Mammals
Orangutan
Pongo · also called orang-utan, person of the forest
An orangutan is a great ape that lives almost its entire life in the trees, which is remarkable given that a big male weighs as much as a person. It is also the odd one out among apes: while gorillas and chimpanzees live in groups, an orangutan forages alone, because the forest fruit it depends on is too scattered to feed a travelling troop. Its name means “person of the forest”.
Overview
Orangutans are the genus Pongo, three species — the Bornean, Sumatran, and Tapanuli orangutans, the last recognised only in 2017. They live in the lowland and hill rainforest of Borneo and northern Sumatra and eat mostly fruit, especially figs, tracked across the forest as different trees come into season, with leaves, bark, honey, and insects filling the lean months.
A slow, solitary life
Adult orangutans are largely solitary — a female with her young, a male alone — a pattern thought to reflect a fruit supply too thin for a group. They are also the slowest-breeding mammal: first birth at ~15, then one baby every 7 to 9 years, each staying with its mother for 6 to 8. Mature males come in two forms — big flanged males with cheek pads and a booming long call, and female-looking unflanged males that can “switch on” flanges within months if the dominant male leaves.
Tools, culture, and nests
Sumatran orangutans use sticks as tools and leaves as gloves and umbrellas, and different populations have different learned traditions. Every orangutan builds a fresh sleeping nest high in a tree each night, sometimes with a pillow and roof.
Status
All three species are Critically Endangered. The Bornean orangutan (~100,000) has more than halved in 60 years; the Sumatran numbers ~14,000; the Tapanuli — in one forest block, under 800 animals — is the most endangered great ape. The cause is the clearing and burning of rainforest for oil palm and pulpwood, plus killing as crop pests and the capture of infants for the pet trade.
Orangutan: common questions
Why are orangutans solitary when other great apes live in groups?
It comes down to food. Gorillas eat abundant, evenly spread leaves and stems, and chimpanzees eat fruit but supplement it by hunting and by feeding in large fig trees, so both can support cohesive groups. Orangutans depend heavily on fruit in a forest where fruiting is patchy and irregular, with long lean periods broken by sudden gluts, and a large-bodied ape moving through the canopy cannot afford to share a scarce, scattered resource with a permanent troop. So adult orangutans forage alone, with females accompanied only by their dependent young. They are not antisocial; individuals recognise their neighbours, tolerate each other at big fruiting trees, and young orangutans and sub-adult females sometimes travel together for a while, but the default is solitary.
What is the difference between a flanged and an unflanged male orangutan?
They are two developmental paths for adult males of the same species. A flanged male has fully developed secondary sexual features, wide fleshy cheek flanges, a large throat sac, a heavy build up to twice the weight of a female, and the ability to give the long call, a series of roars and bubbling groans that advertises his presence and dominance. An unflanged male is sexually mature and can father offspring but retains a female-like appearance and is much smaller, and he mates by roaming widely and seeking out females, sometimes coercively. A male can remain unflanged for many years, and the development of flanges appears to be socially triggered, switching on within a few months when a resident flanged male dies or leaves, which suggests staying unflanged is a strategy rather than simply immaturity.
Are orangutans as intelligent as chimpanzees?
They are in the same league, though tested somewhat less because they are solitary and hard to observe. Orangutans in the wild make and use tools, plan routes through the canopy, and learn population-specific techniques for extracting difficult foods, and captive orangutans are notorious for problem-solving and for dismantling their enclosures, undoing bolts, picking locks, and hiding tools for later use. They also do well on tests of memory, imitation, and understanding of cause and effect, and some have learned to use symbols or sign-based communication. Their intelligence seems geared toward the practical challenges of finding and processing food and moving safely through a three-dimensional forest, and toward remembering, over years, where and when particular trees will fruit.
Why is palm oil linked to orangutan decline?
Oil palm grows best in exactly the warm, wet lowland conditions that support the richest rainforest, and much of the recent expansion of plantations in Indonesia and Malaysia has come from clearing and draining that forest, including the peat swamps where many orangutans live. When forest is felled or burned for a plantation, the orangutans in it lose their food and shelter, become stranded in shrinking fragments, and are often killed when they raid the young palms or are caught trying to cross open ground. Palm oil is in a huge range of foods and products, and demand is still rising, so the pressure on orangutan habitat continues, although certification schemes for deforestation-free palm oil and the protection of key forest blocks are attempts to slow it.