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Kaplan G  (2012)  Orang-utans and the Rainforest.  ECOLOGY.INFO 26

Orang-utans and the Rainforest

Gisela Kaplan
Centre for Neuroscience and Animal Behaviour
School of Biological, Biomedical and Molecular Sciences
University of New England
Armidale, Australia

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

We humans have had a contradictory and problematic relationship with primates, especially the great apes. We have hunted, killed and in many instances used them as objects for our amusement (Eudy 1994). In some of our cultures, great apes are still considered simply as resources: as good food, as sources of artefacts that confer strength and special powers to their owners, and as objects for display. In others, great apes are valued even less, being regarded as pests or as dangerous competitors for living space (Kaplan & Rogers 1995). The idea that apes may be important for ecology is more recent, as is the kind of popular adulation now in vogue that emphasises how closely we are related to the great apes. Some of the modern romanticism about great apes is also linked to particular schools of thought or, simply, to a market niche in eco-tourism (see below).

One aspect of human-ape encounters, however, is very special: the moment of a voluntary contact by a great ape with a human. Some of these singular events have dramatically changed the attitudes of individual humans to great apes and led to projects specifically for the welfare of apes. Going one step further, they have also inspired animal rights based on a new understanding that apes are sentient beings. My own experience in Borneo with a juvenile orang-utan (Pongo pygmaeus), who decided to adopt me as its foster mother, may not equal the dramatic experiences of others but was entirely unexpected, overwhelming and ultimately life changing. A small orang-utan that clamoured up onto my lap and expressed in a multitude of little gestures that it felt comfortable, warm and wanted to be loved was, if nothing else, utterly irresistible (Kaplan and Rogers 2000). Often, it is this quality of the great apes that has spurred modern researchers to learn more about them and to form new attitudes which, in turn, have changed the questions they asked and the goals they have pursued.

In this report, I review the ecology of the orang-utan. The orang-utan occurs only on the Southeast Asian islands of Sumatra and Borneo where it lives in tropical rainforest. Like its African relatives, the gorilla (Gorilla gorilla), chimpanzee (Pan troglodytes) and bonobo (Pan paniscus), the orang-utan is primarily diurnal. However, while all great apes climb trees, the orang-utan is the most arboreal of all and spends large amounts of its time in the trees.

General Behaviour

Life Cycle

Orang-utans have long childhoods, long life-spans and a low reproductive rate. They may live as long as forty years in the wild and as long as sixty in captivity. Like human primates, orang-utans have very extended ‘childhoods’. Young orang-utans stay with their mother at least up to the age of five, before they begin to seek their own company and independent feeding grounds. Females tend not to reproduce until they are about 12-15 years of age (but, like humans, they are sexually mature much earlier) and males may need even longer to gain full adult status (Galdikas 1984, 1985abc). Full adult status depends in part on whether another adult male is in the area or not (presence suppresses full development, absence speeds it up). Among mammals, or any animals, this is a very long period of development and a time for learning on how to survive in a very complex and difficult environment. At best, a female will optimally produce four live offspring during her lifetime but this figure reduces substantially when circumstances are unfavourable (Galdikas and Wood 1990). An adult female will not become pregnant again until her current offspring has reached independence (Galdikas 1995), and seems to abstain from sexual partnerships altogether when there is a scarcity of food or a stress inducing problem (such as logging, fires). Many young orang-utans have been orphaned due to illegal activities. This is highly problematic as rehabilitation is therefore a long drawn-out affair and difficult to achieve.

Social Organization

The social organization of orang-utans differs substantially from that of other apes. Orang-utan offspring are not shielded and surrounded by many relatives (as is the case in chimpanzee, bonobo and gorilla societies) who could teach them different things. Their main social unit is the mother with her offspring, usually one offspring at a time, and the death of a mother is usually catastrophic for the infant. Larger congregations of orang-utans generally occur only at favorite fruiting trees, usually a fig (Ficus spp.) or durian tree (Durio zibethinus). However, adults typically spend no more than 10 percent of their time in social encounters with each other (Galdikas 1985ab), at least not on Borneo, and most of this contact is between consorting males and females. Males have very little contact with other males, and such contacts are generally hostile (Hornaday 1885; Galdikas 1995). Subadult males usually choose females for company (Galdikas 1984, 1985c).

Social organization may also vary between different populations of orang-utans (Mitani et al. 1991). For example, it seems that Bornean orang-utans are semi-solitary most of the time, while Sumatran orang-utans are semi-solitary only some of the time and tend to be more group orientated. Delgado and Van Schaik (2000) and Van Schaik (2003) argue that semi-solitariness is a consequence of feeding competition. Where food is abundant and available on a relatively stable basis, orang-utans may form parties that are comparable with those formed by chimpanzee societies. Supporting this idea is the fact that the large number of fig trees available to orang-utans on Sumatra, are correlated with congregation numbers of orang-utans there. Unfortunately, we do not have more than anecdotal information about orang-utan social groupings on Borneo at a time when fig density there was similar to that of the last remaining reserves on Sumatra.

Differences in social organisation could also be related to other factors, such as predation hazard. For example, the presence of tigers (Panthera tigris) on Sumatra could explain the more stable formations of orang-utan groups there, compared to Borneo where tigers are absent.

Nest building

Adults feed and sleep alone, unless an adult female has an offspring. Sleeping arrangements include nest building (Rayadin & Saitoh 2009). Other ape species also build nests, but this behaviour is particularly extensive in orang-utans. A nest provides support and comfort for sleeping, shelter from heavy rain, and convalescence when an orang-utan feels ill. In the later part of pregnancy, a female may build several nests a day. (Davenport, 1967). The nest needs to be a relatively solid structure to support the weight of an adult female (about 40 kg) and her offspring, or that of a male weighing up to 80 kg. Nests may be built at great heights, and if the construction is flimsy, the individual may fall to its death. Some have argued that construction of nests should be regarded as tool use. Orang-utan young practise nest building from the age of 14 months to about 5 years of age, at which time they can usually construct good solid nests.

Offspring usually prefer to sleep together with their mother in a nest that she has prepared for the night. However, there comes a day when the mother no longer tolerates the growing youngster in her nest and denies it access. On these terrible days, one can witness the persistent, dramatic and very noisy temper-tantrums of the energetic offspring, sometimes lasting through most of the night. In this battle of wills, the young may win for a protracted period. However, because the weight of the growing youngster can cause the leafy nest to collapse, the juvenile is eventually forced to make its own bed (Kaplan and Rogers 2000).


We know relatively little about orang-utan communication although this field is growing (Rogers and Kaplan 2000). Communication consists of a variety of signals, including body posture, facial expression, gesturing, touching, and eye and eyelid movements, as well as vocalisations (Marler and Tenaza 1977). Much of the communication between mother and infant appears to occur by touching or in gestures. The mother signals for the infant to return to her body before she moves off by extending a limb, usually a leg, to touch the infant. The infant returns to cling to her body instantly (Kaplan and Rogers 1995). It is conceivable that the gesture is accompanied by low intensity vocalizations that cannot be heard by observers. In mother-infant interactions eye contact may be quite important. As with chimpanzees, juveniles may beg for food from the mother by shifting their gaze back and forth between the mother’s eyes and the food item (Bard 1990).

One of the first systematic descriptions of vocalizations of orang-utans was provided by MacKinnon (1971). About 18 different calls were described, mostly concerning distress or warning (MacKinnon 1971), including the 'fast call' emitted only by males after conflict or other contact (Galdikas and Insley 1988). Infants produce a low intensity purring, contentment call (Kaplan and Rogers 1994). The most distinctive vocalization of orang-utans is the 'long call', a high intensity bellow, produced by first filling the cheek pouches with air and then expelling this air over the vocal chords for a prolonged period. The long call can be heard over great distances, and is commonly considered to be a territorial call either to keep other males away (Mitani 1985) or to attract sexually receptive females into the male's territory (Galdikas and Insley 1988).

Orang-utan behaviour is, however, not immutable. It is clear from studies of orang-utans in captive environments and of orang-utans on Sumatra (Rijkseen and Meijaard 1999; van Schaik et al. 2004) that, under certain conditions, they may even adapt to living in close proximity to other orang-utans. Sumatran orang-utans are more group oriented than Bornean orang-utans now, but it remains an open question what their social life was like before the most crucial events for orang-utan decline occurred; i.e. the onslaughts of killing sprees of orang-utans (from the late 19th century onwards) and the large scale logging activities (from the 1960s onwards). What we perceive in their behaviour now may already be forms of adaptations to a shrinking habitat and other unrelenting pressures that impinge on their lives today.

Continue reading this review on Page 2.

The photograph at the top of the page was taken at Sepilok Orang-utan Sanctuary in Sabah (Borneo), Malaysia, by Brad Stoney (Australia).

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