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Orangutan activity budgets Monthly variations and the effects of body size parturition and sociality.

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American Journal of Primatology 18:87-100 (1989)
Orangutan Activity Budgets: Monthly Variations and the
Effects of Body Size, Parturition, and Sociality
JOHN C. MITANI
The Rockefeller University Field Research Center, Millbrook, New York
The activity budgets of individual orangutans were investigated at the
Kutai Reserve, Indonesia. Activity profiles within and between individuals were compared to examine monthly variations in feeding patterns,
potential energetic constraints imposed by large body size and parturition,
and the costs of sociality. Animals showed monthly changes in travelling,
feeding, and resting patterns. Monthly increases in travelling and feeding
were associated with marked reductions in the time spent resting. Interindividual variations in activity budgets did not exist among animals of
the same age-sex class. Activity patterns differed, however, as a function
of age and sex. Adult females and subadult males travelled and fed significantly longer than an adult male. Parturition had predictable effects
on activity; one female reduced her feeding and travelling immediately
following parturition. Adult male orangutan sociality appears to be limited by travel costs. Associations with females forced a male orangutan t o
travel significantly more compared with periods in which he was solitary.
The male did not lose an appreciable amount of time feeding when accompanying a female.
Key words: rain forest phenology, frugivory, sociality costs, sex and
activity, travel costs.
INTRODUCTION
The manner in which animals allocate their time to various activities has been
employed to investigate several aspects of primate biology. Since anatomical, nutritional, reproductive, and social factors affect the activities of animals, examinations of intra- and interindividual changes in activity budgets can yield insights
into diverse problems including seasonal feeding patterns, energetic constraints
imposed by large body size and parturition, and the costs of sociality [e.g. Waser,
1975; Altmann, 1980; Wrangham & Smuts, 1980; Watts, 19881.
Orangutans are large arboreal apes currently restricted to the vanishing rain
forests of Borneo and North Sumatra in Southeast Asia. Field studies reveal that
orangutans differ from other anthropoid primates in important features of grouping and mating behavior [see review in Rodman & Mitani, 19873. In contrast to
most other anthropoids, which live in stable social groups, adult orangutans are
primarily solitary, with animals associating for only temporary periods. Adult
female orangutans occupy small overlapping home ranges, varying in size from
Received for publication January 9, 1989; revision accepted April 2, 1989.
Address reprint requests to John Mitani, Dept. of Zoology, University of California, Davis, CA 95616.
Millbrook, NY 12545.
0 1989 Alan R. Liss, Inc.
88 I Mitani
0.40-6.0 km2; males move over larger areas, which usually contain the ranges of
multiple females. The solitary nature of females, coupled with long periods of
gestation and lactation, limits the spatio-temporal availability of fertile females. A
promiscuous mating system characterized by intense male-male competition results. Adult male orangutans are more than twice as heavy as adult females, and
this extreme sexual dimorphism in size is attributable in part t o intrasexual selection.
Sexual dimorphism in size, prolonged periods of gestation and lactation, and
variable grouping patterns make orangutans inviting subjects to explore the costs
of large body size, motherhood, and sociality through examinations of changes in
activity budgets. The slow and deliberate movements made by large-bodied orangutans in their arboreal habitat permit recording their behavior in detail and facilitate the investigation of activity budgets methodologically. Nevertheless, the
solitary nature and wide-ranging movements of orangutans have precluded previous investigators from sampling the activity of individuals regularly and systematically. Researchers have often been forced to combine observations of multiple individuals over several months in their analyses of orangutan activity
budgets [MacKinnon, 1974; Rodman, 1979; Galdikas, 19881. Since interindividual
and seasonal variations are commonly exhibited in the activity patterns of many
primate species [see review in Clutton-Brock, 19771, samples previously used in
the analysis of orangutan activities are subject to biases.
One goal of this paper is to present monthly samples of the activity budgets of
individual orangutans. These samples control for the potentially confounding effects of seasonal and interindividual differences, and are used to investigate
monthly, interindividual, and intraindividual variations in activity. The relationship of monthly variations in activity to potentially relevant ecological and social
factors is explored. Interindividual differences in activity budgets serve as the
basis to investigate the costs of large body size in orangutans. The costs of parturition and orangutan sociality are examined through analyses of intraindividual
variations in activity.
METHODS
Study Site and Subjects
Observations of orangutans were conducted a t Mentoko camp in the Kutai
Reserve, East Kalimantan, Indonesia, from July, 1981 through January, 1983. The
study area covered 3 km2 of primary, mixed, lowland dipterocarp forest. A more
detailed description of the study site can be found elsewhere [Rodman, 19781.
During the study period, orangutans lived a t a density of approximately four
animals per km2. Although previous long-term field studies of orangutans found
mature animals to be primarily solitary [Rodman, 1973; MacKinnon, 1974; Horr,
1975; Rijksen, 1978; Galdikas, 19851, I observed males and females in frequent
mating associations as well as alone [Mitani, 1985a, b]. Females occupied small
overlapping ranges averaging greater than 1.5 km2. Female ranges were located
within larger male ranges of undetermined size.
Resident and non-resident animals composed the study population. Individuals encountered regularly and repeatedly formed the resident population and included one adult male, three adult females with their offspring, and two subadult
males. All resident animals were recognized individually. I identified and followed
an additional six adult males, two subadult males, and one solitary adult female,
and observed an undetermined number of other unrecognized subadult males during the study period; these animals constituted the non-resident population.
Orangutan Activity Budgets / 89
TABLE 1. Observation Days of Orangutans*
Age-sex
J A S O N
Individual class
BC"
LU
DO
SI
so
BB
DU
LE
AM
AM
AM
AF
AF
AF
SM
SM
0 2 0 2 3 1
0 0 0 0 0
0 0 0 0 0
5 7 5 5 7
0 3 0 0 0
2 1 0 0 0
0 0 0 0 0
0 0 0 0 0
Month
D J F M A M
J
J
3
0
0
7
9
0
0
0
1
0
0
6
0
0
0
0 1
0 0 0 0 0
8 1
5 2 0
0 0 1 3 0
0 0 0 5 0
0 0 1 2 2 0
0 0 6 0 0
3 1 2 7 1 2 5
2 5 0 8 0
0
0
0
7
0
0
5
0
4 6
0 0
0 0
6 0
0 5
0 0
5 0 1
0 0
4
0
0
5
0
0
4
0 1
5
0
0
7
0
0
7
1 2
A
S
O N D J
0
0
7
0
0
0
5
0
0
0
0
4
0
0
5
0
*Only days in which animals were followed for the entire day are included in the table. Age-sex class abbreviations: AM, adult male; AF, adult female; SM, subadult male. Monthly observations began in July, 1981 ( = J)
and continued without interruption until 18 months later in January, 1983 (=J).
"There is no reason to suspect that the animal BC in this study was the same individual bearing that name in
Rodman's (1973) earlier study.
General Procedures
With the aid of two field assistants, I accumulated over 3,900 hours of observations of orangutans. Most observations were of individual animals followed from
the time they rose in the morning until the time they went t o sleep. I report here
only observations of individual orangutans followed throughout a daily cycle
(Table I). With the exception of adult males, all observations are of resident animals. Because of the difficulty of consistently finding wide-ranging, solitary orangutans, individual animals were located on an opportunistic basis. After an animal
was found, it was followed from dawn to dusk by me or by an assistant. In general,
I followed animals from the time they began activity in the morning until some
variable hour past mid-day. A field assistant then relieved me and continued
observations until the subject constructed its nightly nest and went to sleep. This
protocol allowed us to follow individual orangutans for prolonged periods, and it
was not unusual to continue followings of animals for as long as 1 month. We
occasionally conducted simultaneous followings of two orangutans. On those days,
I observed one animal while my field assistants followed another individual. We
voluntarily terminated observations of orangutans when we switched efforts to
follow another individual or left them to conduct concurrent research on sympatric
gibbons [Mitani, 1985~1.
Methods of Sampling Activities and Definitions of Behaviors
Because of their large size and predominantly arboreal habit, orangutans
move very slowly and deliberately. This mode of locomotion permitted us to record
transition times of an individual's activity to the nearest minute [Rodman, 19771.
I defined broad and discrete behavioral categories, so that animals could occupy
only one activity at any time. We recorded the following five activity states:
1. Feeding. Feeding bouts included the handling, processing, and swallowing
of food items, as well as short movements within a feeding tree or area.
2. Travelling. Travelling included all locomotor behavior with the exception of
short movements during feeding.
3. Resting. Resting included activity periods in which animals were not feeding, travelling, vocalizing, or mating.
4. Mating. Mating involved interactions between males and females in which
animals attempted or completed copulations.
90 I Mitani
TABLE 11. Cumulative Activity Budgets of Individual Orangutans*
Individual
BC
LU
DO
SI
so
BB
DU
LE
No. days
40
16
11
76
31
9
80
56
Age-sex
class
Travel
Feed
Rest
Mate
Vocalize
AM
AM
AM
AF
AF
AF
SM
SM
8 2 4
9*2
9 26
13 t 7
925
8+5
11 2 5
9t 4
28+8
41t13
53 5 2 7
5 2 ? 13
3 8 t 13
3 8 2 17
4 8 2 15
5 3 2 14
63210
5O-Cl7
37+32
34 2 13
5 4 + 16
55+ 19
3 8 5 15
3 6 2 14
0+2
1 + 1
1+2
223
O t l
1?1
2 t 3
1 2 2
*The table shows the percent time spent in each activity. Means 2 1standard deviation are shown. Age-sex class
abbreviations a s in Table I.
5. Vocalizing. Vocalizing included times during which the animals emitted
sounds or calls.
Mating and vocalizing constituted only a small fraction of the activity budget
of each individual (Table 11). For this reason, I restrict the following analyses to
comparisons involving travelling, feeding, and resting activities. Orangutans typically rose around 0600 hours and were rarely active following 1800 hours [cf.
Rodman, 19791. To standardize calculations among individuals, activity budgets
were computed on the basis of a 12 hour day between these two times.
Samples and Methods of Analysis
Monthly variations in activity budgets. Observations of one adult female,
one adult male, and one subadult male spanned sufficient lengths of time to investigate monthly variations in individual activity budgets.
Interindividual variation in activity budgets. Interindividual variations
in the activity budgets of orangutans were examined between individuals of the
same age-sex class (subadult males), between males and females, and between
individuals of different ages. To control for the effect of seasonality, the activities
of individuals were sampled within 2 weeks of each other. The effect of social
factors were controlled by comparing the activities of solitary individuals or by
comparing their activities during days in which they associated with the same
individual.
Intraindividual variation in activity budgets
Costs ofparturition. During the course of this study, the birth of an infant
male was observed. A comparison between the mother’s behavior on the 4 days
prior to birth and her behavior on the day of birth and the 7 subsequent days
permits an examination of the effect of parturition on female activity patterns.
Costs of grouping. Several times during the same week, it was possible to
compare the activity of an animal one day when it was alone with its behavior on
another day when it accompanied another animal. These matched-paired samples
provide a unique opportunity to examine the effects of grouping on activity while
controlling for the potentially confounding effects of seasonal changes in time
budgets. Grouping imposes costs on animals that may be manifest in activity
budgets as a decrement in feeding time [e.g. Wrangham and Smuts, 19801 or as an
increment in travel time [e.g., Waser, 19771. Accordingly, I compare the feeding
and travel budgets of solitary individuals with their activity during associations
with others.
Orangutan Activity Budgets / 91
Statistical analyses. Data conformed to the assumptions of the analysis of
variance [Sokal & Rohlf, 19811, and parametric statistical tests were employed t o
analyze monthly and interindividual changes in activity budgets. The Wilcoxon
matched-pairs signed-ranks test [Siegel, 19561 was used to compare the activity
budgets of solitary males with their activity profiles while they accompanied females. All statistical tests were two-tailed.
RESULTS
Individual Activity Budgets
Table I1 presents the cumulative activity budgets of eight orangutans for
whom sufficient samples exist (see Table I). These data form the basis for examining monthly, interindividual, and intraindividual variations in activity profiles.
Monthly Variations in Activity Budgets
Figure 1 illustrates mean feeding, travelling and resting times for three animals. Feeding and resting times exhibited significant heterogeneity among
monthly samples in the adult female and subadult male (one-way ANOVA P <
0.05 for the four comparisons). A closer examination of these data reveals a peak
in feeding during the last 2 months of the study. Increments in feeding during this
time were apparently due to fruiting of an important orangutan food source, Durio
sp., and were offset by corresponding decrements in resting (Fig. 1).In contrast,
other periods of high feeding activity (female: October, 1981, May, 1982; subadult
male: July, 1982) were not associated with high levels of frugivory; foraging time
increased as animals ate large quantities of bark and leaves.
The adult male showed a different pattern of monthly variations in activities.
Travelling and resting times showed significant heterogeneity over 6 months (oneway ANOVA P < 0.05 for both comparisons), while feeding activity remained
relatively constant. A significant decrease in travel time during one month
(March, 1982) was compensated by an increase in resting. This difference could
have been related to social rather than ecological factors. The male followed adult
females and matched their activity profiles in all samples, with the exception of the
month during which he travelled little (see below). Irrespective of the precise
causes of monthly variations in activity, these results underscore the need to
control for seasonal and social effects in the analysis of inter- and intraindividual
variation in activity budgets.
Interindividual Variations in Activity Budgets
Subadult males. Sufficient observations existed to compare the activity budgets of two subadult males during two different months (Table 111). Pairwise comparisons between activities each month did not reveal any differences between the
males (one-way ANOVA P > 0.15 for all comparisons).
Sex differences in activity
Adult females and subadult males. Observations of two animals during 2
months provide the basis for comparing the activity budgets of adult females and
subadult males (Table IV). Paired comparisons between travelling, feeding, and
resting activities of animals did not reveal any significant differences (one-way
ANOVA P > 0.60 for all comparisons).
Adult females and adult males. Comparisons between the activity budgets
of two adult females and one adult male (BC) showed differences in the proportions
of time spent in all activities (Table IV). The adult male spent significantly less
time travelling and feeding and significantly more time resting than the adult
92 i Mitani
feed
ee
rest
20
0
80
l b
feed
rest
trave1
I
Ja
1
1
I
1
1
I
I
1
F e R p My J1 A u S e O c No De J a
feed
0
No
De
Fe
Mr
QP
My
month
Fig. 1. Monthly variations in mean travelling, feeding, and resting times. a: adult female (SI).b subadult male
(DU). c: adult male (BC). Sample sizes for each individual in each month can be found in Table I.
females in two monthly samples (December, 1981 and March, 1982; Table IV);
similar differences in feeding and resting activities were found in an additional
month (May, 1982). The activities of one other male-female pair during one month
did not differ (LU-BB; Table IV).
Orangutan Activity Budgets / 93
TABLE 111. Comparison of Subadult Male Orangutan Activity Budgets*
Month
Individual
No. days
Travel
Feed
DU
7
5
11
5
823
9 2 3
8 2 6
12 2 2
56 + 16
44 2 11
48 2 13
48 2 9
May, 1982
LE
August, 1982
DU
LE
*The percent time in each activity is shown. Means
?
Rest
35 2
43 2
43 2
37 2
15
11
13
9
1 standard deviation are displayed.
TABLE IV. Sex Differences in Orangutan Activity Budgets*
Month
Individual
May, 1982
DU
SI
January, 1983
March, 1982
DU
SI
BC
SI
BC
May, 1982
BC
December, 1981
so
SI
September, 1982
LU
BB
No. days
4
5
5
4
10
5
6
5
5
5
4
5
Age-sex
class
SM
AF
SM
AF
AM
AF
AM
AF
AM
AF
AM
AF
Travel
Feed
Rest
8 2 4
8 2 4
1424
9+3
7 2 4**
14 2 7
3 2 2**
10 2 5
52 2 7
59 2 16
65 2 10
66 2 14
30 2 6**
58 2 11
2 5 2 4**
45 2 9
29 + lo**
59 2 16
44 2 17
33 2 11
40 9
32 f 14
21 f 8
26 2 6
62 2 5**
28 2 15
72 2 3**
45 2 12
62 2 9**
32 2 14
42 f 15
61 ? 15
825
8 2 4
13 2 5
725
*The percent time in each activity is shown. Means % 1standard deviation are displayed. Age-sex class abbreviations a s in Table I. Comparisons of activities between individuals in the same month: one-way ANOVA.
**P i0.01.
Effect of age on activity: adult and subadult males. Results of comparisons between the activity budgets of adult and subadult males were similar to
those found between adult females and males. In contrast to adult males, smaller
subadult males spent significantly more time travelling and feeding and significantly less time resting (Table V).
Intraindividual Variation in Activity Budgets
Costs of parturition. Travel and feeding times dropped dramatically on the
day of birth relative to the mean times on the 4 days prior to birth (Fig. 2). Travel
and feeding activities remained low for 2 days following parturition, and increased
thereafter (Fig. 2).
Costs of grouping: effect of sociality on activity
Adult males with adult females. The feeding activity of one adult male
(BC) did not change as a function of grouping (Table VI). His travel time, however,
increased, and resting time decreased significantly during associations with females (Table VI). The effect of grouping on travel budgets was not restricted to this
single male. A smaller sample involving another male (LU) showed a trend toward
increased travel time during associations with females (Table VI).
Subadult males with adult females. The activity budgets of two subadult
males did not appear to be similarly affected by the presence of females. Travel,
feeding, and resting budgets did not differ between periods when subadult males
were alone and when they accompanied a female (Table VI).
94 I Mitani
TABLE V. Age Variations in Orangutan Activity Budgets?
Month
May, 1982
May, 1982
Individual
No. days
BC
DU
BC
5
7
5
6
5
10
4
5
LE
LU
LE
July, 1982
September, 1982
LU
DU
Age-sex
class
AM
SM
AM
SM
AM
SM
AM
SM
Travel
Feed
Rest
825
8 k 3
8c5
9 5 2
452"
1025
1425
1427
29 2 lo**
56216
29 2 10*
4329
37 2 12
49 2 12
47 2 19
51 2 16
62 2 9**
34 2 15
62 2 9*
45 2 9
60 2 14"
41 2 12
37 2 14
29 2 20
tThe percent time in each activity is shown. Means -t 1 standard deviation are displayed. Age-sex class abbreviations as in Table I. Comparisons of activities between individuals in the same month: one-way ANOVA.
*P < 0.05.
**P < 0.01.
rest
c,
G
0
40
feed
x
20
travel
-
0
- 4 - 3 - 2 - 1
0
1
J.
day
2 . 3 4
5
6
7
p a r t ur 1 t 1 o n
Fig. 2. Mean travelling, feeding, and resting times of an adult female (SO) before, during, and after parturition.
Before parturition = days -4 to -1. Parturition = day 0. After parturition = days 1-7.
DISCUSSION
Monthly Variations in Activity
Orangutans are primarily frugivorous [MacKinnon, 1974; Rodman, 1977; Rijksen, 1978; Galdikas, 19881, and prior field studies have suggested that seasonal
variations in fruit availability affect the feeding and travel patterns of animals
[MacKinnon, 1974; Rodman, 1977; Galdikas, 19881. The results of this study revealed a peak in feeding activity that correlated with fruiting of one important
orangutan food source. Although the fruiting phenology of the forest appeared to
affect orangutan feeding activity, it did not do so consistently; other periods of high
feeding activity were not associated with frugivory. In these cases, the availability
of fruit may have influenced activity profiles in complex ways. For example, the
orangutans fed more when they depended heavily on leaves and bark, presumably
because fruit was scarce. These vegetative parts usually take a longer time to
process, thereby increasing the total time spent feeding [e.g., Hladik, 19771. In
addition, social factors may have been an important determinant of activity patterns. Increases in the time spent travelling by an adult male orangutan could be
Orangutan Activity Budgets / 95
TABLE VI. Effect of Sociality on Orangutan Activity Budgets?
Individual
BC
LU
DU
LE
No. days
9
Age-sex
class
Travel
Feed
Rest
AM
Alone
With female
5 5 4**
10 5 4
28 6
29 k 10
*
66 + 8*
58 + 12
6 2 3
11 4
30 k 19
53 + 15
65 + 19
3 4 ? 14
8+2
49 5 8
49 + 18
41 C 7
42 C 16
*3
*5
36 + 11
5 0 ? 13
54 2 14
38 12
4
AM
7
Alone
With female
SM
Alone
With female
6
*
10 * 4
SM
Alone
With female
10
11
*
*
+Thepercent time in each activity is shown. Means 1standard deviation are displayed. Age-sex class abbreviations as in Table I. Comparisons of activities between solitary and social animals: Wilcoxon matched-pairs,
signed-ranks test.
*P < 0.05.
**P < 0.01.
related to his association with adult females, which spent a greater proportion of
time in this activity.
The effect of ecological factors on orangutan activity patterns has also been
reported to vary between studies. Rodman’s [19771 earlier study of the Kutai
orangutan population revealed that two resident females increased their feeding
time during a period when they fed relatively little on fruit. No changes in travel
patterns over seasons were apparent. In contrast, MacKinnon [19741, working in a
floristically similar mixed, lowland dipterocarp forest, reported that orangutans
increased feeding time and decreased travel time during seasons when fruit formed
a major part of their diet. Observations of orangutans in a floristically depauperate
peat swamp forest appear to conform to the pattern at MacKinnon’s site; when
orangutans fed on fruit, they travelled shorter distances than other times [Galdikas, 19881.
These conflicting results are difficult to evaluate. One problem is that these
studies have not controlled for variation in activity budgets due to other factors.
For example, previous studies have not clearly defined whether observations of
animals during associations with conspecifics have been combined with observations of solitary animals in activity budget analyses. In light of the result reported
here that social as well as ecological factors may affect activity patterns, it is
necessary to control for the effects of grouping during examinations of orangutan
time budgets. An additional complication is that differences between studies may
reflect sampling biases due to the small number of animals observed in each
population. The wide-ranging habits of orangutans make it difficult to monitor the
behavior of more than a few individuals; given these circumstances, it is necessary
to entertain the possibility that the activity profiles of sampled animals may not
reflect those of the population.
Methodological problems are not limited to studies of orangutans. Other analyses of primate activity profiles have failed to consider the effects of seasonal,
interindividual, and intraindividual variations in activity [e.g., van Schaik et al.,
1983; Milton, 1984; Harrison, 19851. For example, van Schaik et al. [19831recently
provided data suggesting that individual long-tailed macaques suffered a loss in
96 / Mitani
feeding time by travelling in larger groups. Nevertheless, their analysis combined
observations of several animals over time, and it is unclear whether controlling for
seasonal and interindividual differences in activity would affect this result.
Effects of Body Size on Activity
Due to the energetic demands of large body size [Kleiber, 19611, one might
expect adult male orangutans to feed more than smaller adult females and subadult males. Comparisons between animals differing in body size, however, revealed that adult females and subadult males fed for longer periods than one adult
male. While this result is consistent with observations of several other primate
species in which males spend less time feeding than females and subadults [see
review in Clutton-Brock, 19771, it differs from the prediction based on energetic
considerations and previous observations of Kutai orangutans, which showed that
one adult male fed more than adult females [Rodman, 19791.
One possible explanation for these differences is that adult male orangutans
feed at faster rates than adult females and subadult males and thereby gain similar amounts of energy in shorter periods of time. Observations of other primate
species suggest that males tend to feed faster than females [Pollock, 1977; CluttonBrock, 1977; Fossey & Harcourt, 19771, and preliminary data indicate the same
may be true among orangutans [Mitani, unpublished observations]. Alternatively,
the high feeding activity of females and subadult males reported in this study may
reflect important energetic factors unrelated to body size. For example, pregnancy
and lactation may increase the feeding requirements of adult females. In this
context, it is of interest that in one comparison, a pregnant female (SO, March,
1982, Table IV) fed significantly more than an adult male. Similarly, subadult
males, which grow two times larger during development, may require additional
food. Finally, it is important to note that the feeding profile of adult males presented here, based largely on a single animal, may not be representative of orangutans. The total daily time spent feeding by the adult male (BC) in this study is low
compared with that of males observed previously [cf. Rodman, 1979; Galdikas,
19881. This male was the highest-ranking animal in the study population, however, and showed no signs of suffering from his lack of feeding time.
In summary, feeding rates and the activity budgets of additional animals
varying in body size and reproductive condition require further study. In the absence of these data, the age and sex class differences in feeding budgets reported
here will remain problematic.
Effects of Parturition on Activity
Examination of the behavior of an orangutan before, during, and after birth
revealed that parturition affected female activity patterns in two ways. First, there
were marked reductions in feeding and travelling. Second, these reductions corresponded to an increase in the time spent resting. Currently available qualitative
accounts of parturition in pongids [Stewart, 1977; Goodall & Athumani, 1980;
Galdikas, 19821cannot be compared directly with the results presented here. Data
regarding maternal activity budgets of baboons during parturition, however, are
consistent with one pattern found here; baboon mothers spend a considerable
amount of time resting on the day of birth [Altmann, 19801. Further observations
are clearly needed to evaluate this trend.
Costs of Sociality
Orangutans are unique among anthropoid primates in that they are essentially solitary animals. The only enduring social grouping consists of a mother and
Orangutan Activity Budgets / 97
her dependent offspring [see review in Rodman & Mitani, 19871. The factors leading to the evolution and maintenance of asociality in orangutans have been the
subjects of continuing discussion [Rodman, 1973; MacKinnon, 1974; Rijksen, 1978;
Galdikas, 1985; Sugardjito et al., 19871. Arboreality, large body size, and feeding
habits appear to contribute in important ways to the evolution of the solitary
lifestyle of orangutans. Due to their large body size and arboreal habit, orangutans
are subject t o minimal predation. They subsist on a diet composed of sugar-rich
fruits, distributed in discrete and widely-separated trees [MacKinnon, 1974; Rodman, 1977; Rijksen, 1978; Galdikas, 19881. In the absence of predation, this resource base appears to favor solitary foraging by females [Wrangham, 19791. The
spatial dispersion of females combines with their highly restricted reproductive
availability over time to intensify intrasexual competition among males and to
create strong selection pressures leading to a highly promiscuous mating system
[Mitani, in press]. The extreme sexual size dimorphism found in orangutans is one
manifestation of intense male-male competition for mating access to females [see
review in Rodman & Mitani, 19871, and this size dimorphism has been invoked to
play a role in the maintenance of orangutan asociality through its effects on activity patterns.
Based on his observations of Kutai orangutans, Rodman [1979] proposed that
metabolic costs associated with large body size force adult males to feed for significantly longer periods than adult females. In contrast, small body size was
hypothesized to permit adult females to range more widely and to feed more selectively for shorter durations on higher-quality food items, which are separated by
long distances. Viewed within this framework, differences in activity patterns
maintain spatially segregated sexes due to the increased foraging and travel costs
imposed on males during associations with females.
While Rodman’s [19791 earlier observations of the activity budgets of one adult
male and two adult females at Kutai are consistent with this interpretation, additional field research has not replicated his findings. Observations by MacKinnon
[19741 and Galdikas El9881 suggest that male and female orangutans do not differ
substantially in the time they spend feeding. These studies, however, neglected t o
control for the effects of potentially confounding variables. Specifically, observations of different animals have been combined over long periods, with the result
that seasonal and interindividual variation in activity patterns have not been
taken into account. When such variability was controlled in this study, one adult
male was found to spend significantly less time feeding than smaller-bodied adult
females and subadult males. An additional comparison revealed that a n increase
in the time spent travelling was the primary cost of sociality imposed on an adult
male orangutan [cf. Waser, 19771.
These data, suggesting that travel costs may limit adult male sociality, are not
inconsistent with an alternative hypothesis that direct feeding costs due to periods
of low fruit availability force orangutans to forage alone. In support of this hypothesis, recent data from a Sumatran population purported to show positive correlations between the availability of fruit and grouping events in orangutans [Sugardjito et al., 19871. These correlations are unconvincing given the variables used
to measure both fruit availability and grouping. First, an independent measure of
fruit availability that was relevant to orangutans was not provided. Periods of high
fruit availability reported in this study corresponded to large amounts of “fruit-fall”
from primarily three species, Mallotus spaerocarpus, Macaranga dipenhorstii, and
Aglaia speciosa; fruit from only the last species is preferred by orangutans, however.
Second, grouping events were defined as “The occasions on which the focal animal
left a food tree together with other individuals and stayed with them within a
98 I Mitani
distance of 30 m for at least half an hour.” Nevertheless, orangutans are slow
moving and usually do not travel more than a mean distance of 20-30 ml% hr [see
review in Rodman & Mitani, 19871. Associations over short distances during short
periods of time, therefore, do not provide a biologically relevant measure of sociality
in orangutans. These observations do not rule out the possibility that the high
frequency of grouping events recorded during “fruit” seasons in the Sumatran
population represented passive aggregations of animals near large food sources.
Measures of groups persisting over longer periods of time, ranging patterns, and the
spatial as well as temporal availability of fruit will be required to evaluate critically
the feeding and travel costs of sociality in both Sumatran and Bornean orangutans.
In addition, larger samples involving more individuals are needed to test the
generality of all patterns reported in this population of Bornean orangutans.
CONCLUSIONS
1. Orangutans exhibited monthly variations in activity patterns. Monthly
increases in travelling and feeding were associated with decreased levels of resting.
2. Activity budgets of individuals of the same age-sex class did not differ. In
contrast, the activity patterns of animals varied as a function of age and sex. An
adult male orangutan rested more while travelling and feeding less than adult
females and subadult males.
3. Parturition had predictable effects on the activity patterns of a female
orangutan. Reductions in feeding and travelling characterized the behavior of this
animal after giving birth.
4. Increased travel costs appear to limit adult male orangutan sociality. One
adult male travelled significantly more when associating with females than when
travelling alone. This male did not lose an appreciable amount of time feeding
when accompanying a female.
ACKNOWLEDGMENTS
Field work was sponsored by the Indonesian Institute of Sciences, the Indonesian Directorate General of Forest Protection and Nature Conservation, and the
Indonesian Association of Zoological Parks. Research was supported by grants
from the NSF BNS-8022764, the L.S.B. Leakey Foundation, the Wenner-Gren
Foundation, the National Academy of Sciences, the University of California, and
by fellowships from the Regents of the University of California. Additional funding
was provided by U.S.P.H.S. grants RR00169 and RR00166 to the University of
Washington and the California Primate Research Center. Preparation of this
manuscript was made possible with the support of grants from the Harry Frank
Guggenheim Foundation, the National Geographic Society, the L.S.B. Leakey
Foundation in conjunction with Wildlife Conservation International, and the NSF
BNS-8805275. I thank Sdr. Bastar and Yum for field assistance; Peter Rodman,
Chuck Darsono and Orville Smith for logistic aid in the field; John Oates for
supplying a reference; and Peter Rodman, Karen Strier, and three anonymous
reviewers for comments on the manuscript. Finally, I am especially grateful to the
Darsono family for providing the relaxed and peaceful atmosphere in which the
original draft of this paper was written.
Orangutan Activity Budgets I 99
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