Behavioural ecology of the extinct papionines

R. I. M. Dunbar Department ofAnthropology, lZliuersity College London, Gower St, London WCIE 6BT, U.K. Received I April 1991 Revision received 15 October 1991 and accepted 15 October 1991 I;evmrds: papionids, thcropiths, behavioural ecology, group sizts.

Behavioural papionines

ecology

of the extinct

A model of the time budgets of baboons is used to explore the behavioural ecology of extinct species of baboons. The model allows us to determine the maximum ecologically tolerable group size that a population could sustain in a particular habitat without overtaxing its time budget. This in turn allows us to determine the range of environmental parameters under which a given species could survive. The simulations indicate that maximum ecologicall) tolerable group sizes would have been significantly lower in habitats where individual species were not present than in those habitats where they are known to have occurred, suggesting that populations would have found it difficult to survive in just those habitats where they appear to have been absent. The models suggest that the ecological relationship between the papionids and the theropiths has always been complementary rather than competitive. Finally, the analyses suggest that Dinopithecus may have been ecologically more similar to the gelada than the baboons. Journal

ofHuman Evolution ( 1992) 22,407- 42 1

introduction Attempts to reconstruct the behavioural ecology of extinct species have generally involved (1) qualitative assessments ofecological niche based on general morphology (e.g., Szalay & Delson, 1979), (2) determination of specific traits based on extrapolation from allometric scaling relationships for modern primates (e.g., Kay & Simons, 1980; Kay & Covert, 1984)) or (3) assessment of likely traits based on cladistic analyses of living primates (e.g., Andrews & Aiello, 1984; Wrangham, 1987). I n all these cases, taxa are assumed to behave in a species-typical way. However, the accumulation ofdata from field studies of living primates over the past two decades or so has revealed that most primate taxa exhibit considerable variation in their demographic, social and ecological patterns across the range of habitats that they occupy (see, for example, Smuts el al., 1987; Dunbar, 1988). This flexibility of response to changing environmental conditions is particularly characteristic of the higher primates, and especially so of the Old World monkeys and apes. In the light of this, Tooby & DeVore (1987) have argued that we need a more sophisticated modelling approach that is capable of reproducing the fine-tuned responses of animals to the many different variables that influence their behaviour. This paper presents an attempt to move in the direction recommended by Tooby & DeVore (1987). A model of the time budgets of baboons is used to determine the largest group size that extinct papionine populations could have tolerated without putting their time budgets under stress. This tells us two things: one is the limits on group size, and the other is where the animals could have lived. In addition, we can of course also specify the minimum time budget allocations. This approach is based on the assumption that the amount oftime an animal has to devote to different categories of activity is determined mainly by environmental parameters, the length of the dayjourney and group size. There is now a considerable amount ofquantitative data on the behavioural ecology of different populations of the same taxon, and this makes it possible to determine equations describing each of the main time budget components (see Dunbar, 1992, in press a). Since the amount of time available during the day is limited, we can then use the equations that define the time budget to determine the maximum 0047-2484/92/4-50407

+ 15 $03.00/0

0 1992 Academic

Press Limitrd

408

R. I.M. DUNBAR

ecologically tolerable group size. This is defined as the largest group size at which all available spare time is allocated to essential activities (see Dunbar, in press a). The conventional approach to palaeo-ecology has been to assume that species behave in a characteristic species-specific way, in all habitats, under a wide range of environmental conditions. The approach here differs from this in that it is population-specific rather than species-specific. That is to say, I consider the environmental characteristics of individual habitats and ask how well animals with given dietary and body size characteristics might have coped in that particular environment. The focus is on the one taxonomic unit that has real biological validity: the population of animals living in a particular habitat. Species, on this view, are simply populations that happen to share: (1) a set of ecologically relevant characteristics (notably body weight), and (2) a particular bauplun (in respect ofdietary and reproductive specialisations). One by-product of this approach is that we can ignore the niceties of taxonomic classifications and concentrate instead on the factors of real ecological importance to particular populations of animals. Given that we know that a taxon was able to survive in a particular habitat by virtue of its having left fossils, we can go on to ask how well it might have coped with that environment. Doing so should allow us to look more closely at the processes influencing a given taxon’s ability to invade and colonise a range of habitats that differ in key environmental respects. If we can identify points at which a particular taxon would have found it difficult to survive, we may then be able to infer something about the factors that are likely to have driven changes in body size and/or bauplan. This, in turn, should allow us to say something more concrete about why particular taxa went extinct when they did. First, a model developed for Pupio baboons (Dunbar, in press a) is used to explore the geographical distribution of the extinct papionids of the genera Parapapio and Papio. Then, the distribution of Pupio species are compared with those of the theropiths (genus Theropithecus), using a similar model based on data from the extant gelada (Dunbar, 1992). Methods Time budgets consist of four main categories of activity: feeding, moving, resting and social interaction. The factors that are likely to influence the amount of time that an animal has to devote to each of these include: group size, the length of its day journey, the ambient temperature of the habitat and the distribution and richness of its food resources. Rainfall is known to be a reliable predictor of primary productivity in sub-Saharan habitats (e.g., Le Houeron & Hoste, 1977; Rutherford, 1980; Deshmukh, 1984; McNaughton, 1985). Three measures of rainfall are used in these analyses: (a) total annual rainfall, (b) the number of dry months in the year (i.e., months with less than 50 mm of rainfall), and (c) the evenness with which rainfall is distributed across the months of the year (using Simpson’s evenness index; see Dunbar, in press u) . The equations for Pupio are based on an analysis of data from 18 populations. Fourteen of these were used to derive equations for the main components, and the remaining four were used to check the validity of these equations. A stepwise multiple regression procedure is used to determine the best-fit equations for each of the variables. Figure 1 summarises the main causal relationships in this system of equations. The equations themselves are given in Table 1. These equations account for a high proportion of the variance in the observed

PAPIONINE

1

Figure 1. Flow diagram between variables.

Table 1

--+

1

size

+

for the Pa@ socioecology

409

ECOLOGY

I

model, showing

I the main functional

relationships

Equations used in tbe simulations of Papio time budgets

Equation

Variable

Feed (‘I,,)

Move (T,,) Social

(%)

Rest (70) Day journey (km) Rainfall diversity No. ofdry months

In(F)

=(0.401n(M’)-1.16) x (6.87+ 4.081n(z) -0.951n( 7) -0.29lnj O.l61n(J)) In(M) =(0.95-0.331n(M’)) x (2,20+ 0~161n(Jv)+0~221n(V) S=4.53+0.08X In(R) =0.977.921n(<+O.601n( V) In(J) = 1.34+0.781n(N) -0.471n(P) z= 0.48 + 0+041T- 0.00 172 V=7.96-3.02P

V) +

N = Simpson’s index of monthly rainfall dispersion; V= number of months with less than 50 mm of rain each year; T= mean ambient temperature (“C); P=mean annual rainfall (mm); W=mean adult body weight (kg); Jv= group size.

variables (despite a range of values that exceeds an order of magnitude in many cases), and they predict the time budgets and day journey lengths of the test populations to within about 0.4 standard deviations of the observed values on average. The equations for the gelada model are based on a sample of three populations. Although we can say nothing about the statistical significance of the equations in this case, they appear to predict group sizes over a wide range ofhabitats with a surprising degree of precision (Dunbar, 1992) and I take this as sufficient justification for using them, albeit with an appropriate dose ofcaution.

410

R. I. M. DUNBAR

These equations are used to determine the maximum ecologically tolerable group size by iteratively determining the largest group size at which all spare time would be allocated to other activity categories. In order to do this, it is assumed that feeding and moving times are determined by the environmental and demographic state variables for the population. Although social and resting times are influenced by both environmental conditions and the time allocations to feeding and moving, our problem is to determine the minimum amount of time that animals ought to devote to resting and social activities. It is assumed that this minimum is set by the group size in the case of social time (on the grounds that social interaction is the glue that holds a group together; see Dunbar, 1991). If animals devote too little time to social interaction, their groups will be unstable, and hence, will fragment easily. This assumption seems to be justified; in the sample of baboon populations, groups that devoted less time to social interaction than they ought to according to the model were more likely either to fragment while foraging or to undergo fission. In the case of resting time, it is assumed that the minimum amount of resting time is a function of environmental heat loads (reflected in ambient temperature) and the density ofcover in the habitat (itself a function of rainfall diversity). The regression in this case was derived simply by obtaining the best-fit equation to these two variables alone. Although resting time may be important for digestion, it seems that the requirements in this respect are very much lower for frugivores like baboons than for folivores (see Dunbar, 1988: Figure 6.1). In order to extend this analysis to extinct species, it is necessary to take body weight into account. Since the original regression analyses assumed that body weight is constant across baboon populations, we can correct the values for feeding and moving time predicted by the equations by scaling them as follows: F, = F x

W”‘404/B0.404

M,, = M x B@333/ W0.333

where Fis the percentage of time spent feeding, M the time spent moving, W the mean adult body weight of the target population and B the mean adult body weight for the sample of populations from which the equations were derived (see also Dunbar, in press c). For Pupio, B = 17.58 kg, and for the gelada, B = 16.5 kg (see Dunbar, in press a,b) . The scalar for feeding time takes both the metabolic costs of thermoregulation and the increased efficiency of digestion into account (see Demment & van Soest, 1985)) while that for moving compensates for the increased stride length of larger animals (see Peters, 1983). Table 2 gives estimates of body weight for the extinct taxa, as well as details of the sites at which they have been found. The conventional practice of assigning individual populations to specific species has been followed, in order to allow the results that follow to be related to current taxonomic classifications. However, it should be remembered that precise taxonomic ascriptions are in fact irrelevant to this analysis; our concern here is with individual populations of animals, not with higher taxonomic units. For present purposes, Fleagle’s (1988) taxonomy is used. Body weights are those given in Fleagle (1988), except for the two Pupio species where body weights have been estimated in the light of Szalay & Delson’s comments on their relative body sizes. Site distributions are taken mainly from Szalay i3t Delson (1979); only those sites are listed where it has been possible to obtain estimates of environmental parameters. The climatic parameters for habitats are essentially those given by Carr (1976), Bonnefille (1983)) Shipman & Harris (1988) and Vrba (1988). Shipman & Harris (1988) provide

PAPIONINE

Table 2

411

ECOLOGY

Body weight and habitat variables for extinct papionids

(A) Southern

African sites

Sterkfontein levels

Makapan levels

Swartkrans levels

Kromdrai

Bqdy size Species

Taung

(kg)

X

19 23 30 7

Parapapto jonesi Pp. broomi Pp. mhitei Pp. augusticeps Papio robinsoni P. irodi Gorgopithecus Dinopithecus Rainfall (mm) Temperature (“C)

l-4

1-4

5

1

5

X X X

X X X

2

X

x

X

x x

x x

cl9 12 41 77

X

x x ?

700 23.2

?

300 17.2

800 20.8

800 20.8

800 20.8

150 18.1

800 20.8

(B! East African sites

E. Turkana

Omo Body size

Hadar

Laetoli

(kg)

3My

3My

19 17 3

X

Species Pp. jonesi Pp. ado Pp. sp. indet. D. ingens Rainfall (mm) Temperature (“C)

X indicates

habitats

3My

2My

IMy

3My

2My

Olduvai 1My

AMY

‘MY

X X x

77 700 26.6

2 ;I

700 28

x X 640 20

X 300 22

700 28

X 300 25

300 22

1100 25.4

300 21.4

where species occurred.

estimates of temperature and rainfall for a number of key sites by matching fossil fauna1 distributions at these sites to the extant faunas for a number ofAfrican habitats; the current climate for the area that has the most similar fauna1 profile is taken as the best estimate of the palaeo-climate at a given horizon, on the assumption that habitats characterised by similar fauna1 (especially ungulate) profiles are likely to have similar vegetational conditions. Where necessary, mean ambient temperature for any given contemporary habitat can be determined from its altitude and latitude using the equation given by Dunbar (1992; see Table 7 below). In cases where no data were available on the palaeo-climate at a given horizon, it has been estimated by adjusting the present climate for that site by the difference in global temperatures between the present time and the time of horizon, using the graph given by Prentice & Denton (1988: Figure 24.7b). Since these sites are mainly Ethiopian rift valley sites, rainfall for these habitats was assumed to be 700 mm a year prior to 2.5 My and 300 mm thereafter (following Carr, 1976). Palaeontologists will inevitably disagree about the validity of the climatic estimates determined by these authors. So far as the present exercise is concerned, the accuracy of the values is probably not important providing the figures are in the right general area. In any case, my purpose here is mainly to illustrate a method; it is ultimately up to the

412 Table 3

R. I. M. DUNBAR

Maximum ecologically talerabk group size predicted by tke equations in Table 1 for P 2&kg baboon living under various climatic collditions

Mean temperature Rainfall (mm)

100

500 900 1300 1700 2100 2500 2900

palaeontologists predictive tool.

to produce

(“C)

0

5

10

15

20

25

30

35

40

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 1 4 8 14 16 3 0

23 39 47 51 48 32 2 0

54 78 88 91 83 57 5 0

78 Ill 131 144 149 131 31 0

14 38 71 113 163 211 171 32

0 0 0 0 0 0 202 275

0 0 0 0 0 0 0 0

the data that are appropriate

for the model’s precision

as a

Results Ecology of extinct papionids

Table 3 gives the maximum ecologically tolerable group size for an average sized baboon (i.e., one with a mean adult body weight rounded up to 20 kg) predicted by the equations given in Table 1. These results suggest that baboons of this body weight cannot live in habitats with mean ambient temperatures below about 12°C or above 32°C (except under exceptionally wet conditions), or in habitats that receive more than about 2300 mm of rainfall a year (unless temperatures are in the range 30-35°C). These limits are imposed partly by environmental factors influencing thermoregulation and forage quality (and hence the amount of time that needs to be spent feeding and resting) and partly by the spatial dispersion of the baboons’ main food sources (the bush layer of the vegetation) (see Dunbar, in press a). In order to assess the impact of body size on the range of habitats that baboons can occupy, the simulation was rerun for a range of body sizes between lo-90 kg. The results are given in Table 4 as the range in rainfall within which baboons of different body weights can maintain a maximum tolerable group size of at least 20 animals, under a number ofdifferent temperature regimes. Twenty can be considered to be the minimum viable mean group size for a population because relatively few natural groups ofextant baboons are smaller than this (Dunbar, 1988: Figure 7.8). Note that this value is the mean value for the population; to obtain this mean, at least some groups would have to be very much smaller (and very few stable baboon groups contain fewer than 10 animals; see Dunbar, 1988: Figure 7.11). These results suggest that, as body size increases, the animals become confined to an increasingly smaller range of habitats, concentrated on those with a mean annual temperature of 25°C. On the whole, rainfall has much less impact on the ecologically tolerable zone than might be expected. The maximum tolerable group size also tends to decline as body size increases (Table 5). This suggests that animals of smaller body size will be able to exploit cooler habitats more successfully than larger ones. At first sight, this is

PAPIONINE

Table 4

Body weight (kg)

10 20 30 40 50 70 80 90

Table 5

413

ECOLOGY

Range in rainfall valuem within which papiomids of different bady size could sustaia a maximum group sire greater tbaa 20 animals, under Merent temperature regimes

Range in annual

(mm) at mean temperature (“C)

15

20

2.5

O-2400 O-2200 300- 1900 800- 1300 _

c-2500 O-2400 o-2200 O-2100 100-1900 700-900

O-2600 O-2600 O&2400 O&2400 O-2300 O-2200 100-2100 100-1900

10

1500-2400

rainfall

30

>300 200-3000 20&2400 300-2600 3OS2600 400-2400 50&2500 500-2400

35

> 2300 > 2300 > 2300 > 2300 12400 > 2400 > 2400 > 2400

Maximum emlo+ally tolerable group sizes for papkmids of different body weight under di&rent climatic regimes

Body weight (kg)

Mean temperature 0

10

15

20

25

30

35

10 20 30 40 50 70 80 90

1 0 0 0 0 0 0 0

40 16 8 4 2 1 0 0

84 51 32 21 14 7 5 4

127 91 65 48 35 20 15 12

188 149 120 98 80 55 46 38

262 220 184 159 137 103 91 80

353 275 217 173 138 111 98 87

‘Maximum group size predicted rainfall lOlF-2900 mm.

for all habitats

(“C)’

within the range in annual

surprising, since it is the inverse of the usual relationship between body size and temperature (Bergman’s Rule). The validity of Bergman’s Rule has, however, been the subject of heated controversy in recent decades (Schmidt-Nielsen, 1984). In any case, Bergman’s Rule is a consequence of thermoregulation, whereas the results in Table 4 also reflect foraging costs; large animals need absolutely more food than small animals. The results suggest that the lower resource densities in cooler habitats set a tighter constraint on the baboons than the energetic costs of heat loss. Note that the simulation results given in Table 4 also suggest that there is an upper limit on the body size of baboons at about 90-100 kg. Animals larger than this would not have been able to meet their time and energy budgets in most African habitats (see also Lee & Foley, in press). Next is considered the maximum tolerable group sizes that would be predicted for specific populations of extinct papionids. Since the model predicts maximum group sizes for extant baboon populations that match those observed quite closely, we should expect to find that

414

R. I. M. DUNBAR

P. irw’

“Pp. broom; Pp. whifei Pp. jonesi P robimm

Figure 2. Mean maximum ecologically tolerable papionid species at sites where fossils are present

maximum

group

sites where

it did. Figure

sizes are much

taxa for all sites where region

where

geographical

regions;

absent

(Most

for example,

to occur,

sites where

sites in southern group

Africa,

four rather the Olduvai

at the same site (where

least 500,000 maximum when

years:

it was present

chance

(binomial

These the ability

see Table

ecologically

results

successive

species

group

group

at that same location

(Table

size at

than they are at sites where

time horizons

size was smaller

from it was it was

Dinopithecus ingens and Gorgopithecus

are typically

at an earlier separated

2). In all but one of these cases (with one additional

tolerable

discrete

at sites where

sites. In all cases, the mean

as a fossil are smaller

at

area, the

for species that are only known

major, both ofwhich are taxonomically on the periphery of the papionids. In 12 cases, a species was recorded as present at one time horizon, but absent later horizon

than

the same geographical

into

size was determined

African

are the two giant

did not occur

size for each of the extinct

fossil sites cluster

In other words,

the taxon was not recorded The only exceptions

taxon

group

with all those sites within

the maximum

the set of southern

by the Papio model for individual

an extinct

maximum

of the relevant

a set of closely associated

only from within

recorded.

at sites where

area and N.E. Ethiopia.)

Africa,

group sizes predicted

( l ) and absent (Cl).

the mean

it was known

it did not.

Omo/Turkana southern

lower

2 compares

Gorgopifl Oinopith

when

the species

6). This difference

tie), the

was absent

is unlikely

or

by at than

to be due to

test: P= 0.0 12). suggest

ofbaboons

that climatic to survive

variables

in particular

are likely to have had a significant habitats.

However,

it seems unlikely

impact

on

that all the

extinctions observed in the fossil record can be attributed solely to climatic effects. There appears to have been an increase in the size of baboon species following the 2.5 My climatic “event”

(Figure

3)) whereas

survived

better

in the cooler

that

some major

gradient

selection

that would

the simulations drier

pressure

otherwise

terrestrial

herbivores

increased increasing

body size to reduce body size to exploit

are, however,

equally

became

plausible

would

climates drove

body

have favoured locked

suggest

characteristic weight smaller

into an arms

that smaller of the later

upwards body

race with

species period.

against

size. One

would

have

This suggests

an environmental

possibility

the carnivores;

is that

the

as herbivores

the impact of predation, so the predators responded by the new range of prey sizes available. Other explanations so far as the available

evidence

is concerned.

PAPIONINE

Table 6

415

ECOLOGY

n4suimum~tol~blegroupdzeforindividerl~ species at sped& locations where they occur in one time horizon but not in 8nother Species

Location

Parapapio broomi Pp. broomi Pp. jonesi Pp. jonesi Pp. jonesi Pp. whitei Pp. whitei Papio izodi P. robinsoni P. robinsoni Dinopithecus Dinopithecus

Makapan Sterkfontein Makapan Sterkfontein Swartkrans Makapan Sterkfontein Sterkfontein Sterkfontein Swartkrans Swartkrans Omo (Shungura)

80,

Present

Not present

68 52 79 63 63 43 29 85 63 63 3 5

30 22 30 28 22 11 11 42 63 28 2 6

0

60 -

0 Figure 3. Estimated

I

I

Before 2.5 My

After 2.5 My

body sizes for papionid

species that occur before and after the 2.5 My climatic event.

On the other hand, the later climatic “event” at around 1-O My may have been solely responsible for a further bout of extinctions, especially among those larger species that had survived the 2.5 My “event’‘-thereby giving rise to the evolution of the smaller modern forms. Unfortunately, we cannot determine when the later large species became extinct with any certainty, since no sites younger than 1.0 My exist. However, this explanation is given some support by the simulation. The global temperature fluctuations during the period since 1.5 My have been considerable, with cold periods yielding global temperatures as much as 2.5”C below current values (Prentice & Denton, 1988). With generally rather dry climates in Africa during these periods (i.e., rainfall as low as 300 mm a year), species larger than 20 kg in body weight would have found it difficult to maintain group sizes larger than the minimum, except where temperatures exceeded 15-17°C and/or rainfall exceeded about 500 mm a year

416 Table 7

R.I.M.DUNBAR Equdons

used in the tsinmlationr for the g&da

Variable

Feed (7;) Move

(9;)

Social (“I,,) Rest (96) Herd size Day journey (km) Temperature Grass cover (%) Grass protein (%)

Equation

In(F)

=(0~401n(W)/l~l3) x (5.940.60ln(T)-0.3lln(QJ) In(M) = (0.93/0.33ln(M)) x (4.75+ 0%26ln(J) -0.48111(C)) S=4.53+0.08X R= -12.24+2.461 ln(Nr) = -3.93+0.88ln(M - 1,29ln(C) In(J) = 1.25+ 1.081n(NJ - 1,29ln(C) T=28.36-0.0048/I-0.18L C=55.9+ 1.25T-0.13P Q,= -26.71+23.901n( 7) -4.84(1nT)z

T= mean ambient temperature (“C); W= mean adult body weight (kg); N= mean group (band) size; NC= mean foraging party (herd) size.

(Table 3). In southern Africa, at least, contemporary temperatures are already in this range, and would thus have been considerably lower at certain points during the Pleistocene. An examination of the predicted time budgets for these populations suggests that the problem in this area lies mainly in the high levels of resting forced onto them by high ambient temperatures. This cost is, of course, independent of body size. Feeding, in contrast, seems to be much less of a problem at low ambient temperatures, almost certainly because the costs of thermoregulation at moderate-low temperatures are offset by the increased nutritional quality of the vegetation under a more benign climatic regime. Only in habitats where the mean ambient temperature drops below 5°C does feeding time begin to rise steeply; at this point, plant growth begins to be adversely affected (see Dunbar, 1992, and references therein). However, since larger animals have a larger absolute nutrient requirement, body size does have a significant impact on baseline levels for feeding time. Although 70 kg animals can expect to gain significant savings on their moving time requirement, this is more than offset by the fact that they will have to spend 5O-7Oo/o oftheir day time feeding, even under the most benign environmental conditions, if they are to maintain metabolic processes (see also Lee & Foley, in press). Ecological relationships between baboons and geladas

Table 7 gives the system equations for the gelada, based on the analyses given by Dunbar (1992). A detailed comparison of the equations for the two taxa reveals that they reflect responses to the same ecological variables; the only differences between the two equation sets are a consequence of the two taxa’s respective dietary preferences for fruits/seeds (mainly a product of the bush layers) and grass, and the fact that these two vegetation layers respond in complementary ways to the same environmental parameters (Dunbar, in press c). This contrast influences both time spent feeding and time spent moving, in the latter case through its effect on inter-patch distances. The only other marked difference between the models for the two taxa lies in the fact that rainfall does not enter into the gelada model as an independent variable; temperature seems to be the main driving variable. This may be a consequence ofdifferences in ecological adaptation, or it may reflect the fact that the sample

417

PAPIONINEECOLOGY

300 (0) 250 200 150 1

IJ.niii

t:# 0

~ 600

1200 1800 2400 30(

3600 4200 480(

300 (b)

-

250 :: 'Z 200 2 g

150

E E' 100 ._ :: I 50 0 0

600

1200 I800 2400 3000 3600 42OQ4000

600

1200 1800 2400 3000

(cl 250

t

0

Altitude

36004200

4600

(m)

Figure 4. Mean maximum ecoIogically tolerable group sizes for extant Papio ( n) and gelada (Cl) at different altitudes, at IO” latitude with 800 mm ofrainfall: (a) under the current climatic regime, and when global temperatures were 2°C (b) cooler and (c) warmer than at present.

of gelada populations is restricted in both its size and its range of rainfall values. However, grass protein content measured in a number of East African habitats correlated only with ambient temperature (Dunbar, 1990). Irrespective of the precise reasons for this difference, it is clear that maximum ecologically tolerable group sizes are greatest for baboons in warmer, drier habitats, whereas they are greatest for the gelada in cooler habitats. This results in the two genera occupying different altitudinal zones, with the gelada in the higher (hence cooler) habitats where temperate grasslands predominate. Figure 4 compares the ecological distributions of the two extant genera, in a particular habitat under current global temperature regimes, as well as when global temperatures were

418

R. I. M. DUNBAR

2°C colder and warmer than at present. It can be seen that as temperatures fluctuate, so the altitudinal border between the two genera moves up and down the altitudinal gradient. At no time does the overlap between the two genera increase significantly. This suggests that Tappen’s ( 1960) ori‘g’ma1 suggestion, that the gelada occupy a retreat habitat in the Ethiopian highlands as a result of competition from the more aggressive Papio baboons, is incorrect. Gelada are simply unable to cope ecologically with the vegetation conditions currently prevailing in grasslands below about 1500 m altitude in eastern and southern Africa. They would be confined to the high altitude moorlands of Ethiopia even if the Papio baboons did not exist. With their striking (and longstanding) specialisation for a grazing niche (see Jolly, 1972; Lee-Thorpe et al., 1989), they probably never have experienced significant ecological competition from other large terrestrial primates. Theproblem of Dinopithecus The one striking exception that emerges from these analyses is Dinofiithecus ingens. The model suggests that this giant species (estimated body weight ca. 77 kg) could not have sustained group sizes larger than 3-5 animals at the sites where it has been found. Even for an animal whose massive size would have greatly reduced its susceptibility to predation (see Dunbar, 1988), such small maximum group sizes would imply that the animals would have been foraging alone most of the time. This hardly seems credible, given that even adult male African buffalo are at significantly greater risk from predators when on their own than when in herds (Sinclair, 1977). The only alternative would be to suggest that Dinopithecus was arboreal, and thus could pursue a solitary way of life like the extant orang-utan. Although there is at present no postcranial material that can be uncontroversially assigned to Dinopithecus (Szalay & Delson, 1979) the associated fauna1 evidence would suggest dry open habitats rather than forests at the sites where this species was found. Moreover, as Szalay & Delson (1979) observe, all the Cercopithecidae of moderate-large size are in fact terrestrial. Taken together, these observations would tend to make an arboreal habitus seem unlikely. In contrast, the smaller Gorgopithecus major would have been able to sustain group sizes which, although small (about 14 animals), are probably not ecologically intolerable for an animal of this body size (about 40 kg). The results of the analyses thus prompt us to ask how it might have been possible for Dinopithecus to have sustained larger groups in these habitats. There would seem to be at least three possibilities. One obvious explanation is that the models are wrong in some key respect. This would seem to be the least likely explanation given that the models work so well for extant baboon populations occupying an enormous range ofhabitats. The only dimension on which the simulations extrapolate beyond the observed range is body weight; the body size for Dinopithecus lies so far outside the observed range ofbody weights for extant Papio populations that extrapolation in this case must be subject to some uncertainty. It is always possible that relationships which are linear with respect to metabolic body weight within the observed range become non-linear outside this region. However, since the correction factors for body weight derive from general scaling relationships obtained from typical mouse-to-elephant curves which have a range of body weights far greater than those used in the present analyses (see Peters, 1983), this objection loses much ofits force. A second possibility is that Dinopithecus was locally abundant only in small isolated ecological islands within these habitats (e.g., forest pockets along the banks oflarge rivers and lakes). This would imply that the species was never particularly abundant at any time during

PAPIONINE

Table 8

419

ECOLOGY

Maximum ecologiadly to1elmble group !hc predicted for a 77&g thtropitheciue by the g&da model, under global temperatures 2°C cooler than 8t present

Altitude (m)

200 600 1000

1100 1800 2200 2600 3000 3400

Latitude

(“latitude)

0

5

10

15

20

25

30

0 0 0 0

0 0 0 0

0 0 0 0

0 0

0 0 0

0 0 0

0

0 0 0 0

0

0 10 38 38 0

0 24 44 19 0

8 37 40 0 0

21 44 23 0 0

6 35 41 2 0 0

19 43 27 0 0 0

1 26 44 15

0

its history. That it seems to have had a relatively short lifespan as a species is consistent with this suggestion. However, at least one of the sites at which the species was found (Swartkrans) was probably not associated with a large body of water, suggesting that this explanation is also unlikely. The final, and perhaps most likely, possibility is that it was not an ecological baboon. Since baboons are generalised frugivores concentrating on the bush/shrub layers (see Dunbar, in press a), this more or less rules out all diets except one in the ground-layer vegetation. The obvious alternatives would be a grazing niche similar to that of the gelada or one adapted to exploiting the underground storage organs of certain species that are (at least now) characteristic of open Savannah habitats. Since Dinopithecus had an anterior dentition that resembled that of Pupio rather than Theropithecus (i.e., relatively large incisors), a gelada-like grazing niche seems unlikely. This leaves a diet focused on the larger subterranean storage organs common to certain dicot families, such as the Liliacae and Moracae, as the most likely possibility. Significantly, considerable physical strength is required to remove these items from the ground, and it is possible that using the teeth in the manner ofPupio gives more leverage than using the hands in the way the gelada harvest grass roots. More importantly, perhaps, in the present context, is the likelihood that the densities of these plant species have a different relationship to the primary climatic variables than the bush-layer plants on which Papio depend; this would have a dramatic effect on the group sizes predicted for Dinopithecus. It is plausible to suggest that those climatic factors that determine the grass biomass are also likely to affect the biomass ofother herb layer species, which rely on tubers or bulbs to sustain them through the dry season. Ifso, then the gelada model would be more appropriate. Table 8 gives the maximum ecologically tolerable group sizes for a large (77 kg) animal that are predicted by the gelada model. (Note that rainfall does not influence gelada time budgets; temperature is here decomposed into its two main determinants, altitude and latitude.) This suggests that, under a temperature regime 2°C cooler than the present, group sizes of up to 30 animals would have been sustainable at the southern African sites where Dinopithecus occurred (Swartkrans I: altitude 1700 m, latitude 26”S), with group sizes of about five in those east African habitats where it has been found (Omo, Shungura D-G:

R. I.M. DUNBAR

420

altitude ca. 2000 m, latitude 5”N). (Group sizes would probably have been larger in the latter habitat if the proximity of the lake resulted in denser ground cover; see Dunbar, in press b.) So large an animal would, however, have been rapidly driven to extinction as global temperatures approached their current values once more. Analyses given in Dunbar (in press b) indicate that under current temperature regimes, a gelada as large as this would only have been able to survive at altitudes between 2100-2900 m in southern Africa, and altitudes of 2600-3600 m in eastern Africa. If temperatures rose significantly above current levels (as they may have done at around 1.0,0.7 and again at 0.3 My; Prentice & Denton, 1988), an animal of this size would have been quite unable to meet its time and energy budgets, and would have rapidly become extinct. Conclusions I have tried to show how detailed modelling ofthe behavioural ecology ofindividual taxa can throw considerable light on the evolutionary history of its extinct congeners. Although the models themselves seem to work extremely well for living members of the genera to which they apply, the valid extrapolation of these models to extinct populations depends on three key assumptions. First, we need to be able to determine fairly precise body weights for extinct animals; second, we need to be able to determine at least the rainfall and temperature variables for palaeo-habitats; and finally, we need to be able to assume that the extinct taxa were ecologically similar to the extant taxa on which the model is based. The latter would seem to be the most problematic aspect of the exercise. However, the models can be put to good use even in those cases where this last assumption is tenuous. Knowing that a given taxon actually lived in a particular habitat, we can ask whether it could in fact have done so if its ecological niche was similar to the living taxon’s. If the living taxon could not have survived under the conditions prevailing at that site at that time, then it follows that the extinct taxon must have differed in its ecological adaptations. I have, of course, assiduously avoided referring to the continuing debates among palaeontologists as to the validity of inferences concerning palaeo-climatic variables, let alone those concerning the reality of the various climatic “events” that have been identified. I am not in a position to comment on these questions. Instead, I have simply taken the estimates given in the literature at face value. As with all modelling exercises, my concern is simply to evaluate the implications of the causal relationships and leave it to the palaeontologists themseIves to make more precise inferences in specific cases as and when the relevant data become available. The primary value of models is always that they force us to evaluate our assumptions about complex biological processes, thereby drawing our attention to features of the real world that deserve more detailed study. Methods for obtaining more precise estimates of climate at specific habitats wouId seem to me to be one such priority. Ifwe can determine these, the models would allow us to infer a great deal about the actual behaviour of the animals. Acknowledgements I thank Leslie Aiello, Peter Andrews,

Phyllis Lee and Caryl van Schaik for their comments. References

Andrews, P. & Aiello, L. (1984). An evolutionary model for feeding and positional behaviour. In (D. Chivers, B. Wood & A. Bilsborough, Eds) FoodAcquisition and Processing in Primates, pp. 429-466. New York: Plenum Press.

PAPIONINE

421

ECOLOGY

Bonnefille, R. (1983). Evidence for a cooler and drier climate in the Ethiopian

uplands towards 2.5 Myr ago.

Jv’ature

Lond. 303,487-491.

Carr, C. J. (1976). Plant ecological variation and pattern in the lower Omo basin, In (Y. Coppens, F. C. Howell, G. L. L. Isaacs & R. Leakey, Eds) Earliest Man and the Environments in the Lake Rudolf Basin, pp. 432-467. Chicago: University of Chicago Press. Demment, M. & van Soest, P. J. (1985). A nutritional explanation for body-size in ruminant and non-ruminant herbivores. Am. Nat. 125,641-672. Deshmuhk, I. K. (1984). A common relationship between precipitation and grassland peak biomass for east and southern Africa. Afr. J. Ecol. 22, 181-186. Dunbar, R. I. M. (1988). Prima& Social Systems. London: Chapman & Hall. Dunbar, R. I. M. (1990). Environmental determinants of fecundity in klipspringer (Oreotragus oreotragus). A&. 3. Ecol. 2lJ,307-3 13. Dunbar, R. I. M. (1991). The functional significance ofsocial grooming in primates. Foliaprimat. 57, 121-131. Dunbar, R. I. M. ( 1992). A model of the gelada socioecological system. Primates. 33,6!+84. Dunbar, R. I. M. (in press a). Time: a hidden constraint on the behavioural ecology ofbaboons. Behau. Ecol. Sociobiol. Dunbar, R. I. M. (in press b). Socioecology of the extinct theropiths: a modelling approach. In (N. Jablonski & R. Foley, Eds) Theropithecur: The Evolutionary History oja Primate Genus. Cambridge: Cambridge University Press. Dunbar, R. I. M. (in press c). Ecological constraints on group size in baboons. Physiol. Ecol. Fleagle, J. (1988). Primate Adaptation and Evolution. New York: Academic Press. Le Houeron, H. N. & Hoste, C. H. (1977). Rangeland production and annual rainfall relations in the Mediterranean basin and in the African Sahelo-Sudanian zone. 3.Range. Mgmt. 30, 181-189. Jolly, C. J. (1972). The classification and natural history of Theropithecus (Simopithecus) (Andrews, 1916), baboons of the African Plio-Pleistocene. Bull. Brit. Mus. nat. Hirt. (Geol.) 22, l-l 23. Kay, R. F. & Covert, B. (1984). Anatomy and behaviour of extinct primates. In (D. Chivers, B. Wood & A. Bibborough, Eds) Food Acquisition and Processing in Primates, pp. 467-508. New York: Plenum Press. Kay, R. F. & Simons, E. L. (1980). The ecology ofOligocene African Anthropoidea. Inf. 3.Primatol. 1,21-37. Lee, P. C. & Foley, R. (in press). Ecological energetics and extinction ofgiant gelada baboons. In (N. Jablonski & R. Foley, Eds) ‘Theropithecus: A Case Study in Evolutionary Biology. Cambridge: Cambridge University Press. Lee-Thorp, J. A., van der Merwe, N. J. & Brain, C. K. (1989). Isotopic evidence for dietary differences between two extinct baboon species from Swartkrans. 3.hum. Evol. 18, 183-190. McNaughton, S. J. (1985). Ecology ofa grazing ecosystem: the Serengeti. Ecol. Monogr. 55,259-294. Peters, R. H. (1983). The Ecological Implications of Bo4 Size. Cambridge: Cambridge University Press. Prentice, M. M. & Denton, G. H. (1988). The deep-sea oxygen isotope record, the global ice sheet system and hominid evolution. In (F. E. Grine, Ed.) Evolutionary Histoty of the ‘Robust’ Australopithecines, pp. 3833403. New York: Aldine. Rutherford, M. C. [ 1980). Annual plant production-precipitation relations in arid and semi-arid regions. S. Afr. 3. Sci. 76,53-56. Schmidt-Nielsen, K. (1984). Scaling; U’& is Animal Size so Important? Cambridge: Cambridge University Press. Shipman. P. & Harris, J. M. (1988). Habitat preference and paleoecology OfAustralopithecus boisei in Eastern Africa. In (,F. E. Grine, Ed.) Evolutionary History of the ‘Robust’ Australopithecines, pp. 343-381. New York: Aldine. Sinclair. A. R. E. (1977). The.4frican Buffalo. Chicago: University ofchicago Press. Smuts, B. B., Cheney, D., Seyfarth, R., Wrangham, R. W. & Struhsaker. T. T. (Eds) (1987). Primate Societies. Chicago: University ofchicago Press. Szalay, F. E. & Delson, E. (1979). The Evolutionary History of the Primates. New York: Academic Press, Tappen. N. C. i 1960). Problems ofdistribution and adaptation ofthe African monkeys. Curr. Anthrop. 1,91-120. Tooby, J. & DeVore, I. (1987). The reconstruction ofhominid behavioural evolution through strategic modelling. In iI\‘. G. Kinzey, Ed.) The Evolution ofHuman Behauiour: Primate Models, pp. 1833238. Albany: State University of New York Press. V&a, E. S. (1988). Late Pliocene climatic events and hominid evolution. In (F. E. Grine, Ed.) Evolutionary History of the ‘Robust’ :iustralopithecines, pp. 405-426. New York: Aldine. Wrangham, R. W. (1987). The significance of African apes for reconstructing human social evolution. In i W’ G. Kinzey, Ed. i The Evolution of Human Behaviour: Primate Models, pp. 51-71. Albany: State University of NCM York Press.