Bornean orangutan

Name and taxonomy

The word orangutan comes from Malay, and it means person of the forest. The genus name Pongo comes from a Bantu word, mpongo, for a large primate, which once described chimpanzees. The species name pygmaeus comes from the Greek word for a dwarf, an odd choice for a large ape. Carl Linnaeus first described the animal under the name Simia satyrus, which means satyr monkey. The type specimen, the holotype, is kept in the British Museum in London.

The species belongs to the family Hominidae, which also holds gorillas, chimpanzees and humans. Within that family it sits in the subfamily Ponginae, the only great ape group native to Asia. Three subspecies are recognised: P. p. pygmaeus in Sarawak and western Kalimantan, P. p. wurmbii in the south, and P. p. morio in the north-east. The split is not settled, because the southern form may be closer to the Sumatran orangutan than to the other Bornean forms. Some authors treat P. p. morio as a synonym of P. p. pygmaeus.

Evolution and relatives

The orangutan line and the human line parted about 14 to 18 million years ago. Fossils show that orangutans once lived on the Asian mainland, up into southern China, and on Java and Sumatra. The first complete fossil skeleton came from Hoa Binh Province in Vietnam and dates from the late Pleistocene. That animal differed from a living orangutan only in having a smaller body in proportion to its head.

The Bornean and Sumatran orangutans were treated as one species until 1996, when studies of mitochondrial DNA separated them. The two split about 400,000 years ago, but a small amount of gene flow continued after that. The Bornean line diverged from the Tapanuli orangutan about 674,000 years ago, and the Tapanuli form was named as a species in 2017. Bornean orangutans carry less genetic diversity than Sumatran ones, because their population went through a sharp decline in the Middle Pleistocene. Orangutans share about 97% of their DNA with humans.

Appearance

The Bornean orangutan has a heavy body, a bulging belly and arms that can be up to 1.5 metres long. A wild male measures 120 to 170 centimetres and weighs about 75 kilograms on average. A female measures 100 to 120 cm and averages about 38.5 kg. Captive animals can grow far heavier, and one obese male named Andy weighed 204 kg in 1959. The skin is grey, and the coat is coarse, shaggy and orange to brown, often darker than a Sumatran orangutan's.

The face is bare apart from a beard and moustache, and infants have pink faces that darken with age. Pale rings of skin around the eyes stay visible through adolescence. The hands and feet both grasp, and the thumb is short, so food is picked with the fingers and palm. A fully mature male grows fatty cheek pads, a much larger throat sac and bigger canine teeth than a female. Males come in two adult forms: a large flanged male and a smaller unflanged male that looks more like a female.

Habitat and range

The species lives only on Borneo, in the Malaysian states of Sabah and Sarawak and in most of Indonesian Kalimantan. Its presence in Brunei is unconfirmed. Habitat loss has broken the range into patches, and the ape is rare in the south-east of the island. It is also absent from the forest between the Rajang River in Sarawak and the Padas River in Sabah. In 2014 researchers found about 50 orangutans in South Kalimantan, a province with no earlier record of the species.

The best habitat is mature riverside forest, freshwater swamp, peat swamp and lowland dipterocarp forest. Swamp forests fruit more regularly than dry forest on the hills, so they hold the highest densities. Most animals live below 500 metres, and the species reaches about 1,500 m in montane forest. It uses old-growth forest and also secondary forest, and it travels long distances to reach trees in fruit. Large rivers and mountain ranges act as barriers, because orangutans cannot swim. GBIF holds records from 19 countries, but that count includes zoos, not the native range.

Diet and feeding

Fruit makes up more than 60% of the diet, and the ape moves through the forest to follow the fruiting season. Wild figs and durians are favourites, and the full menu runs to more than 400 kinds of food. Leaves, shoots, seeds, flowers, sap, inner bark, honey, fungi and bird eggs fill out the list. It also eats insects such as termites, ants and caterpillars, and even spider webs. On rare occasions it takes a small vertebrate, and it has been seen eating dead fish and catching live ones.

Borneo has poor soils, so fruit crops swing sharply from year to year. In El Nino droughts the orangutan falls back on inner bark, and its heavy jaws and thick enamel suit that hard food. In good years it gorges on fruit and dipterocarp seeds and lays down fat for lean months. It drinks from fruit and from water that collects in tree holes. In some areas it eats soil, which probably helps to neutralise toxins in its plant food.

Behaviour and social life

The Bornean orangutan is active by day and spends most of its time in the canopy. It feeds in the morning, rests in the middle of the day, then feeds and travels again until dusk. Adults live alone for most of the year, and the Bornean species is even less sociable than the Sumatran one. Forests on Borneo produce less food, so animals that fed together would compete. Two or three animals with overlapping ranges may meet for a short time, and larger groups gather only at a mass fruiting.

A dominant flanged male holds a large home range that overlaps with the ranges of three or four females. He tolerates no other adult male nearby, and unflanged males keep quiet and out of his way. An adult male range covers about 2 to 6 square kilometres. A young female settles next to her mother's range, while a young male travels much farther. Average density across Bornean forests is about 1 to 3 animals per square kilometre.

Every evening the ape builds a sleeping nest from bent branches, with a base and a leafy mattress. Some nests get a roof against rain, a pillow or a blanket of leafy twigs. Young orangutans learn the skill by watching their mothers, and they practise on play nests. The species uses tools. A leafy branch serves as an umbrella or a fly swat, a pad of leaves holds a spiny durian, and a stick scratches the back. Bornean orangutans walk on the ground more often than Sumatran ones, probably because Borneo has no tigers. On the ground an adult walks on its fists, not its knuckles like other great apes.

Communication and senses

The main signal is the long call, which only a flanged male gives. It lasts one to two minutes, and the throat sac helps it carry for about 1 kilometre, or several km in still air. The call tells other males where he is and draws in females ready to mate. An unflanged male that hears it moves away, and the sound may even hold back his own development into a flanged male.

A fast call follows a fight between males. In a small group the apes smack their lips, and a frightened orangutan pushes its lips out and screams. Orangutans have fewer social calls than other great apes, which fits their solitary lives. Like other great apes, the orangutan relies mostly on colour vision to find ripe fruit in the canopy. Captive animals recognise themselves in a mirror, plan ahead and can deceive.

Reproduction and life cycle

Males and females meet only to mate, and there is no fixed breeding season. A female's cycle runs 22 to 30 days, and she is most fertile when fruit is plentiful. Females prefer flanged males and answer their long calls, though a smaller unflanged male often forces a mating. A female can dodge a flanged male in the canopy because she is far lighter, but an unflanged male matches her size and speed. A pair may stay together for several days before they part.

Gestation lasts about 8.5 months, and a single infant is born, though twins are recorded. The newborn clings to its mother's belly with its fingers in her hair, and it nurses every three to four hours. At about four months it takes soft food from her lips, and later it rides on her back. Weaning is usually complete by about four years, and the youngster stays with its mother until the next birth. Full independence comes at about seven years, and both sexes then leave the mother's range.

A female first ovulates between about six and 11 years, and gives birth for the first time at about 14 to 15. Births come 6 to 8 years apart, one of the longest intervals of any mammal. In a lifetime a female rears about four or five young, and captive females reach menopause at about 48. A male is unflanged from about 8 to 15 years, and most grow flanges between 15 and 20. Wild animals live 35 to 45 years, and the captive record is 59 years.

Predators and threats

An adult Bornean orangutan has no natural predator, because Borneo has no tigers. A clouded leopard can kill a young orangutan, and a crocodile is a risk at the water's edge. People are the real predator, and even hunting at a 2% yearly rate is more than the population can bear. A 2011 survey of 6,983 people in 687 villages estimated that 750 to 1,800 orangutans were killed in one year in Kalimantan. Most respondents, 73%, knew the species was protected, yet prosecutions are rare.

The greatest threat is the loss of forest to logging and to oil palm plantations. When forest is burned to clear land, orangutans burn with it. Two fires, in 1983 and 1998, burned 90% of Kutai National Park, and its orangutans fell from about 4,000 to about 600. In the 1997 to 1998 fires an estimated 8,000 orangutans died in Central Kalimantan, a third of the species in one year. Young animals are captured for the pet trade, which usually means the mother is killed. Crop raiders near plantations are shot, and one rescued female carried 40 air-rifle pellets.

Population and conservation

The IUCN Red List rates the Bornean orangutan as Critically Endangered, and CITES lists it on Appendix I. About 104,700 animals remain, far more than the 14,000 or so Sumatran orangutans. Even so, the total is less than 14% of the population that lived on Borneo before the middle of the 20th century. Numbers fell by more than half in the last 60 years, and the fall continues at the pace of forest loss. Sabangau National Park holds about 6,900 animals, but its peat forest is itself at risk.

No more than 16% of the remaining habitat is protected, and most orangutans live outside reserves in forest slated for logging or oil palm. Malaysia and Indonesia both protect the species by law, and Sarawak lists it as a totally protected animal. Rescue centres take in orphans and teach them forest skills before release. Sepilok in Sabah opened in 1964 as the first rehabilitation centre, and groups such as the Borneo Orangutan Survival Foundation run others. iNaturalist holds 3,449 observations of the species.

Relationship with humans

People and orangutans compete for the same land and sometimes the same durian fruit. Palm oil brings in more than a tenth of Indonesia's export income, and its plantations replace orangutan forest. Plantation staff have paid hunters for dead orangutans, and one case in 2011 involved at least 20 animals. Because the two species are so alike, orangutans can catch human diseases, so contact with people is itself a threat.

Ecotourism at rehabilitation centres brings money to conservation groups and to local communities. Zoos hold Bornean, Sumatran and hybrid orangutans, and a seven-year zoo study found that happier animals lived longer. Wild orangutans usually avoid people, but an ape defending fruit will on rare occasions attack. In both Indonesia and Malaysia the species now serves as the public face of rainforest conservation.

In culture and myth

In Malay the name means person of the forest, and the ape was long seen as a kind of human. Malay and Bornean tales describe forest people who refused speech, because a creature that talks can be made to work. Linnaeus named the animal after the satyr, the half-human forest spirit of Greek myth. The idea of a silent forest person probably grew from the ape's human-like face and hands.

Today the orangutan stands for the rainforest itself in campaigns against palm oil and illegal logging. Rescue centres such as Sepilok and Nyaru Menteng have made orphaned orangutans a familiar image on television. As with the giant panda, the ape's gentle face likely helps it to win public support that less appealing species do not get.