Orangutans

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Orangutans

Orangutans are large, long-haired, red apes. There are two species of orangutans: the Bornean orangutan (Pongo pygmaeus ), which inhabits lowland primary forests of Borneo, and the Sumatran orangutan (P. abelii ), which is found in a small area in the mountains of northwest Sumatra. Their distribution was once much more widespread, extending throughout the tropical forests of Southeast Asia. However, these apes are now endangered, mostly because of the clearing of their forest habitat to develop agricultural land, along with the effects of timber harvesting and hunting. Orangutans are related to the great apes of Africa, the gorilla and the chimpanzee. In the Malay language, orangutan means “man of the woods.”

Physical characteristics and habits

Orangutans are sexually dimorphic, with adult males being about twice the weight of females. The height of the male is about 54 in (137 cm), it weighs 130-200 lb (60-90 kg), and has an arm span of 7-8 ft (2.1-2.4 m). The long, coarse, coat of orangutans is typically reddish brown, but is bright orange in juveniles, and can be maroon or dark brown in some adults. The fur is especially long and shaggy over the shoulders, where it may reach up to 18 in (46 cm) in length. Orangutans lack the strong brow ridges of chimpanzees and gorillas, and have a dish-shaped face that is hairless and usually black; young animals have pinkish skin on the muzzle and around the eyes. As the males mature, they develop deep throat pouches which extend under the arms and over the shoulder. Cheek flanges of fatty tissue are present in both species. Male orangutans from Borneo have huge cheek flaps, resembling horse blinders, while males from Sumatra have relatively longer, oval faces with cheek flanges that extend sideways and a well-defined moustache and beard.

Orangutans are largely arboreal and spend little of their time on the ground. Although these apes are slow-moving and cautious, their long arms and hooked hands and feet provide an effective means of moving rapidly through the forest canopy. Orangutans are versatile climbers, using a modified over-arm brachiation as a mode of locomotion. Leaping or jumping over any distance is uncommon. Orangutans use their flexible joints and powerful hands and feet to distribute their weight over several small branches, any one of which might not support them. When a gap in the canopy is encountered, they use their weight to swing trees back and forth until the distance can be bridged. Most orangutans will occasionally descend from the trees to the forest floor, though the practice is most common among adult males. Once on the ground, orangutans move using a form of knuckle-walking, with the weight carried on the bunched fists rather than on the knuckles. They rarely walk in a bipedal fashion, and if they do so it is stiff-legged and awkward.

At night, orangutans sleep in nests built high above the ground. Most nests are built in the middle level of the forest canopy. Orangutans prefer to nest in places that afford good visibility, and they change the nest location each night. During the day, less elaborate nests are built for resting and protection against heavy rainfall. Just as humans use umbrellas, orangutans hold leafy branches over their head as a shield from rain.

Diet

Orangutans spend an average of one-third of their day foraging for food. They exhibit a bimodal feeding strategy, feeding most actively in the morning and late afternoon, and resting during the mid-day. Orangutans prefer to eat fruits just before ripening, including the spiny-skinned, pulpy, aromatic durian. As much as 60% of all food eaten is fruit. Other items of the diet include young leaves, shoots, lianas, bark, flowers, wood pith, mineral-rich soil, and small amounts of ants, bees, honey, and wasp galls. Small foods, such as berries and leaf shoots, are picked with their lips or fingers. Large fruits are held in the hands while eating. Water is obtained from succulent vegetation and what can be collected from tree holes. They dip a hand in the water, and then drink droplets from the hairy wrist.

Communication

Orangutans are the least vocal of the great apes, but they do have a system of auditory communication. Their most dramatic vocalization is the long call, given only by adult males. The call has been likened to the sound produced by large volumes of water roaring through steel pipes. The precise function of the long call is not known, but it probably serves to space out adult males in territories. It likely also serves other functions, including a display to attract sexually receptive females, and a signal to inform the community of the location of the dominant male. Males tend to call in bad weather, when another male is visible or calling, when close to a sexually receptive female, or as an element of copulatory behavior. Calls are audible up to 1.2 mi (2 km) from the source.

Other vocalizations include a variety of grunts, squeaks, moans, barks, and screams. Alarmed and agitated individuals produce “kissing” and “gluck-gluck-gluck” sounds that seem to indicate their level of annoyance. These noises are often accompanied by aggressive physical displays, such as shaking and breaking branches. Males use their huge size and other secondary sexual characteristics during intimidation displays. They inflate their throat sac and elevate the hair on their shoulders and arms to make themselves look larger. When threatening, individuals open their mouths wide to show their teeth, and when fearful, they extend their prehensile lips.

Behavior and reproduction

Orangutans are more solitary than the other apes. Adult males interact with other orangutans only to fight over responsive females and to mate. The most common social units are the mother and her immediate offspring, subadult males, and small groupings of adolescents of both sexes. Orangutans are long lived (around 50 years) and have extensive intervals between births. Females mature at about 10 years of age and remain fertile until about 30 years old. Although there is no visible evidence of estrus, when a female is pregnant, her swollen, white genitalia are readily observable. Gestation lasts 264 days. The young orangutan is totally dependent on its mother for food, protection, and transportation during its first year. The young are weaned at about three years and begin to climb and forage for their own food at age four. At six or seven years old, orangutans reach sexual maturity and become independent of their mother.

Conservation of orangutans

For thousands of years, the orangutan has been exploited by humans. Early humans found it an abundant source of food and hunted it to local extirpation in many areas. More recently in Borneo, it has served as a substitute for humans in traditional head-hunting rites. In the 1960s, the population of orangutans was decreased by the collection of young animals for sale to zoos and the pet trade. Despite legal protection by the governments of Malaysia and Indonesia, the capture of young orangutans has not yet been completely halted. There are now an estimated 12,000-15,000 wild orangutans in Borneo, and 4,000-6,000 on Sumatra.

Today the greatest threat to this species is the destruction and disturbance of their habitat. Old-growth tropical rainforest is rapidly being logged and cleared for agricultural development on Borneo, leaving only patches of suitable habitat. In recent decades, wildfires have also destroyed extensive areas of forest habitat. This has happened during years of unusual drought associated with El Nino events, when fires lit by people to develop agricultural land accidentally escape into the wild forest.

The orangutan is extremely sensitive to human intrusions, and as a consequence suffers reductions of an already low rate of reproduction when frequently disturbed. For this reason, research on how it responds to alteration of its habitat, especially selective logging, is necessary. Several protected reserves for this species have been designated in Sumatra and Borneo, but these reserves must be managed effectively and additional habitat must be preserved if the orangutan is to escape extinction.

See also Primates.

Resources

BOOKS

Ciochon, Russell L., and Richard A Nisbett, eds. The Primate Anthology. Englewood Cliffs, NJ: Prentice Hall, 1997.

KEY TERMS

Arboreal —Living in trees.

Brachiation —To swing by the arms from branch to branch.

Dimorphic —Having two distinct forms.

Durian —The tree and fruit of Durio zibethinus, a plant cultivated in Southeast Asia. The fruit is 6-8 in (15-20 cm) in diameter and has a hard external husk covered with coarse spines. Inside, five oval compartments are filled with sweet, custard-like pulp.

Flange —A protruding rim, edge, rib, or collar.

Genitalia —The reproductive organs, especially the external sex organs.

Gestation —The period of carrying developing offspring in the uterus after conception; pregnancy.

Prehensile —Adapted for seizing or holding, especially by wrapping around an object.

Succulent —Having thick, fleshy leaves or stems that conserve moisture.

Galdikas, Birute M. Orangutan Odyssey. New York: Abrams, 1999.

Gould, Edwin, and Gregory McKay, eds. The Encyclopedia of Mammals. 2nd ed. New York: Academic Press, 1998.

Napier, J. R., et. al. The Natural History of the Primates. Cambridge, MA: The MIT Press, 1985.

Preston-Mafham, Ken, et al. Primates of the World. New York: Facts on File, 2003.

Rijksen, H., and E. Meijaard. Our Vanishing Relatives: The Status of Wild Orang-utans at the Close of the Twentieth Century. Dordrecht: Kluwer, 1999.

Romer, Alfred S. The Vertebrate Story. Chicago: The University of Chicago Press, 1959.

Tuttle, Russel H. Apes Of The World: Their Social Behavior, Communication, Mentality, and Ecology. New Jersey: Noyes Publications, 1986.

van Schaik, Carel, and Perry van Duijnhoven. Among Orangutans: Red Apes and the Rise of Human Culture. Cambridge, MA: Belknap Press, 2004.