The behaviour and ecology of wild orang-utans (Pongo pygmaeus)

Between June 1968 and November 1971, 21 months of fieldwork were spent studying wild populations of orang-utans in Borneo and Sumatra. Fifteen hundred hours of daylight observation were made on these animals. In contrast to other higher primates, the dispersed distribution of food and the orang-utan's slow mode of travel seem to favour solitary habits. Over, 80 per cent of foraging units seen contined only one independent animal, either alone or with dependent young. Larger groupings were seen when adolescents joined up with other animals or when males and females formed temporary consortships for mating purposes. Comparison between several populations revealed an interesting mechanisms for the natural regulation of animal numbers. Differences between Bornean and Sumatran orang-utans are discussed in relation to the zoogeography of these two islands.     

Happy orang-utans live longer lives

Nonhuman primate ageing resembles its human counterpart. Moreover, ratings of subjective well-being traits in chimpanzees, orang-utans and rhesus macaques are similar to those of humans: they are intercorrelated, heritable, and phenotypically and genetically related to personality. We examined whether, as in humans, orang-utan subjective well-being was related to longer life. The sample included 184 zoo-housed orang-utans followed up for approximately 7 years. Age, sex, species and number of transfers were available for all subjects and 172 subjects were rated on at least one item of a subjective well-being scale. Of the 31 orang-utans that died, 25 died a mean of 3.4 years after being rated. Even in a model that included, and therefore, statistically adjusted for, sex, age, species and transfers, orang-utans rated as being "happier" lived longer. The risk differential between orang-utans that were one standard deviation above and one standard deviation below baseline in subjective well-being was comparable with approximately 11 years in age. This finding suggests that impressions of the subjective well-being of captive great apes are valid indicators of their welfare and longevity.     

Modelling ranging behaviour of female orang-utans: a case study in Tuanan,Central Kalimantan,Indonesia

Quantification of the spatial needs of individuals and populations is vitally important for management and conservation. Geographic information systems (GIS) have recently become important analytical tools in wildlife biology, improving our ability to understand animal movement patterns, especially when very large data sets are collected. This study aims at combining the field of GIS with primatology to model and analyse space-use patterns of wild orang-utans. Home ranges of female orang-utans in the Tuanan Mawas forest reserve in Central Kalimantan, Indonesia were modelled with kernel density estimation methods. Kernel results were compared with minimum convex polygon estimates, and were found to perform better, because they were less sensitive to sample size and produced more reliable estimates. Furthermore, daily travel paths were calculated from 970 complete follow days. Annual ranges for the resident females were approximately 200 ha and remained stable over several years; total home range size was estimated to be 275 ha. On average, each female shared a third of her home range with each neighbouring female. Orang-utan females in Tuanan built their night nest on average 414 m away from the morning nest, whereas average daily travel path length was 777 m. A significant effect of fruit availability on day path length was found. Sexually active females covered longer distances per day and may also temporarily expand their ranges.     

Agonistic interactions of captive female orang-utans with infants

Female orang-utans with infants were pair-tested for dominance (displacement behavior) and for competitive food-getting. Dominance was quickly established in the first test for each pair, displacements were unidirectional in all tests and dominant animals obtained essentially all food incentives in 3 of the 4 series of tests. Agonistic interactions were similar in most respects to those of monkey, but were not influenced by location of testing, i.e. home cage. Aggressive behavior and food-getting were related to body weight but displacement was not. This development of dominance relationships is similar to those reported for many territorial vertebrates confined to restricted spatial conditions.     

The social behaviour of doucs (Pygathrix nemaeus nemaeus) at San Diego Zoo

A group of six doucs was observed for 200 hours at San Diego Zoo. Patterns of social behaviour (including communicatory patterns) are described and compared with those of other colobine monkeys. Most aspects of the douc's social behaviour appear to be typical for colobines, although an eyelid display may be unique within the sub-family and the copulatory patterns are at least unusual.     

Male bimaturism and reproductive success in Sumatran orang-utans

Although orang-utans live solitary lives most of the time, theyhave a complex social structure and are characterized by extremesexual dimorphism. However, whereas some adult male orang-utansdevelop full secondary sexual characteristics, such as cheekflanges, others may stay in an "arrested" unflanged conditionfor up to 20 years after reaching sexual maturity. The resultis a marked bimaturism among adult males. Flanged males allowfemales to overlap with their home range and often toleratethe presence of unflanged males. However, wherever possibleflanged males actively prevent unflanged males from copulatingwith females. Two competing hypotheses, previously untested,have been advanced to explain male reproductive behavior andbimaturism in orang-utans: (1) the "range-guardian" hypothesis,which asserts that the flanged males are postreproductive anddefend a range in which they tolerate sexually active, unflangedmale relatives; and (2) the "female choice" hypothesis, whichasserts that flanged males tolerate unflanged males in their range because they rely on female preference to favor flangedmales. We investigated these hypotheses and a third hypothesisthat the two male morphs represent co-existing alternativemale reproductive strategies ("sitting, calling, and waiting"for flanged males versus "going, searching, and finding" forunflanged males). Fecal samples were collected from a well-studiedpopulation in Indonesia, and eight human microsatellites wereanalyzed for 30 individuals that have been behaviorally monitoredfor up to 27 years. By carrying out paternity analysis on 11offspring born over 15 years, we found that unflanged malesfathered about half (6) of the offspring. Relatedness betweensuccessful unflanged males and resident dominant males wassignificantly lower than 0.5, and for some unflanged/flangedmale pairs, relatedness values were negative, indicating thatunflanged males are not offspring of the flanged males.     

Genetic signature of anthropogenic population collapse in orang-utans

Great ape populations are undergoing a dramatic decline, which is predicted to result in their extinction in the wild from entire regions in the near future. Recent findings have particularly focused on African apes, and have implicated multiple factors contributing to this decline, such as deforestation, hunting, and disease. Less well-publicised, but equally dramatic, has been the decline in orang-utans, whose distribution is limited to parts of Sumatra and Borneo. Using the largest-ever genetic sample from wild orang-utan populations, we show strong evidence for a recent demographic collapse in North Eastern Borneo and demonstrate that this signature is independent of the mutation and demographic models used. This is the first demonstration that genetic data can detect and quantify the effect of recent, human-induced deforestation and habitat fragmentation on an endangered species. Because current demographic collapses are usually confounded by ancient events, this suggests a much more dramatic decline than demographic data alone and emphasises the need for major conservation efforts.     

The social organisation of a population of Sumatran orang-utans

Female orang-utans in a Sumatran swamp forest live in large, but stable, and widely overlapping home ranges. They preferentially associate with some of their female neighbours, possibly relatives, to form socially distinct clusters that also experience reproductive synchrony. Sexually mature males range more widely than females, but among them the dominant adult male has a relatively more limited range. His ranging and that of the subadult males reflect the local abundance of sexually attractive females. The other adult males tend to avoid these concentrations and focus on areas away from the dominant male. Females show philopatric tendencies. Male-biased sex ratios at birth give way to heavily female-biased sex ratios among adults. This suggests a net loss of males as they mature, due either to excess male mortality (e.g. by male mating competition), excess male dispersal from the population or a combination of both. We conclude that the orang-utan social organisation is best described as a loose community, showing neither spatial nor social exclusivity, consisting of one or more female clusters and the adult male they all prefer as mate.     

Testicle size of orang-utans in relation to body size

Few data are available for assessing the relative testicle size of orang-utans, Pongo pygmaeus, so measures were obtained for 31 individuals of varying age. It was shown that the volume of the testicles, calculated from in situ measures of testicle length and breadth, closely approximates testicle weight when multiplied by the specific gravity of solid tissue. Growth curves for body weight and data published for wild specimens were evaluated to obtain the weight most characteristic of male Pongo, and the ratio of testicle weight to body weight was calculated. The mean ratio for individuals with fully adult stature is 0.034, similar to but smaller than that of humans at about 0.050, and larger than the ratios reported for 5 gorillas at 0.013. The testicles mature faster than the body, however, so the mean ratio for young adult orang-utans is about 0.056 and resembles the ratio for humans more closely than the full adults. The differences between the ratios for a monogamous gibbon species, orang-utans, and humans is accounted for when testicle size relative to the weight of the female is considered. This is consistent with a sperm dilution effect produced by variation in the size of the female reproductive tract. The small relative testicle size of the gorilla is anomalous and requires verification as does the application of female size to scale the testicles. © 1993 Wiley-Liss, Inc.     

Mother-infant relations and infant development in captive chimpanzees and orang-utans

Four chimpanzee (Pan troglodytes)mother—infant dyads and four orangutan (Pongo pygmaeus)motherinfant dyads were studied for the first 11 months of the infants’ lives. For both species, ventroventral contact and nipple contact decreased over time at a similar rate, but total contact decreased earlier in the orangutans and was 50% lower than for the chimpanzees at the end of the study. Social play between the mothers and the infants did not differ in frequency between the species, but orangutans played above the ground and chimpanzees on the ground. Solitary play differed in form between the species and, like social play, reflected their differences in arboreal and terrestrial proclivities. In addition, the orangutans engaged in solitary play considerably more frequently than the chimpanzees during the second half-year of life. The developmental differences in mother-infant contact and solitary play of these apes are consistent with the differences in their speciestypical social organization. The data may reflect, therefore, early development of species differences in the social and relatively solitary natures of chimpanzees and orangutans, respectively. An erratum to this article is available at .     

Hand preferences in captive gorillas,orang-utans and gibbons

Hand preference was assessed in 12 gorillas (Gorilla gorilla gorilla), 13 orang-utans (Pongo pygmaeus abelii), and 9 gibbons (Hylobates lar) by using a floor retrieval task and a mesh retrieval task. Hand preference was also assessed in 8 gorillas and 8 orang-utans by using a task involving the unfastening of a hasp. A bipedal requirement during testing (mesh retrieval task) facilitated detection of hand preferences. A significant left-hand preference was found for the gibbons with 6 of 6 gibbons preferring their left hand on the mesh retrieval task. Similarly, a significant right-hand preference was found for the gorillas with 10 of 12 gorillas preferring their right hand on the mesh retrieval task. The data for the orang-utan suggest a bimodal distribution on all tasks. Since the gibbon and gorilla in the wild engage in bipedal locomotion more frequently than the orangutan, one possible interpretation for these results correlates the degree of bipedal behavior of a species in its natural environment with its readiness to exhibit a unilateral population-level hand preference.     

Assessing reproductive status in orang-utans by using urinary estrone

The aim of this work was to compare normal menstrual cycle patterns of urinary estrone excretion in orang-utans with those of an orang-utan in her late 40s, an orang-utan before and after rehabilitation for anorexia, and an orang-utan during prolonged lactation. A detailed menstrual and sexual activity record was kept for each orang-utan subject. Urine samples for estrone radioimmunoassay were collected daily for two cycles whenever possible. “Mawas,” in her late 40s, was monitored for two cycles in each of 3 successive years. While the normal pattern of estrone excretion was initially seen, some of the later cycles were shorter and appeared to lack the large luteal-phase peak. “Puspa,” anorexic and amenorrhoeic at the age of 9½ years, had very low urinary estrone concentrations. After rehabilitation, followed by a late menarche, she had regular menstrual cycles with a normal estrone excretion pattern. She gave birth to a normal infant in 1990. “Puteri,” who had been lactating for 2 years, had consistently low estrone concentrations during an 8-week monitoring period, with no evidence of cyclic variation in estrone excretion. Daily urinary estrone measurements, together with observation, were found to be useful in assessing the reproductive status of orangutans.