by Nardie Hanson

Created on: May 16, 2010   Last Updated: May 17, 2010

Why is the Sumatran orangutan (Pongo abelii) going extinct?

We all recognise the characteristic red fur, long arms and the thoughtful expressions of the orangutan. There is something in all the great apes that we relate to and recognise as humanistic, we ourselves, Homo sapiens, belong to the same group. So how is it that we are causing their depletion and far too rapid extinction?

There are currently four recognised species of orangutan restricted to the rainforests of the islands of Borneo (P. pygmaeus pygmaeus, P. pygmaeus wurmbii, P. pygmaeus morio) and Sumatra (P. abelii). The Bornean species’ distribution is mainly in the South of Borneo with some in the North-East, spanning two of the three nations that share the island, Malaysia and Indonesia. The Sumatran species is restricted to a small area of two provinces in the North of the island, Aceh and Sumatera Utara. All species are recognised as endangered, but the Sumatran orangutan is the most vulnerable and is listed as critically endangered (the International Union for the Conservation of Nature, IUCN). The most recent population estimates are 7,400 Sumatran and about 45,000 Bornean orangutans (all species).

Orangutans represent the only great ape outside of Africa and are predominantly arboreal, travelling; feeding; mating and nesting in the canopy. The Sumatran orangutan especially, very rarely visits the ground, except maybe to cross particularly large gaps in the tree crowns or where peripheral branches can not support their weight, particularly an issue for large males. They invest a huge amount in their offspring and have very long inter-birth breaks, offspring staying with the mother for around nine years.

The Sumatran orangutan in particular is recognised to have the longest inter-birth break of all great apes except for man. Time with the mother is spent learning how to be an orangutan, how to move around in the unpredictable environment of the canopy; how to socialise; how to find the best foods and what are the best routes through the tree tops. The mother and infant duo is the most common social unit as orangutans are generally solitary. This is mainly due to the sparse availability of their favoured food, fruit. However, during mast years (when the trees produce a particularly high abundance of fruit) and food is plentiful orangutans come together and form strong social bonds and hierarchies, headed by a large dominant male. Mature and dominant males (73-91 kg) are larger than females

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