Left nipple suckling preference in wild chimpanzees

Left Nipple Suckling Preference in Wild Chimpanzees Toshisada Laborutory of Science, Japan.

Nishida of Humun Evolution Studies. Department Kyoto University, Kitushiruku~~w-Oiwukecho,

of Zoology, Faculty Sukyo, Kyoto,

Thirty-four chimpanzee mother-infant pairs were observed in the Mahale Mountains Natiotfal Park, Tanzania. Nipple preferences were determined in terms of the nipple sucked first and by measuring the duration of suckling. Thirteen of fifteen male, but only six of eighteen female infants began to suck from the left nipple more often than the right. All of those who showed a statistically significant laterality (six males, two females) were biased to the left nipple. In terms of the duration, nine of thirteen male, and ten of eighteen female infants showed the left nipple preference. All of those who had a significant laterality (five males, two females) showed a left nipple preference. It is likely that the left nipple preference was facilitated by the mothers’ tendency to support their neonates with their left arm. KEY WORDS:

Chimpanzee;

Nipple preference;

Suckling,

Laterality

INTRODUCTION

P

observations of suckling behavior by wild chimpanzees have not mentioned (Clarke 1977; Nicolson 1977; Hiraiwa-Hasegawa 1990) or supported (Goodall 1968) laterality in nipple contact. This study reports the first evidence of a left nipple preference in a population of wild chimpanzees. Since mothers of humans and great apes have been reported to cradle their infants on their left side, this finding may provide a new development to the “maternal heartbeat” hypothesis (Salk 1960) for the left side cradling preference in humans. revious

METHODS Study Groups and Periods The chimpanzees of the Mahale Mountains National Park, been studied since 1965 (Nishida 1990). The study groups, sisted of 14-29 and 90-100 chimpanzees, respectively. Received

February

18, 1992; revised

Tanzania, have K and M, con-

July 28, 1992.

Address reprint requests to: Toshisada Nishida. Laboratory partment of Zoology, Faculty of Science, Kyoto University, Kyoto, Japan 606. Ethology and Sociobiology 14: 45-52 (1993) 0 Elsevier Science Publishing Co., Inc., 1993 655 Avenue of the Americas, New York, NY 10010

of Human Evolution Studies, DeKitashirakawa-Oiwakecho, Sakyo.

0162-3095/93/$5.00

46

T. Nishida

Observations

wcrc

spring. Behavior (Martin

made

on 34 pairs of habituated

was continuously

and Batcson

mothers

1986). A focal mother was followed

as long as possible

until she was lost or made her sleeping bed in the evening. an ad libitum

basis were incorporated

Study periods consisted (Group

into the analysis

Data collected

on

when relevant.

of one year in 197S- 1976, five months in 1979

K). five months in 1981. four months

1985-19X6,

and off-

recorded on a focal animal sampling basis

four months in 1987-l%%,

in 19X3--1984,

four months in

and three months in 1989 (Group

M).

Most focal females of Group M were observed for 30 hours during each study period.

Total

focal observation

hours were

1300.

A suckling bout was a period with nipple contact during which no interruption

to nipple contact

renewed

lasted for more than one minute.

after such an interruption

bout (XC Nicolson for “reassurance

preference

a suckling

bout since brief contacts

was determined

nipple

pletely (N, in Table

contacts,

appeared

to be

plete data were also incorporated (N2 in Table

One minute

by measuring the duration

breaks.

and changes

of suckling

were observed

com-

I ). The number of such bouts was 736. The nipple sucked

first was also used to determine

statistical

contact

nipple contact.”

bouts in which

was witnessed

Nipple

to be the start of a new

1977). Only nipple contacts lasting more than ten continu-

ous seconds constituted Nipple

was considered

criterion

independcncc

nipple preference.

For this analysis

so long as the initiation

incom-

of the suckling bout

I).

of the bout interval of successive

was enough to guarantee

observations

the

of the same mother-in-

fant pair since ifan infant did not suck for one minute after a bout of continuous sucking, so during playing,

it usually did not return to its mother’s

which

it would

activities

breast for one hour or such as climbing

and

having a fair chance of going to either nipple on the next occasion.

To investigate

the effect of maternal

support patterns offive were recorded. neonate

engage in other

cradling on nipple prefercncc,

Each bout constituted

a stretch of continued

support of the

with one hand. At the start of each new bout of cradling.

was considered her infant’s

hand

mothers carrying neonates and walking on the ground a mother

to be free to support her infant with either hand, because of

central

position

directly

under her belly.

RESULTS Nipple Sucked First Fifteen

male infants were observed

at least once when they began sucking

from their mothers (Table

la). Among

ing only one study period,

six began to suck more often from the left nipple

(LB did so significantly),

seven males who were observed

dur-

and one (PM) did so more often from the right one.

Among four infants who were observed

through two periods,

two (AL,

BG)

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c=-----

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C!

=

-_=Cn=-r

=5=====

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48

T. Nishida

began to suck consistently more often from the left nipple. One individual (KB) began to suck from the right nipple in the earlier period, but did so significantly more frequently from the left in the later period. Finally, one male (MA) changed from sucking from the right to showing no laterality. Three (BB, CP, NC) of the four male infants who were observed through three study periods consistently preferred the left nipple: the remaining male (LT) sucked from the right nipple during the first period. but from the left nipple during later periods. Six male infants showed statistically significant nipple preference in terms of the nipple sucked first during at least one study period (binomial test, p < 0.05); all of these males showed a preference for the left nipple. The above tendency, however, was not apparent among female infants (Table lb). Nineteen female infants were observed to suck at least once from their mothers. Of nine females who were seen during only one period, three preferred the left nipple, five the right nipple, and one sucked evenly from both sides. The only female (MI), however, that showed significant laterality preferred the left nipple. Among nine female infants who were observed through two study periods, one (TL) consistently preferred the left nipple. one (AS) the right nipple. and seven others were inconsistent. None of them showed significant nipple preference. however. One female (AB) was observed during three study periods; she showed a right nipple preference during the earliest period, but showed a significant tendency to suck first on the left side during the later periods.

Nipple Sucked in Longer Duration Thirteen male infants were seen at least once to suck from the beginning to the end (Table la). Four of the five males that were observed in only one study period sucked longer from the left than the right nipple. Four males were observed through two study periods; one male (BG) consistently sucked longer from the left nipple. Another (KB) changed from sucking on the right to the left. Both of these males had significantly more left-biased bouts during later study periods (binomial test, p < 0.05). One male (AL) changed from sucking from the left side to the right. An additional male (MA) changed from sucking from the left to evenly from both sides. Three (CP, NC, LT) of the four males who were observed through three study periods had significantly more left nipple biased bouts during at least one study period. Another male (BB) showed a right nipple preference during an early period, but a left nipple preference during later periods. Nineteen female infants were seen at least once to suck from the beginning to the end (Table lb). Of the nine females that were observed in only one study period, four had more left nipple, and five more right nipple, biased bouts. However, the only female (MI) that displayed a significant difference in preference. showed a left nipple preference. Nine females were observed

Left Nipple Preference in Chimpanzees

through

two study

periods:

three

(SE,

TZ,

TL)

showed

49

a consistent

left

nipple bias, but all the other females were inconsistent in nipple selection, Only one female (BL) showed a significant nipple preference, which was again directed to the left side (p < 0.05). One female changed her sucking pattern from left to right. Only one female (AB) was observed through three study periods. Although she showed a nonsignificant right nipple preference during an early period, she showed a significant left nipple preference during later periods.

Concordance

of Preference

Infants tended to suck longer on the nipple from which they sucked first: On 378 or 89.2% of 424 bouts in which infants sucked first from the left nipple, they sucked longer from the same left nipple, and on 238 or 76.3% of 312 bouts in which they sucked first from the right nipple, they sucked longer from the right side. All of the I3 male infants (binomial test, p < 0.001) and I8 of the I9 female infants (binomial test, p < 0.0001) showed a concordance in nipple preferences (compare fn with pn2 in Tables la and lb).

Ontogeny of Nipple Preference Of the eight male infants who were observed during two or three periods, five consistently showed left nipple preferences and three changed towards a left nipple tendency as years passed in terms of the nipple sucked first. In terms of duration, two showed consistent left nipple preferences, five showed left nipple tendencies, and only one showed the opposite trend (see above). Of the ten female infants who were observed through two or three periods, two consistently sucked first from the left side, five showed left nipple preferences as years passed, and three showed the opposite tendency. In terms of duration, three showed consistent left nipple preferences, four showed left nipple preferences as years passed, and three the opposite trends (see above). Table 2 shows the relationships between ages and preference for one

Table 2.

Relations

Between Ages and Nipple Preferences Male Infants

Age in Months

Female

NFS

NLS

No.

L

R

=

L

O-12 13-24 25-36 37-48 49-60

4 8 3 6 5

4 5 2 5 5

0 3 1 0 0

0 0 0 0

3 3 2 4 3

61-

I

I

0

Total

27

22

4

0 I

I 16

NFS: nipple first wcked. nance. = : no difference.

NLS:

I

NFS

R

=

No.

I I

0 I 0

8 II 5

0 2

2 0

4

3 2

0 8

0

I I

3

30

4

nipple longer sucked.

Infants

L

NLS

R

=

4 6 2

2 0 0

0 I

I I 0

I 0 0

13

14

3

2 5

L: left nipple predominance,

L

R

=

3 7 3 3

4 4

1 0

I

I

I

0

I I

0 0

0 0

18

IO

2

R: right mpple pyedomi-

50

1’. Nishida

nipple.

As they grew older. infants showed a tendency

nipple as measured

by duration

Small infants were not always specialized theless,

as they grow older,

to suck from the left

and initial selection. as left nipple suckers.

not only do they become

Never-

more likely

to suck

longer from the left nipple rather than the right. but they also begin sucking from the left nipple. than their female

Male infants show stronger tendencies

Hand Preference

in Support of Neonates by Mother

Five mothers were observed walking

on the ground.

to support their neonates with their hands when

Three (WO,

WA,

CH) supported

often with their left hand and two (WD, hands. Except with

in these patterns

counterparts.

for CH,

WL)

their charges more

more often with their right

who had a male infant and cradled significantly

her left hand (43 bouts with

left hand and

numbers of bouts were too small to conclude in the other four mother-infant

I5 with

anything

more

right hand).

about individual

the bias

pairs.

DISCUSSION This study shows that infant chimpanzees measured

Who is responsible ers or infants?

for the formation

It is unlikely

neonates

mothers when sucking.

Moreover,

of their neonates

tended to support their neonates Manning

and Chamberlain

observation).

however. preference

with their left hand rather than the right.

(1990) reported

by their mothers

that nine of ten captive

are likely to be related

chimpan-

cradling

pat-

to the left nipple

in chimpanzees. 1964: Rheingold

and Keene.

1965: Manning

there has been no data whether

to the maternal are less likely

preference

heartbeat

(Salk

to feel anxiety

1960. 197.1:

1991). Unfortunately,

human infants show the left nipple

or not. This will be an important

Left side cradling

area for further

study.

in humans has been discussed 1973). It is likely

when

cradling

because neonates who have been imprinted in their uterus feel more reassured left side.

A

from this study suggests that mothers

Most human females hold their infants on the left side (Salk Weiland

by

change the position

hold their infants with their left arms. Therefore.

terns of neonates preference

and need to be supported

mothers occasionally

cradling

moth-

to select nipples at

from right to left or vice versa (personal

data set on maternal

zec mothers

of the left nipple prcfercncc.

are helpless,

as

of sucking.

that infants have the ability

birth since chimpanzee

limited

display a left nipple preference

by the nipple sucked first and the duration

that chimpanzee

their

infants

in relation mothers

on the left side

to the heartbeat

of their mothers

when they are cradled

on the mothers’

51

Left Nipple Preference in Chimpanzees

1 gratefully

acknowledge

and Technology.

the cooperation

K. N. Hi+

sawe. and their staff at the Mahale and L. A. Turner and revision

who cooperated

of the English.

comments.

Thi\ Program

hlO43017.

62041021.

(No\.

Wildlife

Mountain\

Wildlife

in the collection

404420.

63043017.

by grants

504319.

of Tanrania Commission

Research

Institute,

Research

for Science

and E. Tarimo,

I

Centre.

E. Mas-

thank T. Tsukahara

of the data. J. C. Mitani for useful comments

and W. C. McGrew

study was financed

Kehearch

of A. S. Msangi

of Serengeti

and M. J. A. Simpson under the Monbusho

56041019.

57043014.

for their constructive International

58041025.

Scientific

59043022.

600410?0.

01041057).

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