Orangutan activity budgets Monthly variations and the effects of body size parturition and sociality.код для вставкиСкачать
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  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 REFERENCES Altmann, J. BABOON MOTHERS AND INFANTS. Cambridge, Harvard University Press, 1980. Clutton-Brock, T. Some aspects of intraspecific variation in feeding and ranging behaviour in primates. Pp.539-556 in PRIMATE ECOLOGY. T. Clutton-Brock, ed. New York, Academic Press, 1977. Fossey, D.; Harcourt, A. Feeding ecology of free-ranging mountain gorilla. Pp. 415447 in PRIMATE ECOLOGY. T. CluttonBrock, ed. New York, Academic Press, 1977. Galdikas, B. Wild orangutan birth at Tanjung Puting Reserve. PRIMATES 23500510,1982. Galdikas, B. Orangutan sociality at Tanjung Puting. AMERICAN JOURNAL OF PRIMATOLOGY 9:lOl-119, 1985. Galdikas, B. Orangutan diet, range and activity at Tanjung Puting, Central Borneo. INTERNATIONAL JOURNAL OF PRIMATOLOGY 9:l-31,1988. Goodall, J.; Athumani, J. An observed birth in a free-living chimpanzee (Pan troglodytes schweinfurthii) in Gombe National Park, Tanzania. PRIMATES 21545-549, 1980. Harrison, M. Time budget of the green monkey, Cercopithecus sabaeus: some optimal strategies. INTERNATIONAL JOURNAL OF PRIMATOLOGY 6:351-376, 1985. Hladik, M. A comparative study of the feeding strategies of two sympatric species of leaf monkeys: Presbytis senex and Presbytis entellus. Pp. 323-353 in PRIMATE ECOLOGY. T. Clutton-Brock, ed. New York, Academic Press, 1977. Horr, D. A. The Borneo orangutan: population structure and dynamics in relationship to ecology and reproductive strategy. Pp. 307-323 in PRIMATE BEHAVIOR: DEVELOPMENT IN FIELD AND LABORATORY RESEARCH, Vol. 4, L. Rosenblum, ed. New York: Academic Press, 1975. Kleiber, M. THE FIRE OF LIFE. New York, John Wiley & Sons, 1961. MacKinnon, J . The orangutan in Sabah today. ORYX 11:141-191, 1971. MacKinnon, J. The behaviour and ecology of wild orangutans (Pongo pygmaeus). ANIMAL BEHAVIOUR 22:3-74, 1974. Milton, K. Habitat, diet, and activity patterns of free-ranging wooly spider monkeys (Brachyteles arachnoides E. Geoffroy 1806). INTERNATIONAL JOURNAL OF PRIMATOLOGY 5:491-514,1984. Mitani, J. Sexual selection and adult male orangutan long calls. ANIMAL BEHAVIOUR 33:272-283,1985a. Mitani, J. Mating behaviour of male orang- utans in the Kutai Reserve, Indonesia. ANIMAL BEHAVIOUR 33:392-402, 1985b. Mitani, J. Gibbon song duets and intergroup spacing. BEHAVIOUR 9259-96, 1985c. Mitani, J. Experimental field studies of Asian ape social systems. INTERNATIONAL JOURNAL OF PRIMATOLOGY in press. Pollock, J. The ecology and sociology of feeding in Zndri indri. Pp. 37-69 in PRIMATE ECOLOGY. T. Clutton-Brock, ed. New York, Academic Press, 1977. Rijksen, H. A FIELD STUDY OF SUMATRAN ORANGUTANS (Pongo pygmaeus abelii Lesson 1827). Wageningen, H. Veenman & Zonen, 1978. Rodman, P. Population composition and adaptive organization among orangutans of the Kutai Reserve. Pp. 171-209 in COMPARATIVE ECOLOGY AND BEHAVIOUR OF PRIMATES. R. Michael, J . Crook, eds. London, Academic Press, 1973. Rodman, P. Feeding behaviour of orangutans of the Kutai Nature Reserve, East Kalimantan. Pp. 383-413 in PRIMATE ECOLOGY. T. Clutton-Brock, ed. New York, Academic Press, 1977. Rodman, P. Diets, densities and distributions of Bornean primates. Pp. 465-478 in THE ECOLOGY OF ARBOREAL FOLIVORES. G. Montgomery, ed. Washington, DC, Smithsonian Institution Press, 1978. Rodman, P. Individual activity patterns and the solitary nature of orangutans. Pp. 234255 in THE GREAT APES. D. Hamburg, E. McCown, eds. Menlo Park, Benjamin Cummings, 1979. Rodman, P.; Mitani, J . Orangutans: sexual dimorphism in a solitary species. Pp. 146154 in PRIMATE SOCIETIES. B. Smuts, D. Cheney, R. Seyfarth, R. Wrangham, T. Struhsaker, eds. Chicago, University of Chicago Press, 1987. Siegel, S. NONPARAMETRIC STATISTICS FOR THE BEHAVIORAL SCIENCES. New York, McGraw-Hill, 1956. Sokal, R.; Rohlf, F. BIOMETRY. New York, W.H. Freeman, 1981. Stewart, K. The birth of a wild mountain gorilla (Gorilla gorilla beringei). PRIMATES 18:965-976,1977. Sugardjito, J.; teBoekhorst, I.; van Hooff, J . Ecological constraints on the grouping of wild orangutans (Pongo pygmaeus) in the Gunung Leuser National Park, Sumatra, Indonesia. INTERNATIONAL JOURNAL OF PRIMATOLOGY 8:17-42,1987. van Schaik, C.; van Noordwijk, M.; de Boer, R.; den Tonkelaar, I. The effect of group size on time budgets and social behavior in wild long-tailed macaques (Macaca fuscicularis). BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY 13:173-181, 1983. 100 f Mitani Waser, P. Monthly variations in feeding and activity patterns of the mangabey, Cercocebus algigena (Lyddeker). EAST AFRICAN WILDLIFE JOURNAL 13:249-263, 1975. Waser, P. Feeding, ranging and group size in the mangabey Cercocebus albigena. Pp. 183-222 in PRIMATE ECOLOGY. T. Clutton-Brock, ed. New York, Academic Press, 1977. Watts, D. Environmental influences on mountain gorilla time budgets. AMERI- CAN JOURNAL OF PRIMATOLOGY 15: 195-211,1988. Wrangham, R. On the evolution of ape social systems. SOCIAL SCIENCE INFORMATION 18:334-368,1979. Wrangham, R.; Smuts, B. Sex differences in the behavioral ecology of chimpanzees in the Gombe National Park, Tanzania. JOURNAL OF REPRODUCTION AND FERTILITY (SUPPLEMENT) 28:13-31, 1980.