in class assignment
1975b). If birth dates for many individuals reached
criteria are thus combined
The most recently published
grounds,
into two stages-prime
to assess rates of development,
because individuals
different field sites are compared
individuals
into one of four specific age
physical and/or behavioral
can in turn be readily segmented
with an added measure of intuition) are likely to be inconsistent
can be readily placed and adult-on
ages of older
are largely resolvable.
using growth
captive studies of the same primate species because captivity
can be
rates obtained
can dramatically
from
accelerate
both physical and behavioral development.) But for purposes of demographic analysis, standardized age intervals are essential. We have accordingly chosen to use an interval of 4 years for each of the three lowest age grades listed above, an interval of 14 years for the prime adult grade and an interval of 10+ years for the old adult grade. Thus, infant covers a span of O-4 years, juvenile 5-8 years, adolescent 9-12 years, prime adult 13-26 years? and old adult 27-38 + years. The upper terminus on this scale is close to the estimated age of Flo, the oldest Gombe chimpanzee. As the sample is very small for wild chimpanzees, this upper figure remains flexible. The 12-year marker for achieving adulthood represents an average for males and females. When the two sexes are treated separately, female adulthood is reached at about 11 years, the mean age at first pregnancy, and male adulthood at about 13 years. In order to maintain
consistency
in all analytical
procedures,
all named chimpanzees
have been assigned to age classes on the basis of the number of years elapsed since their
564
G. TELEKI ET AL.
known or estimated dates of birth.
This is in some cases an over-simplification
able data, but will have to serve until field studies covering
of avail-
several generation
spans are
completed. 4. Chimpanzee The chimpanzees
inhabiting
Gombe
National
Population
Units
Park can be viewed in terms of three basic
types of numerical units : a park population, a study population, and a social community. TWO of these units-the park and study populations-are probably somewhat artificial in their construct, the first because park boundaries were drawn long before the ecological and ethological needs of resident chimpanzees were known and the second because long-term
banana provisioning
probably
altered the structure
of natural
units
within
the
park system. The three types of units must therefore be clearly defined in relation to one another and to other units described in the literature before the structural (size, composition) and dynamic (mortality, natality, chimpanzee society are examined in detail. All chimpanzees Some chimpanzees
living
within
occasionally
Gombe
migration)
National
range beyond
characteristics
the park pop_dution.
Park constitute
the park borders,
of Gombe
sometimes
for several
days at a time, but today this unit is probably a near or complete reproductive isolate surrounded on all sides by physical barriers: the rift escarpment and lake shore to the east and west, and human to the east (Teleki, completed,
settlements to the north and south as well as beyond
1973a).*
but it probably
A detailed
census of this park population
consists of about
100-150
chimpanzees
the rift
has not yet been (Lawick-Goodall,
1968). The park population some 20-50
members
These communities productively
contains (Teleki,
at least three social communities, each of which includes 19753),
can be spatially
nor behaviorally
and perhaps
and socially
isolated
because
one or two smaller units as well.
distinguished,
but they are neither re-
some migration
occurs
among
them at
Gombe and elsewhere (Lawick-Goodall, 19756; Nishida & Kawanaka, 1972). Due to the interdisciplinary and international nature of field research on chimpanzees, the literature abounds with synonymous
labels referring to a fundamental
organizational
unit of chimpanzee society that contains some 30-80 members and occupies a given home Thus, range. Such ranges may be discrete or overlapping, stable or seasonally mobile. the natural
“social
called a group a unit group Goodall, “troop”
unit”
(Reynolds (Nishida,
19758).
originally
described
& Reynolds, 1968),
and a social community
In a primatological
level of organization,
by Itani & Suzuki (1967) has also been 1965), a regional population (Sugiyama, 1969))
framework
(Teleki,
1974, 19756;
this chimpanzee
whereas in ethnological
Lawick-
unit is comparable
to
terms it is perhaps most closely The matrifocal unit, which has
allied to a “band” level of human social organization. been structurally and functionally described for several non-human primate societies (Teleki, 1975a), is therefore not, as Lawick-Goodall (1968) once postulated, the only lasting social nexus among wild chimpanzees.
* The eastern range of Pan troglodytes today runs parallel to the western arm of the Great Rift Valley, with several narrow belt that extends from Lake Albert to the latter, the chimpanzee range foilows the to Mt. Lubangagulu (6’35’s) in Tanzania, inland (Kane, 1972).
populations inhabiting a Lake Tanganyika. Along eastern shoreline at least no more than 30-40 km
GOMBE
The Gombe
Stream
Research
larger social communities
within
CHIMPANZEE
Centre
565
DEMOGRAPHY
is located
within
the home range
the park, one that is hereafter
referred
of one of the
to as the “Ka-
kombe
Community” because a valley having that name lies near the center of this home range (see map in Teleki, 19753). B ecause most or all of the Kakombe community presumably became habituated to humans via banana provisioning, and also because most observations between 1963 and 1973 occurred within a small central study zone that covers a map area of about 8 km2, and perhaps as much as 10 km2 surface area, this community is the core of the demographic survey reported here. As will be described in a later section, however, the membership of Kakombe com-
munity was probably augmented by several pseudo-migrants who were attracted to the main study zone by the availability of bananas, and who therefore adopted temporary or permanent
residence
in this area.
This larger demographic
constellation,
which con-
records, constitutes a study pojulation whose If all chimpanzees suspected of having mean size during the decade was 48.1 individuals. tains all named
chimpanzees
pseudo-migrant
status are subtracted
the Kakombe
community
Population opportunity rugged
from the annual
densities cannot The
census records,
the mean
size of
would be about 40 individuals. be precisely
for error is further increased
terrain.
the actual
in the center’s
calculated
from these census figures,
by a problem
in measuring
and the
surface area in such
estimated
map area of the park (32 km2) may be much less than surface area (32 + 6 km2), and a similar discrepancy may exist (8 km2) and the surface area (8 + 2 km2) of the central study zone.
topographic
in the map area
Moreover, estimates of crude and specific densities should reflect differences in patterns of habitat utilization, and preliminary data on ranging habits at Gombe suggest that several km3 of the steep, rocky terrain near the escarpment
crest are not routinely
Given this variability only be roughly 2.2-3.1
in census and area measurements, Starting with 100 chimpanzees estimated.
and the s.d. about 2.94.0;
and the s.d. to 4.3-6.1. community
exhibit
for the entire
with 150 chimpanzees,
utilized.
crude and specific densities can in the park, the cd. is about the c.d. would rise to 3-334.6,
Within the central study zone, the study population
a c.d. range of 4.8-6.0,
park region.
which falls within
and Kakombe
the 2.2-6~1
As most or all of this zone is heavily
range
utilized,
given
the specific
density should be similar to the above figure.
Pojwlaiion size : the annual census The
annual
size of the Gombe
high of 62 members records
include
study population
to a 1973 low of 39 members
96 chimpanzees
: 87 named
fluctuated (Figure
considerably-from
a 1966
1).
census
The
1963-1973
individuals, 9 unnam-ed births and fetal a very high turnover during these years:
deaths. But the study population experienced only 4 females and 9 males (13.5 % of 9’)o were present for the entire decade. births, 33 postnatal plus 6 fetal deaths, and a net gain of 2 immigrants between
With 30 1966 and
1972 (when migration was probably least affected by provisioning), the study population’s composition differed considerably from year to year. Indeed, given these gross internal changes it is remarkable that the population maintained as much numerical and compositional stability as shown in the accompanying graph. In order to estimate the probable size of the Kakombe community,
the average number
566
G. TELEKI
ET AL.
Figure 1. Temporal fluctuations in the Gombe study population (solid line) census data, and in the estimated size of the Kakombe community (broken line), shown in relation to various banana provisioning schedules (A-E).
PROVISIONING
::I, , . , , , , , , . ,964
,965
,966
1907 1968 ,969
1970 197,
1972 ,973
YEARS
of suspected
pseudo-immigrants-i.e.
nanas-can
be subtracted
corrections
those entering
are made only between
was most consistent.
the local home
from the annual census totals.
these trends and provisioning
in Figure
1, these
1966 and 1972, the period when banana provisioning
This provides a crude comparative
within both natural and artificial
range to seek ba-
As depicted
measure of demographic
units,. The coincidental
schedules
(see stages A-E
and/or in Figure
trends
causal links between 1 and Table
9) are
discussed in later sections. Pojudation When
composition : age and sex classes
the mean annual membership
line (48.1 =
of the study population
13.1 ‘A juveniles,
age classes break down as follows: 17.0 ‘A adolescents, 54.7 y0 adults. The adult/subadult
entire study population is 1.21. Sex ratios within the study population differences
among
100.0 females,
is used as a numerical
100 %), the four major
the four age grades.
while the age-specific
infants, 85.3 for juveniles,
base
15.2 % infants, ratio for the
vary little on an annual basis but reflect major The 1963-1973 mean sex ratio is 94.7 males to
sex ratios in the four main classes are:
141.2 for adolescents,
and 94.8 for adults.
breaks down into 83.6 for prime adults and 200.0 for old adults.
65.9 for
The latter class
Thus, while the sexes
seem to be at or near parity within the study population
as a whole, and also within the adult class as a whole, there are marked differences in the remaining classes. For example, males are greatly exceeded by females in the infant and juvenile classes while the reverse is true in the adolescent and old adult classes. These discrepancies suggest that sexspecific factors regulate survivorship within age-classes, all the more so in regard to excess adolescent and old adult males because female births exceeded male births by 50.0 % during the 1963-1973 period (see Table 1). Two population pyramids have been constructed from the baseline census data to further illustrate these features (Figure 2). Pyramid A depicts only percentage membership in various age classes, and shows that a fairly even distribution exists in all but the two youngest classes. Pyramid B adds a sex-class dimension to this age distribution, and thereby shows that females decrease rather evenly during the middle ages (9-24 years) and more sharply at later ages (25-40 years), while males decrease sharply during early
GOMBE
CHIMPANZEE
Figure 2. Percent distribution of age-classes (pyramid A, top) and of age- and sex-classes (pyramid B, bottom) in relation to total population size. Both pyramids reflect small percentages of young individuals (infants and juveniles) in the Gombe study population.
567
DEMOGRAPHY
A
Tzi -
33-36
1
I
29-32 25-28
I
y21-24
z ;;r;-, -
I
13- 16 9-12
1
7 o-4 -
I IO
5
0
15
1 20
PERCENT
ZG -33-36
d
6
2
29-32
25=18 21-24
y
1
z17-20 13-16 9-12 5-8 O-4 -
I I I I f
1
0
5
IO
5
IO
PERCENT
(13-20 years) and more gradually at later ages (2 I-40 years). A preponderance of old adult males is clearly illustrated in pyramid B. There is unfortunately little merit in speculating about the natural causes behind these phenomena because differences
adulthood
in male and female survivorship selective
effects of provisioning
in the 21-32 upon
year brackets may simply be related
the mortality
and migratory
to
patterns of the two
sexes. When compared to standard pyramids for wildlife populations (e.g., Qdum, 1971), the chimpanzee study population seems to reflect neither marked stability nor potential for rapid expansion.
In fact, when the topheavy appearance
of the chimpanzee
pyramids
is viewed in the context of the epidemics recorded at Gombe, the indication is that considerable time is needed to achieve reproductive recovery from such major crises. The slow
maturation
contribute
and
reproduction
to a recovery
chimpanzees,
rates described
in the
following
lag which may span an entire generation,
section
could
which, for the Gombe
is an average of 19.6 years.
be typical for wild chimpanzees,
A topheavy structure may, on the other hand, for the K-group pyramid at the Mahali site also leans in
a topheavy
in pressj.
Although
direction
(Nishida,
there is still no precise scale for gauging
in wild ape populations,
normality
of age-sex distributions
an urgent need exists for field techniques
whereby
the repro-
ductive status of a potentially threatened primate population can be monitored. Data on the composition of the Kasakati (Izawa, 1970) and Mahali (Nishida, in press) study populations supplement those available from Gombe, and the combined figures can be used to develop an “optimal” matrix of age/sex composition panzee unit. These three sites are selected for comparison proximity and ecological similarity.
for an hypothetical chimdue to their geographical
As shown in Table 2, the three study populations exhibit some variation in percentages of immature and mature males and females, but the variation is well within what would
568
G. TELEKI
Table
2
ET AL.
Percentage distribution of individuals, by age and sex, for three chimpanzee population units in western Tanzania, combined to depict an hypothetical population with an optimal composition
Gombe (study population) Males Females Mature Immature Samples :
Kasakati
Mahali
(Z group)
(I( group)
Males
0.27 0.28 0.23 0.22 N = 4% 1 mean over 10 years
B Males
Females
0.20 0.30 0.25 0.25 N = 36 members over 2 years
Average Unit
Females
0.21 0.34 0.09 0.36 N = 26.3 mean over 8 years
Males
Females
0.23 0.20
0.30 0.27
Note: the marker age separating immaturity years in each study population.
be expected
if age and sex distributions
varied randomly
(x2 =
and maturity is about 12
1.61, p = O-90 at 5 degrees
of freedom). Also shown in the table is an average, or optimal, matrix which reflects a preponderance of both immature and mature females. Comparison of these figures with the Gombe
matrix
suggests that the Gombe
study population
is somewhat
deficient
in
both categories. Even if the Gombe study population continuing
percentages
reproduction
additional
measure
to counteract
populations
deviations
managers
could
monitored
and to ensure
if and when necessary, from
need cause no immediate
should be carefully
from
then
regional
determine
long-term
development
become
alarm,
in the coming
it is evident
that this
years in order to evaluate
survival.
Steps could then be taken,
of further
discrepancies.
When
data
available, or local
this tentative model can be refined to Field primatologists and wildlife norms.
the need for protective
action
on a more
pragmatic
basis than is now common. 6. Chimpanzee The analytical disciplines Gombe
techniques
of human
data-small
death dates, etc.-are
Population
Dynamics
applied in this section have been selectively
demography numerical
and wildlife
samples,
in someways
biology.
continuous
borrowed
However,
census records,
imprecise
unique to the field study ofnon-human
tions, and do not always allow for application primatologists
have not yet developed
our approach
is often experimental.
of standard
the analytical Demographic
from the
the nature
demographic
of the
birth
and
primate populatechniques.
As
tool kit needed for this special task, research on non-human primates is
clearly in need of development and refinement, and this is but a first step in that direction. The raw data upon which analyses of mortality, natality and migratory patterns are based
appear
the 1963-1973 Mortality
in Figure
3, which
is a graphic
summary
of all events recorded
during
study period.
patterns
During that decade 33 postnatal deaths were known or estimated to have occurred within the study population, the causes running a gamut from contagious disease to fatal accidents. There were 6 additional cases of fetal death, representingl5.4 ity recorded, but these will be discussed with natality patterns.
% of the total mortal-
569
GOMBE CHIMPANZEE DEMOGRAPHY
Figure 3. Raw data on mortality (top), natality (middle) and migratory (bottom) patterns in the Gombe study population during 1963-1973.
0 0
The
distribution
adolescents, chimpanzees,
48.5 o/oadults). with mortality
the susceptibility wastage.
of postnatal
deaths
but highly variable
of infants
is close to parity
Deaths (ii=331
n
Fetal Deaths
0
Live Births
Emigration
12
(n=6)
Deaths
Postnatal
Immigration
14
54.5 % females)
Fetal
(n=6)
(8 =30)
(n=23) In= 18,
along sex lines (45.5 % males,
along age lines (33.3 % infants, 9.1 % juveniles,
Mortality
in the first four years of life probably to both
No proof has been obtained
9.1 %
is heaviest in the infant and adult classes of Gombe direct
(e.g. disease)
from East African
being exacerbated
and indirect
by
(e.g. orphanage)
studies that carnivore
predation
contributes to chimpanzee mortality (Teleki, 1973a,d), although West African populations are heavily preyed upon by humans (Harrisson, 1971; Sabater Pi & Groves,1972; Gartlan,
1975).
Diseases,
parasites,
causes of nearly all wild chimpanzee
genetic
abnormalities
and injuries
are the likely
deaths.
An epidemic of some paralytic disease (probably poliomyelitis) and two severe bouts of respiratory illness (probably influenza) struck the study population in 1966, 1968 and 1969. Directly or indirectly-that is, via infection or orphanage-these events accounted for most of the 23 postnatal deaths logged over this 4-year span. In fact, some 30 ‘A to 40 % of the mortality recorded at Gombe during the 1963-1973 period can probably be attributed to pathogenic the estimate underscores
causes. Even if some cases have been incorrectly diagnosed, the high susceptibility of this population to contagious diseases.
570
G. TELEKI
Control
of epidemics
conservation,
ET AL.
should thus be a key issue in future programs
at Gombe
and probably
elsewhere
of management
and
as well.
Given this insight, it i.s especially unfortunate that so little is known about the epidemiology of wild chimpanzee populations. Some general information on disease symptoms and internal parasites is provided by Lawick-Goodall (1968,1975), who refers to undiagnosed (one
cases of skin ailments
resembling
(perhaps
goitre),
influenza
and peritonitis. Attempts
Ciliates,
nasal
hookworms
congestion
growths
often
as we11 as confirmed and roundworms
to treat the paralytic
on the face and neck
accompanied
by coughing
cases of pneumonia,
have been recovered
disease
(Lawick-Goodall,
enteritis
from feces. 1968)
and a
as Rhinophycomycosis enteromorphae (Roy & Cameron, 1972 ; Roy Given the duration of the Gombe project, the epidemiological in field conditions.
1974), comm.)
noted
Budongo
sparse.
graphic
Forest.
Behavioral
descriptions
1968,
Lawick-Goodall
1971,
of chimpanzee
1975a-c;
& Hamburg,
deaths are somewhat
Suzuki,
1971a;
1974), but such information
Bygott,
more com-
1972;
Teleki,
is of little use in demo-
analysis of the causes of mortality.
No conclusive traction
Other field projects have fared no better: Kortlandt (pers. cases at Beni, and Suzuki (1971 b) observed an amputee at
7 “paresis”
(Lawick-Goodall,
19736;
evidence
or dissemination
distribution
of crude
that an historical mains somewhat middle
and colds),
mange),
diagnosed
data are extremely
mon
lung
or common
have been made
facial fungus,
(one resembling
years,
of a direct
death
linkage
link between
banana
provisioning
of disease has yet been obtained rates so clearly
cannot
circumstantial.
parallels
at Gombe.
the various
and the conBut the annual
provisioning
schedules
be discounted even though the evidence at present reMortality rates were low in the early years, high in the
in the final years of the decade (Figure 4). The crude death rate ranged from a maximum of 2 17*4/ 1000 in 1969 to a minimum or 19.6/1000 in 1972. Computed from the 33 cases backed by conclusive or strongly circumstantial evidence, the mean mortality rate for the decade was 6%0/l 000. If all losses from the study population
and low again
were considered
as deaths
Figure 4. Temporal distribution of mortality and natality rates per 1000 individuals for the Gombe study population. Fetal wastage is excluded from both samples.
(i.e. no migration),
the crude
250
death
rate would
-
Mortality
--__
Natality
200-
:_
ISO-
L B w 2 az
IOO-
SO-
I 1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
GOMBE
reach
11 l-7/1000
The
individuals.
CHIMPANZEE
true rate, which probably
these two extremes, seems high for such a small population. raised mortality
571
DEMOGRAPHY
is further strengthened
by a concentration
lies somewhere
between
The premise that provisioning of all known prenatal deaths
in the same period that postnatal mortality reached a peak (see Table 6 for a non-parametric runs test of fetal wastage). Abnormally high and frequent aggregations of chimpanzees within a small area, accompanied by increased aggressive limited food resource, may well have raised prenatal and postnatal exceptional
levels between
1967 and 1970.
Various
introduced
and transmitted
quite effectively
in these conditions
(see summary and/or
contagious
competition for a mortality rates to
diseases may have been of high density and stress
of similar cases in other species by Stott, 1969), with imported
bananas
human suppliers perhaps serving as vectors.
Survivorship. Postnatal Gombe
mortality
study population
cases-the published
(Table
can be transformed 3). Despite
into
a current
life table
for the
the structures of a small sample-only
33
computations are enlightening because no survivorship scale has ever been for wild or captive chimpanzee groups from which individuals were not arti-
ficially
culled. Demographic data obtained from captive colonies are neither complete nor reliable (Bourne, 1972; Butler, 1972), and the unpublished demographic report on Kortlandt’s Beni study population (Ku&ten, 1972) attempts to reconstruct chimpanzee
demography Table
3
Age intervals
by mathematically
reshaping human mortality tables and then extrapolating
Current life table based on known and estimated ages at death of 33 Gombe chimpanzees, with both sexes combined. Mean length of life is 13.5 years
Number dead
Apeyears
(years),
xtox+n o-2 1 3AA 5-6 1 7-8 B 9-10 11-12 I c 13-14 15-16 17-18 19-20 D 21-22 23-24 I 25-26
8 3 2 1 S 1 1 1 0 : 2 3 0 2 1 0 1
42.6 34.8 32.4 35.3 40.6 48.4 44.2 39.7 27.4 24.3 26.0 24.8 19.5 12.8 11.0 10.5 5.4 2.0 1.5
Probability of dying in each age interval,
&
Number entering each age interval out of 1000 born, L!
Average years of life remaining to survivors in each age interval %
0.188 0.086 0.062 0.028 0.049 0.041 0.023 0.025 0.036 0.000 0.038 0.081 0.103 0.234 0.000 0.190 0.185 0.000 0.667
1000~0 659.3 550.8 484.7 457.9 414.2 380.9 363.5 345.5 321.1 321.1 297.2 251.0 202.0 118.5 118.5 77.8 51.7 51.7
11.9 15,6 16.5 16.6 15.5 15.0 14.2 12.9 11.5 10.3 8.3 6,9 6.0 5.2 6.1 4.1 3.7 3.1 1.1
Note: to facilitate comparison with other demographic features, the age intervals are also represented in terms of age grades: A, infant; B, juvenile; C, adolescent; D, prime adult; E, old adult.
572
G. TELEKI
to obtain
a fit with wild chimpanzee
ET AL.
population
curves.
Similar
limitations
occur
at
other field sites, such as Mahali, where Nishida (in press) has collected data on natality and infant mortality but has not yet obtained a general picture of postnatal mortality. In contrast ape-years
to the life tables commonly
have
(the d, column) other components
been
used to compute
among
the Gombe
generated
for wildlife and human
the probability
chimpanzees.
in the life table are standard
of dying
The procedures (see, for example,
populations,
for each
age interval
used to generate Deevey,
all
1970).
Abridged life tables for each sex, based on 15 male deaths and 18 female deaths, were then compiled to illustrate differences in male and female life expectancies at various Table
4
Abridged for each 15 males
life table results showiqg average sex, based on kmnwn and estimated and 18 females
Age intervals (years)
O-2 3-s 9-14 15-20 21-26 27-32 33-38
life expectancies ages at death of
Average years of life remaining to survivors in each age interval, e, / Males
\ Females
(years)
(years)
14.3 15.9 16.6 13.3 El.9 6.7 3.0
IO.9 16.2 15.3 13.2 8.4 5.5 3.0
Figure 5. Actual survivorship in the Gombe study population (solid line) during 1963-1973, and estimated survivorship in a hypothetical wildchimpanzee population (dotted line) based partly on Kortlandt’s data from the Beni site. The latter has been adapted from Kuijsten (1972). Both curves are on a logarithmic scale.
20
AGEIN
3”
YEARS
3”
GOMBE CHIMPANZEE
ages (Table
4).
The results indicate
573
DEMOGRAPHY
of life
that the expectation
at birth (e,,) is greater (er5). The dif-
for males than for females, but that parity is achieved by early adulthood
ference arises mainly from a less favorable set of death rates for females between infancy The causes remain unidentified, and may involve physiological, ecoand adolescence. logical and/or
behavioral
factors.
When plotted on a logarithmic
scale, from the I, column
in Table
3, the survivorship
curve shows a rapid drop during the initial 4 years of life and a steady decline over the following age intervals (Figure 5). The curve in general exhibits a remarkably even convex shape that is characteristic the end of the life span (Odum, (1972) is also provided
of populations in which most mortality occurs toward 1971). The survivorship curve obtained by Ku&ten
for comparative
purposes. although
what skewed because fragments of several populations The life expectancy that management particularly
and survivorship
rates exhibited
policies should include contingency
among females, when the population
his rates are proba’bly
some-
were combined. by Gombe
chimpanzees
plans for reducing
suggest
infant mortality
is struck by unforeseen calamities
such
as epidemics. Swift recovery by such a small and isolated population is essential if gaps in the age pyramid are to be averted, and may warrant artificial stimulation (e.g. medication) by wildlife managers. Orphan mortality. At Gombe,
chimpanzee
(Clark, in press), and are physically, mothers
infants nurse
psychologically
for
an
for at least that many
tapers of& offspring
years. Even after the nutritional may hinge mainly upon continuing
survival
mother for several additional patterns within the younger behavioral
of
years (Lawick-Goodall,
1975u-c).
5.2 years upon their
basis of dependence association with the
Mortality
and survival
age classes may therefore be strongly affected by social and
factors (relationships
factors (weather,
average
and socially dependent
within matrifocal
units, etc.) as well as various extrinsic
disease, etc.).
The deaths of G named deaths, are listed in Table
orphans,
all of whom
died within 3.5 years of their mothers’
5. All but one of these (Pepe) died well before the expected
time shown for their respective
ages in the life table.
male whose death was probably
unrelated to being orphaned.)
died within 0.1-2.5
(Pepe was a paraplegic The remaining
adolescent 5 orphans
years of their mothers, and only one (Flint) was older than 4.5 years.
In at least 3 cases (Cindy,
Merlin,
Flint) the symptoms
prior to death included
general
listlessness and lethargy, with severely reduced maintenance activity and social inter-. action. And in the particular case of Flint, who was intensively observed during the 2 5 days after his mother’s death, psychological
distress and depression may well have contributed
to or even caused death, although he also acquired an intestinal infection (Lawick-Goodall, 197%; La-wick-Goodall & Hamburg, 1974), These and other forms of “grieving” behavior reported elsewhere (Teleki, 19736) suggest that wild chimpanzees perceive death among close kin as an emotional events.
issue, and may possess a generalized
concept
of such
Precise prediction of the age at which orphans are likely to survive parental mortality is prohibited by the small size of the available sample. The data merely suggest that a. death/survival transition occurs for most orphans around 5 years of age, but may stretch another 3-4 years in special circumstances. Prior to the 5-year limit, nutritional deficiencies, coupled mortality.
with social and other deprivations,
are the most likely causes of orphan
30 24 24 30 27 26 37
24 26 24 37 27 37 37
December 1964 October 1968 January 1968 December 1964 January 1969 March 1965 December 1972
October 1968 March 1965 January 1968 December 1972
January 1969 December 1972 December 1972
Bessie Vodka Sophie Bessie Oily Marina Flo
Vodka Marina Sophie Flo
Oily Flo Flo
*d = d = -t = ? = ?A = C = ? =
16 26
January 1968 March 1965
F F
F F F
F
M M M
M M
M
M M
M
13.6 16.5 19.5
11.3 11.8 12.6 13.5
4.5 6.3 6.6 7.5 7.6 7.8 8.9
3.2 3.7
14.2 12.1 10.3
4.3 + 0.5 + 0.5 +
16.5 16.5 16.5 16.6 16.6 16.6 16.6
15.0 15.0 15.6 14.2
d + + ? + + d
15.6 15.6
11.9
4.7 + 3.5 d -? 0.5 +
2.5 4.7 4.4 3.5 4.4 8.2 0.1
0.2 d 1.6d
0.1 d
Faben -
Faben Figan -
-
Evered Pepe Faben Figan Fifi -
Pepe Miff Bumble Jomeo Sally -
Sally Sniff -
M 19.5 - --
19.5 16.5
13.6 11.8 19.5 16.5 13.5
7.8 7.5 11.3 12.6
C -
A A A ? A ? ? C A C C -
F F M F -M M M M F ---M M --
M 11.8
? A A
F 12.6 M 6.6 --
Known death, probably related to being orphaned. Known death, probably unrelated to being orphaned. Surviving at end of study period on June 30, 1973. Disappeared, possibly dead. Adoption behavior (described in Lawick-Goodall, 1975c) observed between older sibling and orphan. Contact and interaction, but no overt adoption behavior, observed between older sibling and orphan. Orphan not observed in association with older sibling.
Evered Figan Faben
Jomeo Pepe Sally Fifi
Beattle Sherry Sniff Bumble Gilka Miff Flint
F
Cindy Merlin
1.3
Behavior of older sibling toward orphan?
Circe Marina
F
Sorema
Name of Sex and known or age of suspected older older sibling (years) at age when sibling mother died
Orphan’s expectation of life
24
(years) when mother died
Subsequent survival span (years) of orphaned offspring*
January 1968
spring
spring
Known or estimated age of offspring
Sophie
Off-
Off-
Name of Sex of
Date of mother’s death
Name of mother
Mother’s estimated age at death
Mortality and survival patterns among 17 named offspring who were orphaned (at 1.3-19.5 years of age) when 7 mothers died during the 1963-1973 study period
Table 5
% b t*
!I.+
2
P
UI 2
GOMBE CHIMPANZEE
Whether
adoption
of orphans
by older siblings
uncertain.
The final column in Table
commonly
exhibited
575
DEMOGRAPHY
has a positive influence
5 suggests that adoptive
behavior
also remains
tends to be most
toward the youngest
orphans, but sibling adoption does not seem to promote survival below the 5-year age limit. Nor is there any indication that the sex of In terms of wildlife the older sibling regulates adoptive behavior or influences survival. management, it would be advisable to monitor the welfare reproductive females who
of
head matrifocal units, having large matrifocal
and to investigate the potential units within a social community.
survival
benefits
provided
by
Natality patterns During
the 1963-1973
study period,
36 pregnancies
terminating
in 30 live births and 6
The disparity previously fetal deaths were recorded within the study population. for the sex ratio of postnatal deaths among infants (3 males/S females = 0.375) matched
by the disparity
with a low reproductive
present
in births
(12 males/l8
rate, a slow maturation
females = O-667).
rate, and a low incidence
noted is not
Together
of twin pro-
duction (observed occasionally in captivity but not yet in the wild), the above discrepancies in female natality and mortality rates point to a low growth rate for chimpanzee
populations.
The
picture
is somewhat
incomplete,
however,
because
the sex
of stillbirths and miscarriages was not determined. The annual distribution of crude birth rates is a mirror image of the crude death rates described
previously,
and were therefore
as likely
affected
by provisioning
schedules.
Natality rates were high in the early years, low in the middle years, and high again in the final years of the decade (see Figure 4). The crude birth rate ranged from a maximum of 97.6/1000 the entire
in 1964 to a minimum
IO-year period.
the study population Measures
of infant
of 18~8/1000 in 1967, with a mean of 62*0/1000
As the mean mortality
strayed from equilibrium death
rate
(233*3/1000),
and natality
largely as a result of migratory combined
(466*7/1000), infant mortality risk rate (68*3/1000), 1000) can also be obtained from the census records.
for
rates are close to equality,
infant
plus fetal
and fetal mortality
variations. death
risk rate
rate (43.11
These measures are all based on a reduced infancy span of O-l year, rather than on the usual 4-year interval, to match demographic standards established for human populations. A test was undertaken
to determine
year could be assumed for live births, the Gombe
study population.
whether
a constant
fetal deaths,
probability
and total pregnancy
of occurrence terminations
* These data, when fitted to Poisson distributions
per in
(Cooper,
* The fit of an observed set of a events, classified by their frequency of occurrence (f), can be examined by the chi-square method (Bliss, 1953). This procedure is one-tailed, however, in the sense that it merely gives an accurate probability that the observed unbiased variance (se) is greater than the observed mean (a) of the distribution. In a perfect Poisson distribution, s2 = 2. Another approach was therefore adopted, using this equation as a null hypothesis, in order to obtain a two-tailed test of the conformity of each empirical distribution to the expected distributions. The observed mean (2) was taken as an expected variance with infinite degrees of freedom. (This technique increases the conservativeness of the test.) The observed variance (s”) was then considered in terms of its own degrees of freedom (n - 1). The variance ratio (F) was finally computed, using appropriate degrees of freedom in the numerator and denominator, and the two-tailed probability obtained from a table of F (Lindquist, 1953).
576 Table
G. TELEKI 6
Number of events,
f
Types of events
Live births Fetal deaths Total pregnancies
ET
AL.
Annual frequencies of live births, fetal deaths and total pregnancies in relation to expected Poisson distributions, with a non-parametric runs test of annual fetal death frequencies (see census data in Table 1)
Live births obtained b obt
Number of events,
Live births expected, b=a
Fetal deaths obtained, dObt
Fetal Total deaths pregnancies expected, obtained, dem Pobt
Total pregnancies expected, P exn
5 4
6 3
0 0
0
1
1
2 3 3 1 1 0
: 2 2 1 1 1
Obtained Obtained unbiased mean and variance, expected variance,
?I
s2
30 6 36
2.22 0.27 1.60
i 3.00 0.60 3.60
Fmln_l 1.35 2.22 2.25
P (two-tailed) 0.20 0.19 0.19
Non-parametric runs test when a = number of years without fetal deaths, b = number of years with fetal deaths and u = number of runs yields R, = 5 yrs, nl, = 5 yrs and u,,.,,s = 3.
1972), show that the observed or obtained numbers of pregnancies are all close to the expected numbers required by the Poisson hypothesis (Table 6). It should be noted, however, that the empirical variances are consistently smaller than the empirical means. These results suggest that an underlying constant probability of conception seems to occur within the study population on an annual basis. A non-parametric runs test on fetal deaths between 1966 and 1970 shows that such wastage is significant at the 0.05 level (Table 6). As mentioned previously, most fetal deaths were probably associated with the stresses accompanying banana provisioning. Fertility schedules. Male chimpanzees at Gombe reach puberty at 9-10 years of age, but may not contribute significantly to the reproductive pool of the population for another 3-4 years as a result of social peripheralization until full maturity is achieved (LawickGoodall, 19756). Females typically begin to exhibit sexual swellings of the perineum at 9-10 years of age, and then experience an additional 1-2 years of adolescent sterility; these swellings become larger and more regularly paced during this period, and menarche probably occurs at about II-12 years of age (Lawick-Goodall, 19753). First pregnancies are likely to occur between 11 and 14 years of age (Clark, in press).* Most females are * Most of the dates marking developmental stages differ in wild and captive chimpanzees. Rates of physical, behavioral and probably psychological maturation can be 2 or more years slower in the wild (compare this report to Riopelle, 1963). Body sizes and weights are significantly smaller at Gombe (Teleki, 1973c; Lawick-Coodall, 19756).
GOMBE CHIMPANZEE
DEMOGRAPHY
577
likely to remain reproductively viable until death, although Flo, the oldest member of the study population, ceased to show swellings 2.8 years prior to her death. No verified cases of menopause have to date been recorded among wild chimpanzees. The mean generation span, or elapsed time between birth of a female and birth of her median offspring, is about 19.6 years for a sample of 10 Gombe females with 3 or more recorded births. In order to establish a standard measure of reproductive performance in the following calculations, the onset of a female’s reproductive span (wr) is here assumed to occur at age 10 years because a gestation period of about 8 lunar months can be subtracted from the minimum age at which Gombe females produce their first offspring. Termination of the reproductive span is assumed to occur at 35 years of age because the oldest Gombe female gave birth to her last infant when she was about 33 years of age. As the ages of initial and final parturition may be highly variable in a small population, however, these marker dates should be viewed as conservative estimates. It is possible that the 25-year average reproductive span used here may in some cases extend another IO-15 years. Among the 49 named females present in the Gombe census data, a sample of 27 females experienced ape-years in the lo-35 years age bracket. The surveillance period for this cohort encompassed a total of 137.8 ape-years. Of these 27 females, 20 exhibited either observed pregnancies or associated progeny during the 1963-1973 study period. Of the remaining 7 females, 6 were at or beyond the lo-year age level for only 6 months to 2 years of reproductive life. Only one female (Gigi) fell within the lo-35 age bracket and did not become pregnant at all during the decade. The reproductive performance of Gombe chimpanzees can be estimated from the above observations (Table 7). Given a potential reproductive span (w$ - wi) of 25 years, the data presented in the table yield the total pregnancy experience (O-235 x 25 years = 5.9 pregnancies), the total fertility (0.188 x 25 years = 4.7 live births), and the total lifetime fetal death experience (5.9 - 4-7 = I.2 fetal deaths) per female in the study population. Over the entire 25-year fertility span, the average probability of a live birth is 0.19 per annum for each female. When only the first 5 and last 5 reproductive years are compared, however, the figures show a 33 % drop in fertility, largely because most pregnancies occurred among those females who were IO-20 years of age. Nonetheless, the number of live births expected (be& is in fact close to the actual distribution observed (bobcj) among Gombe females. A chi-square test, measuring chance divergence of obtained and expected live births when the probability per year is 0.19, shows that the divergence is indeed small (x2 = 1*17,p = 0.9 1 at 4 degrees of freedom). Hence, these chimpanzees show essentially no statistical loss of reproductive capability with advancing age. According to the data presented in Table 7, the net reproductive rate (0.130 x 5 = 0.65 female offspring) is considerably lower than the gross reproductive rate (0.577 x 5 = 2.90 female offspring) in the Gombe study population. This discrepancy results from severe mortality in the young female cohort. The neonatal sex ratio in the small Gombe sample contains a scarcity of males, but the life table indicates that the prereproductive mortality of males is more favorable than that of females. Even so, this compensation is not enough to yield as many male offspring as female offspring a generation later. Coupled with the mortality and migratory patterns described in other sections, the above results allow for a guardedly positive prognosis regarding the future survival of the Gombe study population. But the survival balance may well shift into a negative *
O-228 0.193 0.238 0.128 0.154 z= 0.188
27
Live
z= 0.235
0.285 0.242 0.238 0.256 0.154
Live+ dead
Annual probability of birth
10 8 6 2 1
bexe
Live births expected if probability of birth is 0.2 years,
11*
7 0 3 1 0
Male live births observed
0.363
0.133 o*ooo 0.102 0.128 0.000
fi 0.478 0.404 0.301 0.184 0.071
P.
Males living at midpoint of each age interval,
0.119
0.064 0.000 0.031 0.024 o*ooo
Pafs
Agespecific fertility for surviving male offspring,
18
5 a 4 0 1
’
0.577
0.095 0.193 0.136 0.000 0.153
fs
Observed annual probability of live female births,
chimpanzees
Female live births observed
rates of male and female
Observed annual probability of live male births,
and reproductive
0.367 0.299 0.258 0.125 0.011
Pz?
Females living at midpoint of each age interval,
study
0.130
0.035 0.058 0.035 0.000 0*002
Pzf.
Agespecific fertility for surviving female offspring,
in the Gombe
* Note: these figures differ from the 30 births shown in the main census records because the mother of one male infant was not identified and aged in the initial study period.
29*
1376
Totals
6
12 a 7 1 1
3 2 0 1 0
52.7 41.4 29.4 7.8 6.5
Live births,
10 15 20 25 30
Fetal deaths
b obt
Apeyears
Reproductive performances population, 1963-1973
WI= 10 ws = 35
Age of mother,
Table 7
GOMBE
direction
if novel causes
CHIMPANZEE
of mortality-such
be held at a minimum .*
mans-cannot
struck the study population
as diseases The
paralytic
performance
of Gombe
ities is likely to be a long one. to virulent
Birth interuals.
Among
females,
by human vectors,
park boundaries
wild chimpanzees,
from
transient
hu-
epidemics
that
several females of breeding
only briefly
as orphans.
Given
the
the road to recovery from such calam-
If other chimpanzee
diseases transmitted
tions within fixed, narrow
contracted
and respiratory
in 1966, 1968 and 1969 eliminated
age, as well as several young females who survived low reproductive
579
DEMOGRAPHY
populations
are as highly susceptible
the enclosure of small, isolated popula-
may produce the capacity
a major
threat
to compensate
to survival. for high rates of
loss in the lower age classes is related to a shift in birth spacing when dependent offspring die. This mechanism warrants special attention because it carries important implications It has, in fact, been routinely for management of wild populations. ‘%-aise” reproductive rates in captive colonies (Butler, 1972; Bourne, Of the 20 females who produced females experienced various
used to artificially 1972).?
live or dead offspring during the 1963-1973
successive pregnancies.
As the number
period, 11
of successive pregnancies
for
females ranged from 1 to 4 cases, the total sample actually includes 17 paired In 7 cases (41%) the first offspring remained alive when a subsequent one was
events. born, and in 10 cases (59 %) the first died prior to the birth of the second. The average
interval
between
the birth dates of two consecutive
offspring
when each
survives is 5.6 years. According to Clark (in press), Gombe mothers lactate in the absence of perineal swellings for 3-4 years after parturition, then resume their cycles while still lactating
for about 2 more years, and finally wean their offspring at an average
of 5.2 years.
But if this routine
offspring, average
perineal
cycling
sequence
resumes much
of 1.6 years after the first dies.
70 % reproductive compensation (5.6 normal time interval between live births. related
to offspring
sooner and a second infant
survivorship
between Gombe
(Table
mortality
experienced
an
the birth dates of successive
females can therefore
achieve
off-
about
8). to have given birth to their first pro-
period, the mean age at the time of this event was 11.8 years.
As 3 of the 9 recorded cases resulted in prenatal or pregnancies
is born within
is high during the first 2 years
l-7/5*6 = O-696) when offspring die within the The data clearly indicate that offspring spacing
For the 9 females who were known or suspected geny during the 1963-1973
age
by the death of one dependent
As infant mortality
of life, the overall birth interval-measured spring-drops to an average of I.7 years.
is directly
is interrupted
deaths, it is probable
in the early years of the reproductive
risks for the offspring
that first pregnancies, span, involve greater
(x2 = 2.4, p = O-09, at 1 degree of freedom).
* Chimpanzees are especially prized as models in biomedical research, partly because they are susceptible to so many diseases contracted by humans (Harrisson, 1971). There is consequently some risk that contact between humans and wild chimpanzees will follow the pattern of past contact with North American Indian peoples, or present contact with such groups as the Kreen-Akarore of South America. T Despite the extensive medical and nutritional care given to captive chimpanzees, as well as having birth intervals greatly shortened by separation of infants and mothers, the reproductive rate of the Gombe study population compares favorably to breeding rates in captive colonies (Teleki & Baldwin, 1975). Considering the expense of domestic breedina programs for primates (Hobbs, 1975), Their ov&all productivity and effiy ciency should be questioned via comparison with wild populations,
580
G. TELEKI
Table
8
AL.
Birth intervals among successive offspring produced by chimpanzee mothers in the Gombe study population, 1963-1973
Viability of initial offspring when next progeny born
Number of cases, N Range
Dead Alive
10 7
Based
on the fertility
chimpanzees during
ET
a lifespan
however,
schedule
can theoretically
1.0-2.8 3.0-7.6
and birth
be expected
of 40 years.
1.6 5.6
interval
0.6 1.5
data
estimate
the balancing
effect of a reduction
Furthermore,
since old females
achieve
reduced ductive
record
is known
represents
mechanism
exhibited
adaptive
adaptations
6 live offspring,
extreme
this conservative
Given
(Lawick-Goodall,
Birth
or groups
interval
that serve to compensate units,
in the birth
interval
the above
estimate
raised to sexual maturity. female in the Gombe
with 5 surving
of reproduction
by individuals
significance.
beyond
when
their last is further The repro-
census records,
the age of 4 years,
adoption,
compensatory
any compensatory
a chimpanzee
population
has
may be one of many physiological
for a low reproduction
Orphan
serving a similar
and survivorship, within
reduction
at least in those cases observed
social adaptations
habitat.
success among wild chimpanzees.
picture
rate among wild chimpanzees. matrifocal
reproduction,
in a natural
may well die before
1975),
that are successfully
of Flo, who, as the most long-lived
to have produced
probably
major
independence
to about 3-4 offspring
of 6 pregnancies
female are likely to live beyond
dependent progeny
7 and 8), female
of achieved
operate to reduce survivorship
an age of 4 years despite die.
(0.001
a maximum
no more than 5 offspring per reproductive
offspring
6.7
(see Tables
to experience
Th is is not a realistic
because many contingencies
Consequently,
Birth interval in years Mean Standard-----T deviation
rate and a slow maturation
which seems to operate largely within to date,
may in turn be one of many
role.
Migratory patterns Field
investigators
entire chimpanzee the transfer 1970;
of individuals
Nishida
“migration” migratory members
have described
as migration both the seasonal
social communities
from one portion
nomadic
movement
of a home range to another
or small clusters from one community
to another
of and
(e.g., Izawa,
& Kawanaka, 1972 ; Nishida, in press). As used in this report, the term Furthermore, inter-community refers only to the latter phenomenon.
patterns
at Gombe
of one community
seem to include
two patterns:
a temporary form where
members of another for some hours, days, and a more permanent form where various perhaps even weeks before returning home; social ties and eventually full residence are established in a new community, perhaps for a lifetime in some cases. As used here, the term “migration” again refers to only the latter form. An arbitrary
but convenient
meet and join
span of one year of presence
in or absence
from the study
population was used as the minimum duration for assigning migrant status. If all observed departures and arrivals defined in this manner are classified as inter-community migration, then 41 cases of transfer into or out of the study population occurred the 1963-1973 period (see Figure 3). This sample yields an annual immigration
during rate of
GOMBE
2.3 and an emigration
CHIMPANZEE
rate of 1.8 individuals,
581
DEMOGRAPHY
with a net migration
rate of $0.5
chim-
But the validity of these figures is highly suspect because the introduc-
panzees per year.
tion and subsequent reduction
of banana provisioning was probably an integral regulator of migration during certain years. Of the total number of migrants, fully 17 cases (41%) of immigration occurred in 1964 and 1965 while 14 cases (34 %) of emigration occurred in 1973.
Thus, 75 ‘A of the observed
cases occurred
within a mere 30 ‘A of the time en-
compassed by the census records. When the composition of these cohorts is more closely examined, 8 of the 17 “pseudo-immigrants”
the data show that
added at the start of the decade and 8 of the 14 “pseudo-
emigrants” subtracted at the end of the decade were the same individuals. With the exception of 1 female, who was a recent immigrant with only 3.3 ape-years of residence, and of the 5 offspring
born to 3 migrant females who arrived early and then departed
again late in the decade, the final “mass emigration” of 1973 consisted of those who appeared before 1965. It seems noteworthy that staff members of the research center called these 8 individuals
“southern
they were frequently kilometers
chimpanzees”
seen leaving
toward,
to the south of the Kakombe
never seemed to achieve
until the early 197Os, on the grounds
that
arriving from, or travelling in an area some station. Indeed, several of these individuals
full social membership
in the resident community
and often
traveled as a cluster in the main study zone. Coupled
with these qualitative
of the migrants observed another social community. “southern
chimpanzees’
impressions,
If this is corroborated
do not eventually
unit but simply blend into a neighboring
community,
data suggest that many may have belonged
by further field research-i.e.
wherein kinship, consort, coalition
social
intensity of the social bonds that deter-
will be amply demonstrated.
we can at best cautiously
to
if the
establish themselves as an independent
one-the
mine cohesiveness in a natural community now stands, however,
the demographic
at the start and the end of the decade
speculate
As the evidence
that the social nexus within a
and other special relationships
are known
to comprise a matrix of strong bonds, is sufficiently long-lasting and exclusive to withstand years of regular interaction with members of another community. Until 1968 no ‘rcommunity” Ieve 1 o f organization was recognized by observers of the Gombe
chimpanzees.
sistent (Lawick-Goodall,
Only the matrifocal unit had been recognized 1968).
as prevalent and per-
Then, largely due to the flurry of reports produced
the topic of larger social units by Japanese
primatologists
working
on
at other study sites,
Gombe investigators gradually began to view the study po@&on as a social community (Lawick-Goodall, 1973, 1975). Wh en this unit segmented into two unequal portions, while provisioning
was being reduced
fissioning of one Lrcommunity”
during the 197Os, the event was interpreted
into two (Bygott, 1974, in press;
nascent smaller
unit has now been accorded
label “Kahama
community”,
area to the nearby Kahama The ethological
evidence
seem strong, especially
full community
Wrangham,
status, identified
having apparently moved south from the Kakombe ValIey area (Lawick-Goodall, 19753). favoring
this interpretation
when investigators
are looking
of a fissioned community outward,
as the
1975).
This
by the Valley may
so to speak, from the
observation base of the study population; yet the demographic picture presented here suggests that the fissioning interpretation may have been premature. The segmentation of the study population was, in other words, perhaps no more than the departure of a transient group whose members originally and also finally belonged to a neighboring community. Before complete acceptance is given either interpretation, however, the
582
G. TELEKI
available subgroup
ET
evidence should be carefully should be obtained.
In order
to obtain
a more
AL.
reviewed
reasonable
and additional
approximation
data on the “fissioned”
of the normal
scale of inter-
community migration at Gombe, we will now focus on a much smaller sample of transfers into and out of the study population. The equivocal data gathered at the start and the end of the study period will be ignored, and attention given to the 10 remaining cases of permanent migration (possibly 14 cases, if 4 additional dissappearances are counted as transfers) that occurred between mid-1965 and mid-1972. Based on the conservative sample
of 10 cases, the net migration
with an annual
immigration
The discrepancy but, although
(Izawa,
Nishida,
1974;
incomplete
may be due to the attraction
inter-community
societies
rate is calculated
1970;
migration
Sugiyama,
Nishida,
at +0*3
rate of 0.9 and an annual emigration of banana
provisioning
1973;
Nishida
in Kakombe
Valley;
of wild chimpanzee
& Kawanaka,
in press; Pusey, in press), the available
per year
rate of 06 individuals.
appears to be characteristic
1972;
Kawanaka
&
data on rates are probably
and inconclusive.
Yet there are some basic themes that begin to emerge. tion and at least 4 cases-possibly There
chimpanzees
8 cases-of
At Gombe,
emigration
6 cases of immigra-
were logged in seven years.
were no male migrants observed.
scents, 2 adults) 3 adults),
Moreover, of the 6 female immigrants (4 adoleand the possible 8 female emigrants (2 infants, 1 juvenile, 2 adolescents,
7 individuals
were kin-related.
These
data lead to two tentative
generaliza-
tions ; one, that permanent migration among chimpanzee communities is, in contrast to what has been reported for other non-human primate societies (e.g. Rudran, 1973; Packer,
1975),
some migrant
a predominantly chimpanzees
or
even
merely accompany older kin. (Young males become migrants that follow matrifocal units.) have been noted
exclusively
in press).
were initially
and female There
introduced
migrants
were commonly
lone young
are at least 2 cases at Gombe
the tendency bability
When
for avoidance
of permanent
No comparative
(Nishida
two, that offspring who
adults
(11-14
& Kawanaka,
where unnamed
by local males to the study population
several such temporary appearances, estrous cycling probably contributes into new communities.
activity;
could, in the latter situation, sometimes At the Mahali site no adult male transfer
in age) who were often in estrus at the time of transfer Nishida,
female
(36 ‘A of 14 cases) are likely to be dependent
years 1972;
young females
while in estrus, and, after
eventually became permanent immigrants. Hence, to the probability that females transfer permanently
males of different
or conflict
intercommunity
data on frequencies
communities
seems to be higher, transfer
meet, on the other hand, thereby
lowering
the pro-
by males.
of temporary
uersas permanent
migration
have
been reported from any chimpanzee study site. Yet it seems likely that genetic exchange among neighboring communities would occur in both contexts. Indeed, gene flow may be higher in the case of temporary migration because male chimpanzees, who on occasion travel far into the home range of another community at Gombe (Lawick-Goodall, 1968, 1975b), are known to sometimes develop brief consort relationships with sexually receptive females during such sojourns. Paucity of observations on transferring, and on the temporary verms permanent aspects of migration, unfortunately prohibit further speculation about the biological phenomena involved in inter-community migration by wild chimpanzees. detailed theoretical
The biological mechanisms of intercommunity migration are in more need of field investigation if a chimpanzee model of social organization is to achieve significance
for ethnologists
and paleoanthropologists.
583
GOMBE CHIMPANZEE DEMOGUPHY
7. Ecological Regulation of Chimpanzee Populations Seasonal
variation
panzees
in the availability
is one of many factors
(Wrangham,
1975;
Teleki,
of floral and fauna1 resources
affecting
1976).
their ranging
Fluctuations
habits
utilized
in Gombe
in nutritional
by chim-
National
Park
factors are in turn likely
to affect stamina
and health. Individual body weights, for example, can vary as much as 10 ‘A during the annual cycle of seasons. Moreover, seasonal differences in the techniques of food collection and predation, which entail flexible grouping strategies that are presumably
geared
expenditure, resources
exploitation
are being heavily
probably
provides
exploited,
optimum
while minimizing
animal proteins McGrew,
(Suzuki, 1974),
1971~;
regular
1973; Teleki,
to some extent regulate
aggregation
for the transmission
in the consumption
Nishida,
energy Some
population.
at specific sites within the home range;
the resultant
conditions
It is even possible that seasonal variations 1974;
of key resources
are dispersed while others are clustered
when the latter panzees
to maximize
may in turn affect disease patterns within a chimpanzee
of specific
1973a, 1974;
susceptibility
of chim-
of fatal pathogens. foods, such as,
Hladik
& Viroben
to diseases.
Although nutritional and epidemiological regulators are not yet well documented, we can discuss meteorological regulators with somewhat greater precision. A representative S-year
sample
(1968-1970)
of precipitation
cycle of seasons at the Kakombe Gombe
climate
Wrangham,
Valley
have been reported
and temperature
station,
elsewhere
data, showing the annual appear in Figure 6. Many aspects of the
(Lawick-Goodall,
in the park:
4 months of dry conditions
tions (October-May). by 4 months
of long rains.
The monthly
(June-September)
The wet season includes
with less than a month
Seasonal
difference
distributions
tion during 1963-1973
transitions
about
Comparison
throughout
to the negative fetal wastage
However,
encompassed
season, there is some possibility wastage.
distributed
stable for all seasons, although
7 does not include
(July-December)
If live births
often
in the Gombe
study populaof months has
seasonal trends. The samples 1 because the specific dates of
are unknown.
rainy months may contribute in the field.
intervals,
7, in which the normal chronology
of these raw data suggests that deaths
is relatively
of wet condi-
of short rains followed
occur at fairly constant
of deaths and births recorded
while births are more randomly
Figure
1973a;
from year to year.
appear in Figure
3 cases in each category
observed
Teleki,
and 8 months 4 months
been rearranged (as in the previous figure) to emphasize of deaths and births used here differ from totals in Table
gradient
1968;
1975), so it should suffice to note here that two distinct seasons occur annually
are associated with precipitation the year. The mean temperature the slight temperature
because
most such events were not directly
since all 6 fetal deaths
occurred
in a 6-month
by the dry season and the short-rain that climatic
and fetal
deaths
drop in the
effect of high precipitation.
or other ecological
are added together,
portion
period
of the wet
factors contribute
to fetal
then 21 pregnancy
term-
inations were logged between July and December while only 12 pregancy terminations occurred between January and June. As the 21 pregnancies which terminated between July and December may signify a conception the initial months of the wet season, a relationship tion may lie hidden
behind
the relatively
peak some 8 months between conception
even distribution
earlier, during and precipita-
of live births.
Incomplete
584
G. TELEKI
ET AL.
Figure 6. Meterological data (1968-1970 averages) in Kakombe Valley, Gombe National Park. Note that the maximum monthly precipitation occurs during the short-rain period of the wet season, and that the minimum daily temperatures overlap that same period.
“C 35
30
2.5
20
as they are, these data already point toward new questions. Does copulation frequency exhibit seasonal fluctuations ? Do seasonal variations in grouping tendencies, determined perhaps by cyclic availability of food resources (e.g. fruiting trees, termite migrations), affect sexual behavior and, consequently, conception rates? When additional data become available, more refined questions about ecological regulators will undoubtedly emerge. Births are distributed in a relatively even pattern throughout the year. Deaths, on the other hand, seem to be closely associated with precipitation. Fully 87 % of 30 postnatal deaths occurred during the 8-month span of the wet season. More importantly, 57 % of the postnatal deaths occurred during the 4 months of short rains marking the onset of the main wet season. If the mean monthly rainfall is taken as the independent variable, the Pearson product moment correlation between precipitation and mortality is significantly positive (r = $0.74, j =
GOMBE CHIMPANZEE
Figure 7. Monthly and seasonal distribution of general mortality (top), sex-specifii mortality (middle) and live births (bottom) in the Gombe study population during 1963-1973.
585
DEMOGRAPHY
LongRains ..)....
Dry
Plus6 Prenatal
Deaths
A
S
0
N
D
J
F
M
A
M
June
July
A
S
0
N
D
J
F
M
A
M
June
July
A
S
0
N
0
J
F
M
A
M
June
July
N=30
dependent offspring additionally risk contracting diseases from mothers and/or siblings with whom they have regular contact. When monthly mortality is graphed separately for each sex, a slight tendency toward a male peak between October and January versusa female peak between December and March can be noted. It is possible that males are slightly more prone to die during the short rains while females are slightly more prone to die during the early portion of the long rains. Additional data are needed to determine whether this sex-specific difference is significant. 8. Methodological
Contamination
of
the
Gombe
data
As illustrated in Figure 1, the major fluctuations observed in the 1963-1973 census dataand presumably also in the estimated membership of the Kakombe community-can be chronologically matched to stages in the history of banana provisioning (Teleki, 1973a; Wrangham, 1974). When the demographic components underlying these trends are closely examined, the timing of peaks and slumps in mortality, natality and migratory patterns seem to coincide with feeding schedules (compare Figures 1, 3 and 4). Considered independently, annual variations in each demographic variable could have been
586
G. TELEKI
ET AL.
caused by natural factors-i.e. regulators not related to human activities. It is possible, for example, that the mortality rate rose sharply during 1966-1969 due to an exceptionally heavy influx of contagious diseases which spread from neighboring chimpanzee populations into the Gombe area: or that the drop in natality during the same period was merely part of a normal series of sigmoid oscillations that is perhaps common in wild chimpanzee populations. There is, in sum, no concrete evidence proving that banana provisioning was the direct cause of the major changes observed in the Gombe study populations. When all historical information is collated (Table 9), however, it seems plausible that banana feeding-together with its byproducts of heightened tension, frequent competition, and spatial compression of chimpanzees at the main Kakombe station-either produced or exacerbated several trends in the 1963-1973 census records. The extent to which provisioning affected various trends is difficult to determine in retrospect. Of the various consequences mentioned by Wrangham (1974), aggressive competition and contraction of diseases are most pertinent here. Given that correlation and causation are not always identical, it still seems noteworthy that the run of fetal deaths observed between 1966 and 1970 coincide with the most intense period of provisioning at which time inter- and intra-specific aggression were also high. More aggression around and against pregnant females may well have accelerated the prenatal mortality rate. Bananas are not normally available to the Gombe chimpanzees (except by possible raiding of nearby plantations), but this novel fruit became a major food resource for many regular visitors to the main research station. When massive amounts of bananas which contain only l-2-1.5 g protein per 100 g edible matter (Leung, 1968), are consistently fed to apes that are accustomed to an eclectric omnivorous diet (Teleki, 1974, 19753), negative effects upon natality and mortality, perhaps via lowered resistance to disease, might be expected. For the years 1964-1967, the annual records of poliomyelitis in the human population of Kigoma Region included 18, 143, 31 and 7 cases (D. E. Parmas, Regional Medical Officer, pers. comm. ). In August and September of 1966 there were 8 known cases within 15 km of Gombe National Park. As the epidemic of paralytic disease among the Gombe chimpanzees was recognized in September of the same year, it seems likely that human vectors were involved in the transmission of polio to the chimpanzees. If provisioning affected the Gombe study population along such lines, the magnitude of their demographic responses can be measured by calculating population growth during 1966-1972, a time span when the “pseudo-migration” phenomenon least influenced population stability. Applied to this census period, the growth formula* yields a net 10~sof 17.7 chimpanzees in 7 years, or a per annum loss of 2.5 individuals, from an initial total of 66 named chimpanzees. Given that immigration from regions outside the park boundaries is probably minimal these days, it is evident that the study population could not for long sustain such losses without draining neighboring communities. If the net migration rate of +0.5 chimpanzees per year recorded in 1963-1973 included some individuals who were actually residents of the Kakombe Valley area, and were erroneously “added” as they became habituated to human observers, and the +0*3 net annual migration rate recorded in 1966-1972 is a more realistic measure of transfer, the difference * (P* - P,)ZCIp, = populationgrowth between the first census (PI) and the secondcensus(Pa), with a constant(K) of 100.
GOMBE
Table
CHIMPANZEE
Chronology of provisioning and Gombe Stream Research Centre
9
587
DEMOGRAPHY
demographic
Demographic
Provisioning events
events
at
the
events
Stage
Period
A
August 1962March 1965
Introductory period when many Study population rapidly increases in new chimpanzees are habituated size, but trend is deceptive because most additions are not births but adult via irregular provisioning. “immigrants.” Numerous “southern chimpanzees”attracted to feeding area.
B
March 1965August 1967
Experimental and developmental stage when several distribution techniques are tried, with several increases in the frequency and the volume of banana provisioning.
Study population peaks in 1966, then proceeds to drop rapidly. Mortality rate rises sharply in 1966, due largely to epidemic of paralytic disease. Natality rate drops sharply in 1966 and 1967. Run of fetal deaths begins in 1966.
C
August 1967June 1968
Scheduling and volume of provisioning is repeatedly changed in attempt to cope with increasing difficulty of observing large aggregations of chimpanzees, and also with increasing intraand inter-specific competition for bananas.
Study population continues to shrink. Mortality rate rises sharply in 1968, due partly to an epidemic of respiratory illness. Natality rate bottoms out in 1967 and 1968, with nutritional (e.g. excess bananas) and/or behavioral (e.g. aggressive interaction) factors involved. Run of fetal deaths peaks in 1967.
D
July 196% June 1969
Major compensatory actions taken to reduce stressful situation at feeding station, where 80+ primates (baboons and chimpanzees) gather. often Plans made for new feeding systern.
Study population bottoms out in 1969, following a second epidemic of respiratory disease. Mortality rate peaks in 1968, then drops swiftly in 1969 when feeding is reduced. Natality rate still low, but begins to recover in 1969. Run of fetal deaths continues.
E
June 1969July 1973
Provisioning system completely redesigned to supply few bananas at irregular intervals to lone visitors or small groups.
Study population recovers gradually, due this time mainly to births rather than adult additions. Mortality rate bottoms out in 1971 and 1972, and no epidemics occur. Run of fetal deaths stops in 1970. Mass %migration” of “southern chimpanzees” occurs late in 1972.
between
these rates could be an approximate
or more communities
living near the Kakombe
measure of the drainage
experienced
by one
Community.
The Gombe research setting serves to illustrate how crucial a role can be played by procedures and methodologies applied to long-term study of wild primates. Even though habituation through provisioning may initially seem advantageous and harmless, it can lead to unexpected
dangers for the population
under study.
deemed necessary, we recommend that investigators in order to strike a balance between acceptable
If intensive provisioning is regularly monitor demographic trends and excessive disruption. Moreover,
having applied extensive provisioning, Gombe researchers must now evaluate all results and interpretations in terms of the effects wrought by this technique on a plethora of ethological and ecological factors (e.g., Teleki, 1973a). It remains likely, even then, that some authors will persist to ignore or reject Gombe observations (e.g. Reynolds, 19’75).
588
o.
TELEKI
ET
AL.
9. Tracing matology Modern
anthropologists
Paleodemographic
Trends
favor integration
of demographic
who in principle
Via Pri-
research
primatology, ethnography and paleoanthropology nonetheless acknowledge shortage of suitably complete and comparable data in various areas of research the review by Baker and human partly
primate
because
& Sanders,
1972).
populations
The gap between
studies of extant non-human
seems to have grown particularly
field primatologists
have not provided
That
and apes, should to some extent the Gombe
wide in recent
years,
and accurate
longi-
the detailed
tudinal data needed for drawing comparisons with demographic tions. The Gombe research program, together with other monkeys
studies of human populalong-term
close this gap in the coming
data base is in many
respects
in
a severe (see, e.g.,
severely
limited
field studies of years.
in scope,
possessing
neither the temporal does not completely
nor quantitative depth expected by students of human demography, impair their value. The data we have already presented amply deat one level, the necessity and feasibility of conducting demographic research
monstrate,
on wild non-human speculation-in relevant such
primates.
We now venture
order to explore the potential
to studies of modern
primatological
research.
models of chimpanzee
and early hominid
The
population
for assessing the plausibility If the broader
human
aim,
then,
structure
implications
step-to
populations
is to determine
and dynamics
and accuracy
theoretical
to take another
a level of open
utility of these data, to see whether
insights
can be gleaned from
whether
demographic
may provide a new framework
of some evolutionary
reconstructions.
of the Gombe results are to be fruitfully exploited
however, it is essential to stress (a) that the information base is small and is therefore prone to erroneous interpretation, and (b) the representativeness of the data must be evaluated.
Neither
completion
of additional
caveat
considered,
for some demographic
site listings in Baldwin tion has been met. The chimpanzee
can at present field research
& Teleki,
gests that the studypopulation
therefore
conclude
chimpanzee
that
population
albeit with caution,
data
in Table
in several basic parameters. and sex distributions-have
data obtained 19?3),
demographic
results are summarized
be satisfactorily in primate
10.
at other
but cannot
doesnotdiffer
The first requires The
second
can
be
sites have been reported
(see
be fully resolved until the first condi-
now available Despite
eliminated.
demography.
for comparison
obvious
radically
limitations,
with the Gombe
this information
sug-
from other chimpanzeepopulations
More detailed comparisons of two specific parameters-age lent further support to this assumption (see Table 2). We
the Gombe structure
to explore
data
are in many
and dynamics, the relevance
respects
representative
and that the Gombe
of non-human
primate
of wild
results can be used, demography
to re-
search in hominid paleodemography. Several authors have recently presented demographic analyses on extinct hominid and extant human populations, and some have also reviewed the interpretive problems encountered McKinley,
in paleodemographic 1971;
Mann,
studies
1975 ; Dumond,
framework for the Gombe results. As shown in Table 11, chimpanzee
(Acsadi 1975).
& NemeskCri, These
life expectancy,
sistently lower than similar parameters in early hominid Qnly the Choukoutien survivorship (O-320) is slightly
1970;
Weiss,
1973;
reports serve as a comparative
survivorship,
and fertility
and modern human lower than Gombe
are con-
populations. survivorship
Z-group
K-group
Band A
Kasakati
Mahali
Filabanga 43
28*
50
80
48*
49
85
< 1.0
1.3-2.6
1.6-2.4
2.4-4.7
29-4.8t
c. 5.0
5.0-7.0
1.5
1.1
0.8
I .4
1.2
1.5-1.8
1.8 ?
0.5
0.5
0.8
06
0.9
0.5
0.6
0.6
0.8
0.9
0.6-0.7
0.7-0.8
13
67”
20
50*
120
9*
23*
Continuous or intermittent* study period (months)
chimpanzees per kma.
0.5-0.8
at Gombe is 3.6-4.6
Mosaic grassland/ woodland forest
Mosaic grassland/ woodland Mosaic grassland/ woodland/forest
Mosaic grassland/ woodland/forest Mosaic grassland/ woodland/forest
Forest edge/plantation
Rain forest (with selective logging)
t Estimated density range for entire park population
Study population M-group
Gombe
Mahali
-
Beni
group
Socionomic sex ratio
Nishida & Kawanaka (1972), Nishida (1968, in press) Izawa & Itani (1966), Izawa (1970) Nishida & Kawanaka ( 1972)) Nishida (1968, in press), Sugiyama (1973) Itani & Suzuki (1967), Suzuki (1969), Kano (1971)
Reynolds & Reynolds (1965), Sugiyama (1968, 1973), Suzuki (1971) Kortlandt (nd., pers. comm.) Kuijsten (1972) This paper
Source
Picnic Site
age ratio
type
Total male/ female sex ratio
Budongo
Adult/ subadult
General habitat
Names of observed units
Sites Known* or Estimated estimated crude size of density units per kma
Comparative summary of basic demographic data obtained from field studies of chimpanzees in eastern Zaire (Beni site), western Uganda (Budongo Forest), and western Tanzania (Gombe N. P., Kasakati Basin, Mahali Mtns)
Table 10
590 Table
G. TELEKI
11
ET AL.
Comparison of selected demographic traits in Gombe panzees, extinct hominids, and extant human societies Location or organizational level
Populations
Pan troglodytes* Australopithecus Homo erectus
Expectation SurvivorBirth Age-specific of life at ship at interval fertility age x years age x years in years rate for female offspring
Gombe National Park South Africa Choukoutien Maghreb-type; Taforalt, Afalou Gatherer-hunters Proto-agriculturalists
Homo sapiens Homo sapiens* Homo sapiens*
cbii-
k,
e1.5
115
11.9 21.4
12.9 12.7 26.9
0.364 0.650 0.3200.463
5.6 5.8 4.0 3.5
0.128 (B,,) -
16.5 19.0
20.0 19.8
0.500 0.500
3.5 2.5
0.213 (&) 0.174 (&,)
* Extant populations.
(0*364),
and this difference
the column
showing
panzees exhibiting ingly anomalous
seems well within
estimated
birth
an interval
sample
has an opposite
error.
In contrast,
with living chimThe seemtwice as long as that of living humans.
about
australopithecine
an expected
intervals
survivorship
trend,
(O-650) will be discussed below.
These demographic measures have been selected for comparison largely because relevant data are available for a temporally and phylogenetically broad range of populations.
Consequently,
as more complete to accumulate,
there remains
demographic comparisons
of data listed in Table
of other
populations
wise be obtained
line diverged,
may extend knowledge
by paleodemographers.
are no more than rough estimates, hominid human The
and modern primate
comparison nonetheless
human
populations
survivorship
is in reasonable
measures
1 I is not that chimpanzees
stock from which the hominid primate
considerable
is of course limited
adults at an age of about
feasible.
The
are supposed to represent
but rather
significance a population
that profiles of living non-human
of demographic processes that cannot otherEven though some figures shown in the table
well.
in Figure
accord with survivorship
likely to survive beyond
will become
But
continue
the Gombe data are remarkably well aligned with early It remains to be seen whether other non-
fit equally
that
interpretation.
ape and human populations
populations.
curves depicted
interesting
room for erroneous
profiles of monkey,
8 also show that the chimpanzee
in the same set of hominid
by the small sample the
13 years,
life cycle
sizes underlying
of Gombe
profile Precise
these curves.
chimpanzees-who
enter old age by about
40 years-does
populations.
It is
are prime
27 years of age, and are not
not seem to differ greatly
from the life cycle of
australopithecines. It is thus tempting to assume that other demographic features shown by Gombe chimpanzees (e.g. mortality and natality patterns) are also in close accord with similar features once exhibited by australopithecine populations. Because the australopithecine sample has a shortage of fossil material in the lower age brackets and includes remains from a large time span rather than a single resident population, it is extremely difficult to conduct paleodemographic reconstructions (Mann, 1975). Such difficulties may account for the anomalies apparent in Table 11 where the australopithecine
sample
seems to have an exceptionally
high survivorship
rate while
GOMBE CHIMPANZEE
DEMOGRAPHY
591
Figure 8. Survivorship curves for: A, Australopithecusspp., based on a sampleof 120-155individuals mainlyfrom SouthAfricansites(McKinley, 1971; Mann, 1975); B, Homo erectus, based on a sample of 22 individuals from the Choukoutien site (Acsadi Nemesktri, 1970) ; C, Homo sapiens of the Maghreb-type, based on a sample of about 200 individuals from the Taforalt and Afalou sites (Acsadi & Nemeskeri, 1970); D, Pan trogZodytes,based on a sample of 33 individuals from Gombe National Park (this paper).
AGE
IN YEARS
being similar to Gombe chimpanzees in other respects. The Gombe data may here serve to supplement and refine some of the paleodemographic results obtained by McKinley (1971) and Mann (1975). In both Table 11 and Figure 8 certain australopithecine traits, such as the survivorship probability at age 15 years (0.650) and the entire subadult portion of the survivorship curve (A), seem to be out of phase with both the pongid and hominid data. Only when the raw data are statistically smoothed (Weiss, 1973) does the mean lifetime remaining to australopithecines at 15 years of age (eis = 12.7 years) approach the mean lifetime remaining to chimpanzees at the same age (eis = 12.9 years). Accordingly, it is possible that the mean ages at death proposed for australopithecines (18.0-22.9 years by McKinley and 17-2-22-2 years by Mann, the initial figure for the robust form and the latter for the gracile form) were closer to the mean age at death now exhibited by Gombe chimpanzees (13.5 years). The Gombe data may supplement paleodemographic results in other ways as well. The average interval between births at Gombe is 5.6 years, with a total lifetime productivity of 4.7 or fewer offspring. If the 5.8 year spacing proposed by McKinley (1971: 424) is accurate, then the lifetime fertility of australopithecines may also have been within a range or 4.0-5.0 live births per female. These are of course quite tentative estimates of early hominid survivorship and reproduction. They are, however, based on twu sources of data: living nonhuman primates and fossil hominid remains. If a more comprehensive model of chimpanzee population
G. TELEKI
592
ET AL.
structure and dynamics can eventually be generated from a larger sample of field studies, and if that model would prove to be as compatible with evolutionary trends as the current data seem to be, primatology would add a new dimension to demographic research in ethnology and paleoanthropology. Incomplete as it is now, the model already suggests that a gradual increase of life expectancy and survivorship, along with a gradual decrease in birth spacing, may have jointly contributed to a global expansion of hominid populations since their divergence from common primate stock some millions of years ago. If such evolutionary processes can in time be brought into sharper focus through comparative demographic studies on living monkeys, apes and humans, field primatology will have served to expand our theoretical horizons. A more pragmatic point has perhaps been better demonstrated by this report: namely, that much of the demographic data collected by field primatologists can be arranged, analyzed and presented in accordance with standard procedures used in wildlife biology and human demography. This, in our opinion, is an essential step in producing results that will have comparative value in research focusing on intertaxonomic and interdisciplinary problems. 10. Summary Vital demographic
data on Gombe chimpanzees
The following summary of key numerical data pertains, in the strictest sense, only to the Gombe study population. We believe, however, that this population can provisionally be considered as representative of an average chimpanzee social unit, or community, because its structural components fall within the natural range of size and composition exhibited by unprovisioned populations studied at other sites. 1. Population size and composition (10 year sample) Total size :
mean = 48-l range = 23 = 1.21 = 0.95 = 0.86
Adult/subadult ratio Male/female ratio Turnover rate 2. Mortality and natal@ patterns (10 year sample) Crude death rate Infant death rate Crude birth rate Male/female ratio at birth Male/female ratio for infant deaths 3. Migratory patterns (7 year sample) Annual immigration rate :
Annual emigration rate :
= o-07 = 0.23 = O-06 = O-67 = 0.37
mean standard deviation range mean standard deviation range
= = = = = =
0.86 0.90 2 0.57 1.28 3
GOMBE CHIMPANZEE
Net annual migration
= o-29
rate
Adult-male/adult-female Adultlsubadult
593
DEMOGRAPHY
= 0.00
migrant ratio
= 0.43
migrant ratio
4. Reproductivepatterm (10 year sample) Total pregnancies expected in lifetime of an average female Total live births expected Total fetal deaths expected Annual probability Average
= 4.7
birth interval
dies:
mean =
deviation=
1.70 0.60
(in years) when previous offspring lives:
mean= 560 standard deviation= 1.50 = 0.65
rate (female offspring)
Gross reproductive
I.2
= 0.19
of live births per female
birth interval (in years) when previous offspring
Net reproductive
=
in lifetime of an average female
standard Average
= 5.9
in lifetime of an average female
rate (female offspring)
= 2.90
5. Lijk history data (10 year sample) Male life expectancy
(in years) :
Female life expectancy Combined
at birth=
(in years) :
life expectancy
(in years) :
Age (in years) at death: Maximum
15.9
at birth=
10.9
at age 8=
16.2
at birth=
11.9
at age 8=
16.6
mean=
13.5
standard deviation=
12.5
age (in years) at death:
Generation
14*3
at age 8=
span (in years) :
males=
33
*
females=
37
*
mean=
19.6”
standard deviation = 2.12 range= Age (in years) of females at birth of first infant
mean= standard deviation=
Note:
asterisks(*)
mark figures that are probably
of many adult chimpanzees
are only approximately
conservative
8 11*8* 0.34
range= 1 estimates, as the ages
known.
Some hypotheses and conclusions As demographic
investigation
stage of development,
of non-human
the following
primate
populations
selection of comments
is at such a nascent
is offered mainly as a stimulant
to further research. (a) The membership
of a chimpanzee
population
unit may vary from year to year,
or even month to month, but age and sex ratios are likely to hold stable on a long term basis. Based on a sample of 3 discrete populations, stability is estimated to occur when there are about 75 males to 100 females and 90 immature members to 100 mature members. (b) Female births seem to occur somewhat more frequently than male births, but high infant mortality 6
among
females can compensate
to the extent that the sex
594
G. TELEKI
ET AL.
ratio in a given population
unit remains
population
within
unit
dispersed
at or near parity.
a region
can vary
The composition
considerably,
but
of a
a slight
preponderance of females can be expected in an average population sanple. The sex-specific factors involved in differential mortality patterns have not been identified. (c)
Migratorypatternsareprobablyinfluenced on a temporary
byage, sexandkinship.
Adultmalesmay,
basis, migrate as or even more often than adult females,
are less likely to permanently
transfer into other communities.
but males
Subadult
members
of matrifocal units seem to account for a high proportion of the more persistent form of inter-community migration. The dual roles played by “temporary” versus “persistent” migration in genetic exchange among communities require further
study.
(d) Mortality patterns.
patterns Illness
are more and
susceptible
death
season, when precipitation year,
although
dation where. (f)
The
upon
being strongly
survivorship
curve
as in many shared (g) When
human
account
influenced
of the wet and con-
factors.
negligible,
approaches
a markedly
convex
that most mortality This
of deaths per
The effect of pre-
at Gombe
shape
and else-
after the initial
4
occurs toward the end of the life span,
may be one of several
demographic
patterns
by the higher primates. multiple
likely to produce
factors more
are considered, than
3-4
commercial
exploitation
does not exhibit
average
chimpanzee
that will achieve
female
maturity.
to survive prime adulthood.
of wild populations
(h) With a low reproductive adoption of some orphans), and long birth intervals
the
offspring
no more than 2 offspring can be expected
tion
for a high percentage
by climatic
is probably
groups.
are natality
reach daily minima,
mortality
years of life, and indicates
than months
Live births occur throughout the that conception is seasonally influenced.
there is some possibility
chimpanzee
vagaries
the early
the wet season.
Diseases and to some extent accidents year, the former
during
is high and temperatures
tinue at a high rate throughout (e)
to seasonal
are common
is not Perhaps
Accordingly,
must be strictly regulated.
rate (notwithstanding rate, a high infant mortality a slow maturation rate, a limited fertility span in females
between potential
surviving
offspring,
for rapid
population
a wild
chimpanzee
growth.
A long
popularecovery
period can be expected after a population has been struck by calamitous events. Management and conservation programs should therefore focus on isolating populations (i)
from such events,
especially
and on protecting the lives of subadult If the age and sex ratios characteristic
contagious
diseases carried
and adult females. of wild chimpanzee
populations
by humans, maximize
reproductive potential, then similar ratios may be advantageous in captive breeding programs as well. Emulation of natural community size may also be advantageous. Other wild chimpanzee demographic traits should be examined for the purpose
(j)
of designing biologically efficient breeding programs, and demographic, as well as ethological and ecological, factors should be incorporated into such programs. As some demographic traits exhibited by wild chimpanzees seem to be in reasonable accord with human demographic patterns, especially at the early hominid level, the detailed population models that can be constructed by field primatologists may aid in reconstructing hominoid evolutionary processes.
GOMBE CHIMPANZEE
We are indebted
to the Gombe
and the Republic
of Tanzania
during
two years
Stream Research for providing
of field research
Centre,
financial
in Gombe
595
DEMOGRAPHY
the National
and logistical
National
Park;
Geographic
Society
support to G. Teleki
and to Jane
Goodall,
Director of GSRC, for granting unlimited access to the center’s data files during a visit to the station by J. H. Pfifferling. We are also grateful to the many field investigators who contributed
to those records
over more than a decade
of research
on chimpanzees;
and in particular vital
to David Bygott, Richard Wrangham and Patrick McGinnis for providing The advice and comments of Alan Mann, John supplementary information.
Pfeiffer,
Toshisada
Nishida,
Adriaan
Kortlandt,
and Paul Leslie, who read preliminary
Special thanks are due Lori Baldwin, drafts of the typescript, are also greatly appreciated. whose patient encouragement and assistance were invaluable. References
Gombe bibliographic
entries are marked with asterisks.
Acsadi, Gy. & Nemeakeri, J. (1970). History of Human La3 Span and Mortality. Budapest: Akademiai Kiad6 Aldrich-Blake, F. I’. G. (1970). Problems of social structure in forest monkeys. In (J. H. Crook, ed.) Social Behaviour of Birds and Mammals. London: Academic Press. Baker, P. T. & Sanders, W. T. (1972). Demographic studies in anthropology. Annual Reviews in Anthropo1opy 1, 151-178. Baldwin, L. A., Kavanagh, M. & Teleki, G. (1975). Field research on langur and proboscis monkeys: an historical, geographical, and bibliographical listing. Primates 16, 35 l-363. Baldwin, L. A. Patteron, T. L. & Teleki, G. (in press). Field research on callitrichid and cebid monkeys: an historical, geographical, and bibliographical listing. Primates 18. Baldwin, L. A. & Teleki, G. (1972). Field research on baboons, drills, and geladas: an historical, geographical, and bibliographical listing. Primates 13, 427-432. Baldwin, L. A. & Teleki, G. (1973). Field research on chimpanzees and gorillas: an historical, geographical, and bibliographical listing. Primates 14, 315-330. Baldwin, L. A. & Teleki, G. (1974). Field research on gibbons, siamangs, and orang-utans: an historical, geographical, and bibliographical listing. Primates 15, 365-376. Baldwin, L. A. & Teleki, G. (in press). Field research on tree shrews and prosimians: an historical, geographicai, and bibliographical listing. Primates 18. Baldwin, L. A., Teleki, G. & Kavanagh, M. (1976). Field research on colobus, guenon, mangabey, and patas monkeys: an historical, geographical, and bibliographical listing. Primates 17, 233-251. Berger, M. E. (1972). Population structure of olive baboons (Papio anubis (J. P. Fischer)) in the Laikipia District of Kenya, East African Wild&+ Journal 10, 159-164. Bermant, G. & Lindburg, D. S. eds. (1975). Primate Utilization and Conservation. New York: Wiley Interscience. Bliss, C. I. (1953). Fitting the negative binomial distribution to biological data. Biometrics 9, 176-200. Bourne, G. H. (1972). Breeding chimpanzees and other apes. In (W. I. B. Beveridge, ed.), Breeding Primates. Basel: S. Karger. Burton, F. D. & Sawchuk, L. A. (1974). Demography of Macaca sylvanus of Gibraltar. Primates 15, 271-278. Butler, T. M. (1972). Reproductive history of the Holloman chimpanzee colony. Journal of Medical Primatolopy 1, 51-57. “Bygott, D. (1972). Cannibalism among wild chimpanzees. Nature 238,410~11. *Bygott, D. (1974). Agonistic behaviour and social relationships among adult male chimpanzees. unpublished Dissertation. University of Cambridge, England. “Bygott, D. (in press). Agnoistic behavior and dominance among wild chimpanzees. In (J. Goodall & D. A. Hamburg, eds) The Great Apes. (Perspectives in Human Evolution, vol. IV.) Menlo Park: W.A. Benjamin. Chivers, 6). J. (1974). The siamang in Malaya: a field study of a primate in a tropical rain forest. Contributions to Primatology 4, l-335. *Clark, C. B. (in press). A preliminary report on weaning among chimpanzees of Gombe National Park, Tanzania. In (F. G. Poirer & S. Chevalier-Skolnikoff, eds) Primate Socialization, vol. 2. Chicago: Aldine. *Glutton-Brock, T. H. (1972). Feeding and ranging behaviour of the red colobus monkey. Unpublished Dissertation. University of Cambridge, England. Glutton-Brock, T. H. (1974). Primate social organisation and ecology. Nature 250, 539-542. Coale, A. J. (1974). The history of human population. In (S. H. Katz, ed.) Biological Anthropology. San Francisco: W. H. Freeman.
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ET AL.
Cohen, J. E. (1975). The size and demographic composition of social groups of wild orang-utans. Anime[ Behaviour 23, 543-550. Cooper, R. B. (1972). Introduction to Queing Theory, New York: Macmillan. Deevey, E. S. ,Jr. (1970). Life tables for natural populations of animals. In (W. E. Hazen, ed.) Readings in Population and Community Ecology. Philadelphia: W. B. Saunders. Drickamer, L. C. (1974). A ten-year summary of reproductive data for free-ranging Macaca mulatta.F&z primatologica 21, 61-80. Dumond, D. E. (1975). The limitations of human population: a natural history. Science 187, 713-721. Eisenberg, J. F., Muckenhirn, N. A. & Rudran, R. (1972). The relation between ecology and social structure in primates. Science 176,863-874. Fox, R. (1972). Alliance and constraint: sexual selection and the evolution of human kinship systems. In (B. Campbell, ed.) Sexual Selectionand the Descent of Man. Chicago: Aldine. Gartlan, J. S. (1973). Influences of phylogeny and ecology on variations in the group organization of primates. In (E. W. Menzel, ed.) Precultural Primate Behavior. Basel: S. Karger. Gartlan, J. S. (1975). The African coastal rain forest and its primates-threatened resources. In (G. Bermant & D. G. Lindburg, eds) Primate Utilization and Conservation. New York: Wiley Interscience. Harding, R. S. 0. (1973). Range utilization by a troop of olive baboons (Papio anubis). Unpublished Dissertation. University of California, Berkeley. Harrisson, B. (1971). Conservationof Nonhuman Primates in 1970. Basel: S. Karger. Hladik, C. M. & Viroben, G. (1974). L’alimentation proteique du chimpanzee dans son environment forestier naturel. C. R. Academic Science, Paris, Serie-D 279, 1475-1478. Hobbs, K. R. (1975). The feasibility of supplying relatively large numbers of primates for research. In (G. Bermant & D. G. Lindburg, eds) Primate Utilization and Conservation. New York: Wiley Interscience. Horr, D. A. (1975). The Borneo orang-utan: population structure and dynamics in relationship to ecology and reproductive strategy. Primate Behavior 4, 307-323. Horr, D. A. (in press). Orang-utan social structure: a computer simulation. In (G. E. Friedlander & J. Friedlander, eds) The Measures of Man. Cambridge: Peabody Museum Press. Itani, J. & Suzuki, A. (1967). The social unit of chimpanzees. Primates 8, 355-381. Izawa, K. (1970). Unit groups of chimpanzees and their nomad&m in the savanna woodland. Primates 11, l-45. Izawa, K. & Itani, J. (1966). Chimpanzees in Kasakati Basin, Tanganyika. Kyoto University African Studies 1, 73-156. Kano, T. (1971). The chimpanzees of Filabanga, western Tanzania. Primates 12, 229-246. Kano, T. (1972). Distribution and adaptation of the chimpanzee on the eastern shore of Lake Tanganyika. Kyoto University African Studies 7, 37-129. Kawanaka, K. & Nishida, T. (1975). Recent advances in the study of inter-unit-group relationships and social structure of wild chimpanzees of the Mahali Mountains, In (S. Kondo, M. Kawai, A. Ehara & S. Kawamura, eds) Proceedings from the Symposiaof the Ffth Congress of the International Primatological Society, Tokyo: Japan Science Press. Kortlandt, A. (no date). Computation of the longevity and mortality from the age composition of chimpanzees in the wild. Unpublished manuscript. University of Amsterdam, The Netherlands. Kuijsten, A. C. (1972). Een model voor de leeftijdsopbouw van het wild levende chimpanses. Unpublished manuscript. Univeristy of Amsterdam, The Netherlands. Lancaster, J. B. (in press). Sex roles in primate societies and the evolution of the division of labor in man. In (M. S. Teitelbaum, ed) Social and Biological Bases of Sex Di&rences. *Lawick-Goodall, J. van (1965). Chimpanzees of the Gombe Stream Reserve. In (I. Devore, ed) Primate Behavior: Field Studies of Monkeys and Apes. New York: Holt, Rinehart and Winston. *Lawick-Goodall, J. van (1968). The behaviour of free-living chimpanzees in the Gombe Stream Reserve. Animal Behaviour Monograph 1, 161-311. *Lawick-Goodall, J. van (1971). In the Shadow of Man. Boston: Houghton-Mifflin. *Lawick-Goodall, J. van (1975a). Cultural elements in a chimpanzee community. In (E. W. Mensel, ed.) Precultural Primate Behavior. Basel: S. Karger. *Lawick-Goodall, J. van (19756). The behaviour of the chimpanzee. In (G. Kurth & I. Eibl-Eibesfeldt, eds) Hominisation and Behavior. Stuttgart: Gustav Fischer Verlag. *L.awick-Goodall, J. van (1975c). The chimpanzee. In (V. Goodall, ed.) The Questfor Man. New York: Praeger. “Lawick-Goodall, J. van & Hamburg, D. A. (1974). Gombe East, Gombe West. Stanford Magazine 2, 66-70. *Leung, W. T. W. (1968). Food composition table for use in Africa. Manual published by U.S. Department of Health, Education and Welfare, Bethesda, Md. *Lindquist, E. F. (1953). Design and Analysis of experiments in Psychology and Education. Boston: HoughtonMifflin. Mann, A. E. (1975). Paleodemographic aspects of the South African australopithecines. University of Pennsylvania Publications in Anthropology 1, 1-17 1.
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*McGrew, W. C. (1974). Tool use by wild chimpanzees in feeding- upon driver ants. .lournal of Human _ Euoluti~n3, 5011508.. McKinley, K. R. (1971). Survivorship in gracile and robust australopithecines: a demographic comparison and a proposed birth model. American Journal of Physical AnthropoloQ 34,417-426. Nishida, T. (1968). The social group of wild chimpanzees in the Mahali Mtns. Primates 9, 167-224. Nishida; T. (1973). The ant-gathering behavior by-the use of tools among wild chimpanzees of the Mahali Mountains. .lburnal of Human Evolution 2, 357-370. Nishida, T. (in-press). Social structure of chimpanzees in the Mahali Mountains. In (J. Goodall & D. A. Hamburg, eds) The Great Apes. (Perspectives in Human Evolution, vol. IV.) Menlo Park: W. A. Benjamin. Nishida, T. & Kawanaka, K. (1972). Inter-unit-group relationships among wild chimpanzees of the Mahali Mountains. Kyoto University African Studies 7, 13 l-169. Odum, E. P. (1971). Fundamentals of_Ecalogy. Philadelphia: W. B. Sanders. *Packer. C. 11975). Male transfer in olive baboons. Nature 255.219-220. *Pusey, ‘A. (in press). Intercommunity transfer of chimpanzees in Gombe National Park. In (J. Goodall & D. A. Hamburg, eds) The Great Apes. (Perspectives in Human Evolution, vol. IV.) Menlo Park: W. A. Benjamin. Reynolds, V. (1975). How wild are the Gombe chimpanzees? Man 10,123-125. Reynolds, V. & Reynolds, F. (1965). Chimpanzees of the Budongo Forest. In (I. DeVore, ed) Primate Behavior : Field Studies of Monkeys and Apes. New York: Holt, Rinehart and Winston. *Ransom, T. W. (1971). Ecology and social behavior of baboons (Papio an&s) at the Gombe National Park, Unpublished Dissertation. Berkeley: University of California. Riopelle, A. J. (1963). Growth and behavioral changes in the chimpanzee. Zeitschrift fur Morphologic und Anthropologic 53, 53-6 1. Rodman, P. S. (1973). Population composition and adaptive organization among orangutans of the Kutai Reserve. In (R. P. Michael & J. H. Crook, eds) Comparative Ecology and Behaviour of Primates. London: Academic Press. *Roy, A. D. (1974). Rhinophycomycosis enteromorphae occurring in a chimpanzee in the wild in East Africa. American Journal of Tropical Medicine and Hygiene 23, 935. *Roy, A. D. & Cameron, H. M. (1972). Rhinophycomycosis enteromorphae occurring in a chimpanzee in the wild in East Africa. AmericanJournal of Tropical Medicine and Hygiene 21, 234-237. Rudran, R. (1973). Adult male replacement in one-male troops of purple-faced langurs (Presbytis senex senex) and its effect on population structure. Folia primatologica 19,166-192. Sabater Pi, J. & Groves, C. (1972). The importance of the higher primates in the diet of the Fang of Rio Muni. Man 7, 239-243. Stott, D. H. (1969). Cultural and natural checks on population growth. In (A. P. Vayda, ed) Enviroment and CutturaE Behavior. New York: Natural History Press. Struhsaker, T. T. (1969). Correlates of ecology and social organization among African cercopithecines. Folia primatologica 11,80-l 18. Sugiyama, Y. (1968). Social organization of chimpanzees in the Budongo Forest, Uganda. Primates 9, 225-258. Sugiyama, Y. (1969). Social behavior of chimpanzees in the Budongo Forest, Uganda. Primates 10, 197225. Sugiyama, Y. (1973). The social structure of wild chimpanzees: a review of field studies. In (R. P. Michael & J. H. Crook, eds) Comparative Ecology and Behaviour of Primates. London: Academic Press. Suzuki, A. (1969). An ecological study of chimpanzees in a savanna woodland. Primates 10, 103-148. Suzuki, A. (197la). Carnivority and cannibalism observed among forest-living chimpanzees. Journal of the Anthropological Society of Nippon 79, 30-48. Suzuki, A. (19716). On the problems of conservation of the chimpanzees in East Africa and the preservation of their environment. Primates 12,415418. *Teleki, G. (1973a). The Predatory Behavior of Wild Chimpanzees. Lewisburg: Bucknell University Press. *Teleki, 6. (19733). The omnivorous chimpanzee. Scientific American 228, 32-42. *Teleki, G. (1973~). Group responses to the accidental death of a chimpanzee in Gombe National Park, Tanzania. Folia primatologica 20, 81-94. “Teleki, G. (1973d). Notes on chimpanzee interactions with small carnivores in Gombe National Park, Tanzania. Primates 14, 407-411. *Teleki, G. (1974). Chimpanzee subsistence technology: materials and skills. Journal of i&man Evolution 3, 575-594. Teleki, G. (1975a). The evolution of primate subsistence patterns. Paper presented at 14lst Annual Meeting of the American Association for the advancement of Science. New York. “Teleki, G. (1975b). Primate subsistence patterns: collector-predators and gatherer-hunters. Journai of Human Evolution 4, 125-184.
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Teleki, G . (1976). Spatial and temporal dimensions of routine activities performed by chimpanzees of An ethological study of adaptive strategy. Dissertation. Gombe National Park. Tanzaniai -. lJn&ublished _ University Park: Pennsylvania State Univers&. . _ Teleki, G. & Baldwin, L. A. (1975). Breeding programs aim to keep this a planet of the apes. Smithsonian 5, 76-81. Weiss, K. M. (1973). Demographic models for anthropology. Memoirs of the Society of American Archeologists 27, 1-186. “Wrangham, R. W. (1974). Artificial feeding of chimpanzees and baboons in their natural habitat. Animal Behaviour 22,83-93. “Wrangham, R. W. (1975). The behavioural ecology of chimpanzees in Gombe National Park, Tanzania. Unpublished Dissertation. University of Cambridge, England.