Orangutans are the only exclusively Asian genus of extant great ape. The largest living arboreal animals, they have proportionally longer arms than the other, more terrestrial, great apes. They are among the most intelligent primates and use a variety of sophisticated tools, also making sleeping nests each night from branches and foliage. Their hair is typically reddish-brown, instead of the brown or black hair typical of other great apes.
Native to Indonesia and Malaysia, orangutans are currently found only in rainforests on the islands of Borneo and Sumatra, though fossils have been found in Java, the Thai-Malay Peninsula, Vietnam and Mainland China. There are only two surviving species, both of which are endangered: the Bornean Orangutan (Pongo pygmaeus) and the critically endangered Sumatran Orangutan (Pongo abelii). The subfamily Ponginae also includes the extinct genera Gigantopithecus and Sivapithecus.
The word "orangutan" comes from the Malay words "orang" (man) and "(h)utan" (forest); hence, "man of the forest".
Taxonomic classification
  • Genus Pongo
  • Borneo Orangutan (Pongo pygmaeus)
  • Pongo pygmaeus pygmaeus - northwest populations
  • Pongo pygmaeus morio - east populations
  • Pongo pygmaeus wurmbii - southwest populations
  • Sumatran Orangutan (Pongo abelii)

The populations on the two islands were classified as subspecies until recently, when they were elevated to full specific level, and the three distinct populations on Borneo were elevated to subspecies. The population currently listed as P. p. wurmbii may be closer to the Sumatran Orangutan than the Bornean Orangutan. If confirmed, abelii would be a subspecies of P. wurmbii (Tiedeman, 1808). Regardless, the type locality of pygmaeus has not been established beyond doubts, and may be from the population currently listed as wurmbii (in which case wurmbii would be a junior synonym of pygmaeus, while one of the names currently considered a junior synonym of pygmaeus would take precedence for the northwest Bornean taxon). To further confuse, the name morio, as well as various junior synonyms that have been suggested, have been considered likely to all be junior synonyms of the population listed as pygmaeus in the above, thus leaving the east Bornean populations unnamed.
In addition, a fossil species, P. hooijeri, is known from Vietnam, and multiple fossil subspecies have been described from several parts of southeastern Asia. It is unclear if these belong to P. pygmaeus or P. abeli or, in fact, represent distinct species.

Ecology and behavior
Orangutans live in primary and old secondary forests, particularly dipterocarp forests and peat swamp forests. Both species can be found in both mountainous and lowland swampy areas. Sumatran orangutans live in elevations as high as 1500 m (4921 ft), while Bornean orangutans live no higher than 1000 m (3281 ft). The latter will sometimes enter grasslands, cultivated fields, gardens, young secondary forest, and shallow lakes. Orangutans are the most arboreal of the great apes, spending nearly all of their time in the trees. Most of the day is spent feeding, resting, and moving between feeding and resting sites. They start the day feeding for 2–3 hours in the morning. They rest during midday followed by traveling in the late afternoon. When evening arrives, they begin to prepare their nest for the night. Tigers are the major predatory threat to orangutans in Sumatra. Orangutans may also be preyed on by clouded leopards and crocodiles. The former can kill adolescents and small adult females but have not been recorded killing adult males. In Borneo, orangutans are not threatened by tigers and seem to descend to the ground more often than their Sumatran relatives. Orangutans do not swim, although least one population at a conservation refuge on Kaja island in Borneo have been photographed wading in deep water.

Reproduction and parenting
Male orangutans exhibit arrested development. They mature at around 15 years of age by which they have fully descended testicles and can reproduce. However they do not develop the cheek pads, pronounced throat pouches, long fur or long-calls of more mature males until they gain a home range, which occurs when they are between 15 and 20 years old. These sub-adult males are known as unflanged males in contrast to the more developed flanged males. The transformation from unflanged to flanged can occur very quickly. Unflanged and flanged males have two different mating strategies. Flanged males use long calls to advertise their location which attract estrous females. Unflanged males wander widely in search of estrous females and upon finding one, will force copulation on her. Both strategies are successful, however females prefer to mate with flanged males and will seek them out for protection from unflanged males. Resident males may form consortships with females that can last days, weeks or months after copulation.
A two-week old orangutan

Female orangutans experience their first ovulatory cycle between 5.8 and 11.1 years. These occur earlier in larger females with more body fat than in thinner females. Like other great apes, female orangutans have a period of adolescent infertility which may last for 1–4 years. Female orangutans also have a 22-30 day menstrual cycle. Gestation lasts for nine months with females giving birth to their first offspring between 14 and 15 years old. Female orangutans have the longest interbirth intervals of the great apes, having eight years between births.

Male orangutans play almost no role in raising the young. Females are the primary caregivers for the young and are also instruments of socialization for them. A female often has more than one offspring with her, usually an adolescent and an infant, and the older of them can also help in socializing their younger sibling. Infant orangutans are completely dependent on their mothers for the first two years of their lives. The mother will carry the infant during traveling, as well as feed it and sleep with it in the same night nest. The infant doesn’t even break physical contact with its mother for the first four months and is carried on her belly. The amount of physical contact soon wanes in the following months. When an orangutan reaches the age of two, its climbing skills are more developed and will hold the hand of another orangutan while moving through the canopy, a behavior known as "buddy travel". Orangutans are juveniles from about two to five years of age and start to exploratory trips from their mothers. Juveniles are usually weaned at about four years of age. Adolescent orangutans seek peers and play and travel with peer groups while still having contact with their mothers. Infanticide has not been recorded in the two orangutan species like it has in other primate species. Paternity uncertainty, the range patterns of female dispersals and the fact that the ovulation of females depends on food availability may make infanticide by males ineffective.

Orangutans in Indonesia has been protected by legislation since 1931, which prohibits possessing, killing or capturing orangutans.

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