Zoo Biology 6:315-330 (1987)
Social Behavior of a Group of Orangutans
(Pongo pygmaeus) on an Artificial Island in
Singapore Zoological Gardens
Trevor B. Poole
Department of Zoology, National University of Singapore
The behavior of 12 orangutans (three adult males, two adult females, two subadult
males, three adolescent males, and two infant males) was observed on a 450-m2
island at the Singapore Zoological Gardens (SZG). Male orangutans (6-18 years
old) showed less social and solitary play as they aged; adults (over 16 years old)
were not seen to play. As they grew older males increasingly spent less time
making physical contact, but the amount of time they spent in proximity (within
arm’s length) to others increased. Adult females regularly played with other group
members. Contact, allogrooming, and social play showed nonrandom relationships between individuals. Adult females showed the most allogrooming and
contact, adolescent and subadult males the most play. There was no obvious
dominance hierarchy. One adult male spent about 10%of his time walking around
the perimeter of the island. One-year-old infants rarely interacted with other
individuals apart from their own and the other infant’s mother. While orangutans
lead relatively solitary lives in nature, it was concluded that the opportunities for
social contact and play provided by the SZG orangutan island were beneficial to
this species in captivity. Opportunities for social interaction provided the animals
with a means of increasing the stimulus component of their environment, thus
compensating for the inevitable restriction of complexity and unpredictability as
compared with the wild state.
Key words: captivity, environmental complexity, sociality, play, animal welfare
INTRODUCTION
Under natural conditions adult orangutans lead rather solitary lives, contacting
conspecifics only occasionally [Horr, 1975; MacKinnon, 1974; Rijksen, 1975; Rodman, 1979; Galdikas, 1979, 1981a,b]. Rijksen [1978] observed that 46% of Sumatran
orangutan units consisted of a single individual, and, of the 54% of units consisting
of more than one individual, only 17% could be regarded as true social groupings. In
his study area in Borneo, Rodman [1973] found that only 4.3% of all observed
Received for publication January 6, 1986; accepted January 4,1987.
Address reprint requests to Trevor B. Poole at Universities Federation for Animal Welfare, South
Mimms, Potters Bar, Hertshire EN6 3QD, England.
0 1987 Alan R. Liss, Inc.
Poole
orangutan units were real social groupings. In both areas adult orangutans were less
sociable than younger individuals, and both authors treated mother-infant pairs as a
single unit.
The aim of this study was to examine the behavior of orangutans under conditions of high potential social contact, a situation that is atypical for this species but
normal for other members of the family Pongidae. The Singapore Zoological Gardens
(SZG) provided a unique opportunity to observe this rather solitary ape in a complex
social environment.
Keeping orangutans under these conditions raises two issues: first, the extent to
which a relatively solitary ape may have retained the capacity for the elaborate social
communication shown by its closest relatives in the Pongidae and, second, whether
such opportunities for social contact can provide valuable stimulation for a species
that is known to be highly intelligent [Maple, 1980; Rumbaugh and Gill, 19731.
Because of their solitary nature in the wild it has been common in the past for zoos to
keep orangutans in small groups such as mother-infant dyads or in isolation [Duplaix
and Grady, 1980; Wilson, 19821. This research aimed to determine whether orangutans in captivity will seek out or avoid conspecifics if there are free opportunities for
socializing. If orangutans are, by inclination, unsociable, they would be expected to
distance themselves actively from conspecifics. Wrangham [19791, however, has
suggested that the solitary state of the orangutan in the wild might be attributable to
its adoption of feeding strategies, which are incompatible with group living, and the
field data of Rijksen [1978] could be interpreted in this way. If Wrangham is right,
the animal’s unsociability is a matter of necessity rather than inclination.
MATERIALS AND METHODS
Sources of Orangutans
Wild-born individuals in the colony had been captured as infants in Borneo or
Sumatra and either donated to Singapore Zoological Gardens or sent there after
confiscation by the authorities. Details of the origins of Singapore Zoo’s orangutans
TABLE 1. Orangutans in the Singapore Zoological Gardens collection from whom
data were derived
Name
Sex
Origin
Subspecies
Age
Jojo
Bobo
Zabu
Friday
Letchu
Jinak
Hsing-Hsing
Inoki
M
M
M
M
M
M
M
M
wild
wild
wild
wild
wild
SZG
SZG
SZG
c17
c17
c16
c13
Cll
7
7
6
Charlie
Kamal
Girlie
Leala
M
M
F
F
SZG
SZG
wild
wild
1
1
c13
c 19
c, estimated minimum age; SZG, born at Singapore Zoological Gardens.
Comments
Castrated aged 5
Son of Ah Meng
Fostered by Ah
Meng
Son of Girlie
Son of Leala
Lactating
Lactating
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
316
317
are provided in Table 1, and it can be seen that seven of them were wild-born and
five of them were colony-born. The climate in Singapore is similar to that of their
natural habitat.
Of the 12 individuals in the colony who were studied regularly, 11 were of the
Bornean subspecies (Pongo pygmaeus pygmaeus) while one (Hsing-Hsing) was of the
Sumatran form (P.pygmaeus abelii). The male, Inoki, whose mother died during his
infancy, had been fostered to Ah Meng, the mother of Hsing-Hsing, so that these two
adolescent males had been reared together. The two subspecies can be distinguished
on morphological grounds, but there is no conclusive evidence of any consistent
behavioral differences between them [Rijksen, 19781.
Age/Sex Classes of Orangutans
The division of the animals into agehex classes (see Table 1) follows Rijksen
[ 19781. The three females in the colony were breeding adults, and males were divided
into four groups, namely, infants, adolescents, subadults, and adults. The three adult
males all had fully developed cheek flanges and throat pouches.
Orangutan Island
The island onto which the orangutans were released was roughly semicircular,
with the flat side adjacent to the animals’ night quarters. The island was approximately
450-m2 in area and was covered with grass and sedge; there were two large trees
with metal surrounds to the bases of their trunks to prevent the orangutans from
climbing and destroying them. In the center of the area was a large two-storey
climbing frame, which was roofed to provide shade and shelter. The whole island
was surrounded by a moat into which some of the orangutans waded, often when
trying to retrieve food items thrown by the public (feeding by visitors was officially
forbidden). An illustration of the island is shown in Figure 1.
Observational Methods
A group of orangutans, whose composition depended on the decisions of the
keepers, was released onto the island between 0900 and 0930 hours and returned to
their night quarters around 1730 hours. Group sizes ranged from six to ten individuals, excluding infants. Every animal could be recognized individually. Prior to
releasing the orangutans in the morning, keepers scattered shelled peanuts over the
island. They were sometimes provided with objects that could be manipulated, such
as cardboard boxes or large pieces of vegetation.
Before the start of data collection, the daily activity pattern of the animals on
the island was studied. It was found that immediately after their release onto the
island in the morning, social activity tended to follow a period of foraging and
exploration that lasted approximately 15 minutes. Around midday they became inactive and rested during the heat of the day. In the late afternoon they once again
became active before their return to their night quarters. When an individual was not
released onto the island it remained in its night cage, but some individuals (HsingHsing, Inoki, Leala, and Ah Meng) were taken out into the zoo among the public for
photographic sessions during the afternoons.
Observations were conducted during the orangutans’ period of maximum activity, between 0930 and 1200 hours. There were two periods of observation, usually
for 20 minutes with a quarter of an hour gap in between (mode, 20; mean, 20.56;
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Social Behavior of Orangutans
Poole
I II II %1
7
r
Fig. 1. Artificial island for orangutans, Singapore Zoological Gardens (diagramatic representations of
plan and a section not drawn to scale).
range, 20-32). The sampling method used was to survey the whole island at 2-minute
intervals and record what each individual was doing at that point in time. The study
was carried out between March 5 and April 15, 1982, and the duration of recordings
from different individuals ranged from 11 hours 26 minutes (402 time points) to 5
hours (165 time points).
Behaviors were divided into broad categories, which were mutually exclusive.
The behavioral units used are listed and defined in Table 2. Social behaviors were of
two types: those in which there was an actor and a recipient (eg, allogrooming and
copulation) and those in which two actors were regarded as having identical roles (eg,
contact and social play); these differences were taken into account in analysis of the
data. Statistical procedures followed those described by Siege1 [19561.
The different behavioral categories that were recorded are listed in Table 2, but
behaviors 7, 8, 9, and 11 did not occur with sufficient frequency to allow for detailed
analysis; they will be considered separately. Maternal behavior was not scored as
such, but is reflected in the behavioral scores of the two infants. As the object of the
study was to investigate the sociability of orangutans (behavioral categories 1-6 in
Table 2), the data for the two females with infants referred solely to their interactions
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
318
319
with members of the group other than their own infant; thus, the mother-infant dyad
was treated as a single social unit.
RESULTS
The scores for each individual for seven types of behavior are presented in
Table 3 and grouped by agehex class (behavior category 6 was divided into grooms
and groomed). The data for each behavior for each individual are expressed as a
percentage of the total number of time points recorded for that individual. The
categories of behavior were treated as being mutually exclusive and covered all
possibilities; hence the figures represent the percentage of the individual’s total
behavioral repertoire during the whole period of observation.
Male Behavior
For the eight males who were regularly on the island, Spearman rank correlation
coefficients were calculated for six behaviors and the animal’s age (see Table 4).The
results reveal the expected negative correlation between play and age, which is
common to most mammals. The results also show that older males spend more time
alone. This also is reflected in a negative correlation between social play and time
spent alone. The positive correlation between grooming and being groomed presumably indicates the existence of mutual grooming relationships. There is a strong
correlation between the occurrence of solitary and social play, which has been
observed in other species (eg, rats) [Poole and Fish, 19751. The data for “proximity”
are puzzling; it appears that older males spend more time alone and have fewer social
interactions than younger males, but spend more time in proximity to other individuals. Zabu regularly patrolled the perimeter of the island, spending 10.5% of the
observation periods in this activity.
TABLE 2. Behavioral categories and their definitions
1. Alone
2. Solitary play
3. Proximity
4.Contact
5. Allogroom
6. Social play
7. Patrolling
8. Sexual behavior
9. Displaces
10. Maternal
behavior
11. Aggression
Separated from all other individuals by at least one arm’s length
Making vigorous movements such as bouncing, twisting, rolling
over and over, running rapidly around but not making contact
with other individuals (excluding the manipulation of objects)
Two or more individuals are stationary and within one arm’s length
but not in physical contact
A stationary individual who is touching another individual with any
part of the body (excludes allogrooming and social play, which
are scored separately, but includes sexual behavior)
One individual grooms another (the groomer); one is groomed (the
groomee)
Rolling together, noninjurious wrestling, inhibited bites, chase,
repeated touching or pulling an opponent’s fur, slapping. Usually
accompanied by open-mouthed play face
Walking systematically around the perimeter of the island, stopping
at intervals to stare across the moat
Pelvic thrusting by a male against another individual
As one individual walks toward another, the latter gets up and
moves away
Carrying the infant and nursing, retrieving, grooming, and
restraining it
Injurious biting, wrestling, and chasing
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Social Behavior of Orangutans
12
5
7
11
10
13
16
16
12
2
3
3
6
10
6
8
4
4
72
80
57
46
58
11
19
67
66
20
17
11
Proximity
0
0
0
Solitary
Play
78
79
78
Alone
5
9
62
45
7
5
2
1
+
1
1
5
Contact
2
3
0
0
4
0
1
2
0
1
3
+
Given
248
248
402
165
165
402
402
402
314
11
11
20
32
18
8
12
2
4
1
+
+
2
4
2
0
0
+
402
402
402
N
0
0
0
Social
Play
1
2
3
Allogroom
Received
+, <0.5%; N, total number of 2-minute sampling points. Data expressed as percentage of time points sampled. All behavioral categories are mutually
exclusive.
*Excludes behavior toward her infant.
Adult males
Jojo
Bob0
Zabu
Subadult males
Friday
Letchu
Adolescent males
Jinak
Hsing-Hsing
Inoki
Infant males
Charlie
Kamal
Adult females
Girlie*
Leala*
Name
TABLE 3. The frequency of occurrence of the major types of behavior recorded from 12 orangutans on a small moated island
in Singapore Zoological Gardens
o_
(D
'CI
0
0
h)
W
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
321
Resting alone might result from either active avoidance of others or from being
avoided by others, and the data cannot distinguish between these two possibilities.
Direct observation suggested that the adult males, particularly, had a tendency to
move out of the group and sit alone. Often, when they were in proximity to others, it
was on the climbing frame or when Zabu was patrolling. Other individuals did not
actively avoid the adult males but usually ignored them.
Social Behavior of Individuals
An analysis was done to determine the proportion of time (expressed as 2minute point samples) that each orangutan spent with every other individual. This
excluded infants, who spent the majority of their time in direct contact with or in
proximity to their mothers.
To decide whether the amount of time spent with another individual differed
from chance, a null hypothesis was tested that assumed that each individual was likely
to spend equal amounts of its time carrying out any particular social behavior pattern
with every other individual. The null hypothesis also took into account the proportion
of time of the individual’s repertoire, which the particular behavior occupied, and the
number of other animals present.The formulae used to estimate the normal deviate
(z) are shown in Appendix I. The data will be discussed independently for each of the
three social behaviors considered.
Time Spent in Contact
The overall pattern of contact between group members is illustrated in Figure 2
and is based on the values of the normal deviate derived from the binomial tests.
Arrows represent significant departures from random expectancies. The absence of
an arrow does not imply that the two animals did not interact, but only that a
significant departure from the value expected by chance was not observed. The data
show that the ranked order for the individual most sought for contact was as follows:
TABLE 4. Spearman rank correlation coefficientsfor age and seven behavioral parameters for
male orangutans (excluding infants)
Alone
Age
Alone
Solitary
0.68*
Solitary
Play
-0.98**
NS
Play
Proximity
contact
Grooms
Groomed
NS, not significant (one tailed test).
*P < 0.05 (rs = 0.64).
**P < 0.01 (rs = 0.85).
Groomed
Social
Play
NS
NS
NS
NS
NS
NS
-0.91**
-0.73*
0.93**
NS
NS
NS
NS
0.66*
-0.73*
NS
NS
NS
Proximity
Contact
Grooms
0.78*
NS
-0.76*
NS
-0.81*
NS
NS
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Social Behavior of Orangutans
Poole
/ ZASU
INOKI
p < 0.95
P < 0.61
F < 0.981
P< a . 6 m
- - - - - - ->
-*
)-
r-)
Fig. 2. Sociogram showing the proportions of time greater than expected by chance that different
members of the group of orangutans on the island spent in contact. Statistically significant results only
are illustrated.
1) Leala, 2) Jinak, 3) Girlie, 4)Hsing-Hsing, and 5) Zabu. In terms of the amount of
contact with other individuals (excluding its infant) in the individual’s repertoire the
order was as follows: 1) Leala, 2) Jinak, 3) Hsing-Hsing, 4) Girlie, and 5) Zabu.
Thus the two sets of figures were broadly consistent. The fact that the values on the
sociogram for A to B differed from those from B to A can be explained by the
different expectancies that related to the different proportions of the behavior in the
repertoire of the two animals in question.
Allogrooming
The data for allogrooming relationships are presented in Figure 3. Significant
grooming relationships existed between the two females Girlie and Leala and between
the subadult male Friday and the adolescent male Inoki. Jinak often groomed two of
the adult males, and Zabu groomed Leala more than expected. The individuals
showing the most allogrooming were in the order 1) Jinak, 2) Zabu, 3) Leala, 4)
Girlie, and 5 ) Friday, all of whom spent more than 1 % of their time in this activity
(Table 3).
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
322
Social Behavior of Orangutans
323
J
I/
LETCHV
Fig. 3 . Sociogram showing the proportions of time greater than expected by chance that different
members of the group of orangutans on the island spent in allogrooming. Statistically significant results
only are illustrated.
In only two cases were the relationships for allogrooming and contact similar
(ie, Zabu:Girlie and Lea1a:Girlie) so that in most cases grooming did not simply
reflect the amount of time two animals spent in contact, The order of frequency of
being groomed from the highest was 1) Girlie, 2) Zabu, 3) Bobo, 4) Leala, and 5)
Inoki, but too much emphasis must not be placed on this order because the frequencies
were low.
Social Play
The sociogram for social play is illustrated in Figure 4 and presents a different
picture from the behavior previously considered. Social play is shown mainly by
adolescent and subadult individuals, although adult female orangutans do show a low
level of social play. The three adult males showed no play of any kind, whereas the
three adolescent males played more frequently than any others. The most attractive
individuals as play partners appeared to be, first, Jinak and second, Hsing-Hsing,
whereas the proportion of the repertoire spent in play was greatest for Inoki, followed
Poole
JOJO
P < 0.05
------- 3
P < 6.81
)-.
0.881
b-1
P<
P < B.BO01>- I
Fig. 4. Sociogram showing the proportions of time greater than expected by chance that different
members of the group of orangutans on the island spent in social play. Statistically significant results
only are illustrated. Zabu, Jojo, and Bob0 were excluded from the calculations because they showed no
social play.
by Hsing-Hsing and then Jinak. The strong negative correlation between playfulness
and age is probably reflected in this last ordering.
Sexual Behavior
Heterosexual behavior was very uncommon and only observed on two occasions; it was directed to the female Leala, once by Zabu and once by the castrated,
subadult male Friday. Homosexual behavior was observed on four occasions, initiated
by the adolescent male Jinak.
Aggression
No serious, as opposed to playful, attacks were observed between colony
members. Aggression was seen once, outside the routine sampling period, when an
adult male (Rodney), not normally on the island, was allowed onto it and was savagely
attacked by Zabu.
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
324
6
3
59
56
Girlie
Leala
12-13
11-12
Charlie
Kamal
+, < 0 . 5 % .
Mother
Age
(months)
Name
TABLE 6. Behavior of infant orangutans
N, Total number of 2-minute intervals sampled,
Absent
Present
Solitary
Play
Alone
11
21
Alone
14
6
Proximity
6
8
Solitary
Play
13
6
Contact
2
25
0
2
+
+
Groomed
Social
Play
Groomed
With mother
Proximity
Contact
62
13
16
45
7
3
Grooms
TABLE 5. Differences in the frequencies of behaviors shown by Jinak, an adolescent male, when other adolescents and subadults were
present or absent (expressed as percentages)
Social
Play
8
12
88
134
N
M
a
ti
0
3a
0,
s
1.
nl
5
(D
a
v,
8.
E
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Poole
Influence of Age of Companions on Behavior
The behavior of Jinak suggests that the ages of companions influence the social
behavior of this species. On four occasions Jinak was the only adolescent or subadult
on the island so that his companions were all adult males or females (plus infants).
The differences in his behavior in the presence or absence of other immatures are
shown in Table 5. In the absence of other adolescents or subadults, social play was
reduced from 25 to 2% of his repertoire and confined to interactions with the adult
females. This reduction in social play appears to have been compensated for by
increases in other forms of social behavior (contact, grooming, and proximity).
Behavior of Infants
The two mothers spent much time in contact or proximity and mutual grooming.
This provided opportunities for their infants to play together. The overall behavior of
the infants in relation to their mothers is given in Table 6; their few social interactions
with other members of the colony were largely determined by their mothers’ movements. When a mother moved more than 3 m, she collected the infant and took it
with her. As all observations were carried out when the orangutans were active,
suckling was rarely observed.
An infant was only once seen to be carried by an individual other than its
mother. Charlie was carried by Jinak when Girlie’s attention was diverted. The data
in Table 6 show that Kamal spent more time alone, a reflection of the fact that Leala
was a less restrictive mother. The highest proportion of the time the infants spent
playing socially was with one another (Charlie, 67%; Kamal, 54%). The next highest
amount of time was playing with their mothers (Charlie, 18%; Kamal, 24%), then
with the other infant’s mother (Charlie, 8%; Kamal, 4%), and, on a few occasions,
with various adolescents (Charlie, 8%; Kamal, 18%).
DISCUSSION
Sociability of Orangutans
The orangutans released onto the island had ample opportunity to either establish
or avoid social contact with other members of the colony. The data clearly show that
females and adolescent males spent somewhat more than one third of their active
period in close social contact with other members of the colony. Even adult males
spent more than 10%of this time in proximity to other colony members. It is clear,
however, that the older males are less sociable than adolescents or subadults. These
findings are in agreement with field data that have indicated that young males may
travel with a companion and that females with dependent young meet other females
and socialize with them [Rijksen, 19781; adult males adopt a solitary life-style, apart
from consortships with adult females [Galdikas, 19851. The reason for the relatively
solitary life-style of this ape is still a matter of speculation [Wrangham, 1979; Galdikas
and Teleki, 19811, but it has been suggested that their foraging technique may be
incompatible with group living. It is interesting that female chimpanzees also lead a
rather solitary existence [Wrangham, 19791.
The data presented show an animal capable of making selective social relationships, a conclusion that is in agreement with the findings of Edwards [1982], who
found that 51% of the behavior shown by adult orangutans was social as compared
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
326
327
with 22% (or 2-11 % if proximity is excluded) in the present study. The results of the
two investigations are not directly comparable, however, for three reasons: First,
Edward’s social groups were smaller (three to four animals) and only included a
single male. Second, watching other individuals (“monitoring”) was included in
social behavior. Third, the area available to the animals was much smaller, being
only 22 m2 as compared with 450 in the SZG colony.
Both investigations show that orangutans in captivity display a repertoire of
social behavior comparable to that of group-living great apes [for ethogram, see
Maple, 19801. Nadler and Braggio [1974] compared the behavior of groups of captive
juvenile orangutans (aged 3 years 3 months to 4 years 8 months) and chimpanzees
(aged 2 years 5 months to 3 years 10 months) in outdoor compounds. They found that
both species showed high levels of social play and spent similar amounts of time in
bodily contact, although orangutans were more frequently found at distances greater
than 0.6 m from other individuals.
Play seems to occupy an important role in the life of captive juvenile, adolescent, and subadult orangutans; in the wild individuals in this age range are often
observed in small groups, which may provide opportunities for social play [MacKinnon, 1974; Rijksen, 1978; Galdikas, 1981b; Schurmann, 19821. Adult males have
never been seen to play socially in the wild and they showed no social play in the
present study.
Some authors, however, have recorded social play in captive adult male orangutans. Zucker et a1 [1978] observed one playing with a 4-year-old male offspring and,
in six adult male-adult female dyads, the male played with its female cagemate
[Zucker et al, 19861. In both these studies the degree of confinement was greater than
that in SZG, and Zucker et a1 [1986] suggest that the larger and more complex the
enclosure the lower the level of social interaction. Social behavior, and particularly
social play, may be used by the adult orangutan to compensate for the lack of
stimulation and high predictability of confined conditions; alternatively, it may be that
play is induced in adult males by continued close confinement with another individual.
Juveniles and adolescents appear to play socially in all captive environments reported
so far.
The adult male patrols a large home range in the forest, advertising his presence
with loud calling, which is believed to warn off other males and to attract females
[Galdikas, 1981a,b]. The low level of social contact between adult males on the island
may reflect a natural tendency for adult males to space themselves apart. The
amicability of the three males Zabu, Bobo, and Jojo is surprising, but their situation
is abnormal in that they had been reared together. It is not uncommon for adult males
of other species of solitary mammal (eg, polecats; Poole, 1973) to remain amicable
to other males with whom they have been in constant contact from an early age. If
adult male orangutans had been separated for any length of time, however, the
situation might have been very different.
Welfare Considerations
The orangutan island in Singapore probably represents a unique situation. It
raises the question as to whether keeping these apes in this type of accommodation is
good for the welfare of the individual. The most obvious objection to it is that
orangutans are rather solitary in their natural environment, so that keeping them in
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Social Behavior of Orangutans
Poole
an artificial social group might conflict with a need to avoid contact with others. This
view is not supported by the data; the animals actively sought one another’s company,
and the younger individuals spent much time playing together. Even adult males
sometimes sat close to others and were groomed. The animals knew each other and
could select companions or avoid one another at will. Maple [1982] has also pressed
the case for giving orangutans the opportunity for social contact when they are kept
in captivity.
For highly intelligent animals like orangutans the Singapore Zoo’s facility seems
to provide an excellent way of creating an enriched captive environment for this
species. It allows the animals to interact socially or to distance themselves from other
individuals. The data from this short study provide good evidence that orangutans
should be allowed to socialize even after they are adults. If this is to be done,
however, two aspects of the Singapore Zoological Gardens’ facilities would seem to
be of paramount importance. The first point, which has already been mentioned, is
that there should be enough space for the animals to avoid one another should they
wish to do so. The second and probably equally important factor is that adults should
be housed separately at night so that there is no possibility of fights between them in
confined quarters.
CONCLUSIONS
1. Male orangutans in the age range of 6-18 years show a negative correlation
between the amount of time spent in both social and solitary play and in their
age. Adult males (over 16 years old) were not seen to play.
2. There was a positive correlation between the age of males (aged 6-17) and the
amount of time they spent alone. Older males spent less time in physical contact
with other individuals, but there was a positive correlation between their age and
the amount of time they spent in proximity (within arm’s length) to others.
3. Unlike adult males, the two adult females regularly played with other group
members, including their own and the other female’s infant.
4. Contact, allogrooming, and social play all showed nonrandom relationships
between individuals. The closest relationships for allogrooming and contact were
between the two adult females. There were well-defined play relationships between adolescents and subadults. These data showed that orangutans can develop
preferential social relationships with other individuals.
5. The adolescent male Jinak was often on the island without other adolescents or
subadults. On these occasions he showed a marked reduction in social play but
an increase in other forms of social interaction (eg, contact, proximity, and
allogrooming).
6. There was no obvious dominance hierarchy and no aggression between colony
members. Sexual behavior was very infrequent, although a few incidences of
both heterosexual and homosexual behavior between males were observed.
7. One-year-old infants interacted mainly with their own mothers, one another, and
the other infant’s mother. The infant of the less restrictive mother was more
independent.
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
328
329
ACKNOWLEDGMENTS
I am most grateful to Bernard Harrison and his staff at the Singapore Zoological
Gardens for their cooperation and help in making this project possible. I am particularly indebted to Alexander Stalin, who introduced me to the orangutans and shared
his large fund of knowledge of their behavior and previous history. Professor Tom
Lam of the Department of Zoology, National University of Singapore kindly provided
the academic base for the work; his enthusiasm and encouragement were much
appreciated. Last, but not least, I am grateful to my wife, Helen, who provided a
major source of nourishment for Singapore’s mosquito population while collecting
the essential baseline data.
REFERENCES
Duplaix, N.; Grady, L. Is the international trade utans. ARCHIVES OF SEXUAL BEHAVIOUR
convention for or against wildlife? INTERNA- 6:457-475, 1977.
TIONAL ZOO YEARBOOK 20:171-177, 1980. Nadler, R.D.; Braggio, J.T. Sex and species difEdwards, S.D. Social potential expressed in cap- ferences in captive reared juvenile chimpanzees
tive group-living orang utans, pp. 249-255 in and orangutans. JOURNAL OF HUMAN EVOTHE ORANGUTAN. ITS BIOLOGY AND LUTION 3~541-550, 1974.
CONSERVATION. L.E.M. Boer, ed. The Poole, T.B. The aggressive behaviour of individual male polecats (Mustela putorius, M.fur0 and
Hague, Junk, 1982.
Galdikas, B.M.F. Orangutan adaptation at Tan- hybrids) towards familiar and unfamiliar oppojung Puting Reserve: Mating and Ecology, pp. nents. JOURNAL OF ZOOLOGY 170:395-414,
194-233 in THE GREAT APES. D.A. Hamburg, 1973.
E.R. McCown, 4 s . Menlo Park, Benjamin/ Poole, T.B.; Fish, J. An investigation of playful
behaviour in Rattus norvegicus and Mus muscuCummings, 1979.
Galdikas, B.M.F. Wild orang utan reproduction, lus (Mammalia). JOURNAL OF ZOOLOGY
pp. 281-300 in REPRODUCTIVE BIOLOGY OF 175:61-71, 1975.
THE GREAT APES: BIOMEDICAL AND Rijksen, H.D. Social structure in a wild orang utan
COMPARATIVE ASPECTS. C.E. Graham, ed. population in Sumatra, pp. 373-379 in CONNew York, Academic Press, 1981a.
TEMPORARY PRIMATOLOGY. S. Kondo, M.
Galdikas, B.M.F. Wild orangutan studies at Tan- Kawai, A. Ehara, eds. Basel, Karger, 1975.
jung Puting reserve, Central Indonesian Borneo, Rijksen, H.D. A FIELD STUDY OF SUMA1971-1977. NATIONAL GEOGRAPHIC SOCI- TRAN ORANGUTANS. Wageningen, Veeman
& Zonen, 1978.
ETY RESEARCH REPORTS 13:l-10, 1981b.
Galdikas, B.M.F. Adult male sociality and repro- Rodman, P.S. Population composition and adapductive tactics among orangutans at Tanjung Put- tive organisation among orang-utans of the Kutai
ing. FOLIA PRIMATOLOGICA 45:9-24, 1985. reserve, pp. 171-209 in COMPARATIVE
Galdikas, B.M.F.; Teleki, G. Variations in subsis- ECOLOGY AND BEHAVIOUR OF PRItence activities of female and male Pongids: New MATES. J.H. Crook, R.P. Michael, eds. Lonperspectives on the origins of Hominid labor di- don, Academic Press, 1973.
vision. CURRENT ANTHROPOLOGY 22241- Rodman, P.S. Individual activity patterns and the
256, 1981.
solitary nature of orangutans, pp. 235-255 in
Horr, D.A. The Bornean orang-utan: Population THE GREAT APES. D.A. Hamburg, E.R.
structure and dynamics in relation to ecology and McCown, eds. Menlo Park, BenjamidCumreproductive strategy. PRIMATE BEHAVIOUR mings, 1979.
41307-323, 1975.
Rumbaugh, D; Gill, T. Learning skills of great
MacKinnon, J.R. The behaviour and ecology of apes. JOURNAL OF HUMAN EVOLUTION
wild orangutans (Pongo pygmueus). ANIMAL 2: 171-179, 1973.
BEHAVIOUR 22:3-74, 1974.
Schurmann, C. Mating behaviour of wild orangMaple, T. ORANGUTAN BEHAVIOR. New utans, pp 269-284 in THE ORANGUTAN. ITS
York, Van Nostrand Reinhold, 1980.
BIOLOGY AND CONSERVATION. L.E.M.
Maple, T. Orangutan behaviour and its manage- Boer, ed. The Hague, Junk, 1982.
ment in captivity, pp. 257-268 in THE ORANG- Siegel, S. NON-PARAMETRIC STATISTICS
UTAN. ITS BIOLOGY AND CONSERVATION. FOR THE BEHAVIOURAL SCIENCES. London, McGraw Hill, 1956.
L.E.M. Boer, ed. The Hague, Junk, 1982.
Nadler, R.D. Sexual behaviour of captive orang- Wilson, S.F. Environmental influences on the ac-
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Social Behavior of Orangutans
Poole
tivity of captive great apes. ZOO BIOLOGY utans: A developmental perspective. DEVELOPMENTAL PSYCHOBIOLOGY 19:315-326,
1 :201-209, 1982.
1986.
Wrangham, R.W. On the evolution of ape social
systems. SOCIAL SCIENCE INFORMATION Zucker, E.L.; Mitchell, G . ;Maple, T. Adult maleoffspring play interactions within a captive group
18~335-336, 1979.
Zucker, E.L.; Dennon, M.B.; Puleo, S.G.; Ma- of orang-utans (Pongo pygmaeus). PRIMATES
19:379-384, 1978.
ple, T.L. Play profiles of captive adult orang-
APPENDIX
Estimation of Social Contact Between Individuals
The null hypothesis generated an expected value for social contact based on the
following parameters for animals A and B and behavior pattern “I”: 1) expected No.
of interactions for behavior I = EIAB; 2) No. of intervals when A and B were
together = NAB; 3) No. of intervals in a sample = SS; 4) No. of individuals present
in a sample in which A and B interacted = AP,; 5) total No. of intervals for which
A was observed = TA; 6) No. of intervals in which behavior I was carried out by
individual A = BHW.
The expected number of intervals in which animal A could interact with animal
B for behavior I therefore depends first on the proportion PIA of A’s repertoire in
which it showed behavior I so that
To take into account the number of individuals, the average number of conspecifics
(excluding infants) present per unit time AV was calculated according to the following
formula:
If the proportion of A’s repertoire devoted to behavior I is divided by the number of
other individuals present, this will give the proportion of intervals in which orangutan
A would be expected to show behavior I toward individual B, ie, P R ~ B ,
To estimate the expected number of intervals EXIAB in which A does I with B,
formula 3 must be multiplied by the number of intervals that the two individuals spent
together (NAB):
Formula 4 was calculated in the computer program as
Using this expected value, the probability associated with the observed value was estimated using the binomial test [see Siegel, 19561to calculate the normal deviate (2).
10982361, 1987, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/zoo.1430060406 by University Of Southern, Wiley Online Library on [31/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
330