Orangutans

Orangutans are great apes of the genus Pongo; they are the largest living nonhuman primates after gorillas, and one of the most critically endangered. They live only on the islands of Borneo and Sumatra, in Southeast Asia. They are found over most of Borneo, [Page 1786]but in Sumatra they are restricted to the northern quarter, with a few isolated populations some way down the west coast. The name is a corruption of orang hutan (“forest-living person”) and may originally have designated tribal people such as the Penan of Borneo or the Kubu of Sumatra, but it became commonly used in Europe to refer to any ape. Most indigenous peoples refer to them as mawas.

Orangutans have sparse reddish hair covering dark grey skin. The arms are longer, compared to the legs, than any other primate: the intermembral index (arm length as a percentage of leg length) varies between 135 and150%. The face is strongly prognathous (the jaws jut forward); the profile varies from concave to nearly straight; the orbits are tall and oval, without supraorbital tori (thickened brow ridges); the forehead is round and ascends at about 60° behind and above the orbits; the braincase is short, high, and rounded. As in other great apes (gorillas and chimpanzees), the canine teeth are relatively long but thick in males and shorter and smaller in females, but unlike other great apes, the orangutan's upper central incisors are expended and spade-shaped, while the lateral incisors are thin and pointed. The molar teeth are large and flat, with thick enamel that is minutely wrinkled. Fully grown male orangutans weigh from 80 to 91 kg, adult females less than half of this, from 33 to 45 kg.

Source: © Photo by Richard T. Nowitz/National Geographic/Getty Images.

Genetically, orangutans differ from gorillas, chimpanzees, and humans by about 3.1%, implying that their ancestors separated from them before they separated from each other; in other words, orangutans are the third most closely related ape to us. These phylogenetic relationships are recognized by placing them all in the family Hominidae, but in different subfamilies: orangutans belong by themselves in the subfamily Ponginae, while gorillas, chimpanzees, and humans belong in the subfamily Homininae.

Bornean and Sumatran orangutans are very different and are today placed in different species. The Sumatran orangutan (Pongo abelii) is taller and leaner in build, the hair is usually gingery in color (though there is some variation in this), and the face is covered in light downy hair and is an elongated oval shape. The Bornean orangutan (Pongo pygmaeus) is shorter and fatter, the hair is usually dark maroon, and the face is hairless and figure-8 shaped, with deep hollows under the cheeks. The chromosomes have characteristic differences. Molecular clock calculations suggest that they may have separated as much as 1.5 to 2 million years ago.

Orangutans are almost wholly arboreal—except, perhaps, for some old Bornean males—and largely solitary. This does not mean that they lack a social organization; on the contrary, their social relations are highly complex and reflect their prodigious cognitive skills, which seem to be barely, if at all, inferior to those of chimpanzees (like chimpanzees, some orangutans quickly learn to recognize themselves in mirrors). They feed mainly on fruit, falling back on figs during periods when fruit is scarce, and also eat leaves, bark (especially in Borneo),and insects (especially in Sumatra). In one study site, Suaq Balimbing in northern Sumatra, females occupy overlapping home ranges averaging850 ha, whereas fully grown males have [Page 1787]much larger ranges, about 2,500 ha, which overlap those of both females and other males. In Borneo, these home ranges appear to be generally smaller. Females may associate together temporarily, but males do their best to avoid each other. Males who are not fully grown have similarly large ranges, and avoid the full-grown males. There are also nomadic males who may wander from site to site looking either for food or for fertile females.

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