Orangutans ; people of the forest
Mozart , Photo ©Thea Powell/OuTrop
Bornean orangutan - Pongo pygmaeus
Listed as endangered on the IUCN Red list.
Suborder: Haplorrhini
Infraorder: Simiiformes
Superfamily: Hominoidea
Family: Hominidae
Genus: Pongo
Species: P. abelii, P. pygmaeus
Subspecies: P. p. morio, P. p. pygmaeus, P. p. wurmbii
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Geography and range
Originally orangutans ranged throughout Southeast Asia all the way into southern China and were found on the island of Java and in southern Sumatra. Orangutans are now found only on the islands of Borneo – Pongo pygmaeus - and Sumatra with the Sumatran species (Pongo abelii) limited to the northern part of the island. Bornean orangutans – the species which OuTrop are lucky enough to work on - are widely distributed but few number throughout Kalimantan, Sabah, and Sarawak. Orangutans mostly live in peat swamp forests (like Sabangau were OuTrop work), tropical heath forests, and mixed dipterocarp forests.
Orangutan population density has been shown to correlate positively with the level of fruit availability. This means that in forests where a higher abundance of fruit and fewer or less drastic periods of seasonal fruit shortages occur, are preferred. Forests with fruit available on a more general basis have populations of much higher density . Generally speaking,since Sumatran forests seem more productive than forests in Borneo, Sumatran orangutan populations are denser than Bornean orangutan populations. Orangutans living in peat swamp forests live at higher densities than in other forest types.
Orangutans can live up to 50, sometimes 60 years in the wild. Orang-utans feed mainly on fruit, leaves and termites, with some supplementary feeding on bark and other invertebrates. OuTrop research has shown evidence of self-medication in orang-utans, through fur-rubbing with leaf species, or feeding on unusual leaves during periods of illness.
Most individuals spend their entire lives in the trees, although some large male orang-utans can spend a lot of time feeding and walking on the ground (see camera trap photo, left). Female orang-utans generally occupy a distinct home range of about 2km2 centred around key fruit trees, whereas large males range more widely through the ecosystem
Orangutans climb using both hands and both feet to hold onto branches as they move horizontally through the rain forest canopy – it's quite fantastic to watch the strength of their limbs transport such weighs on sometimes seemingly very small trees and lianas! Due to the position of their fingers and toes, these appendages are able to act like hooks. When they move along the ground, orangutans walk quadrupedally on their fists, not their knuckles as is seen in the other great apes. We spend them spend quite a lot of their time on the ground, usually sitting to eat termites or plant pith – but travelling on the ground to move fast and nimbly through the forest. When they are travelling on the ground, it is easy to loose sight of them! Occasionally we seen them moving bipedally, often when on the ground but reaching up to lift themselves into a tree.
Genetically diverging 1.5 million years ago, you can see the differences between the two species of orangutan in their appreance . Bornean orangutans tend to have darker coats which can be orange, brown, or even a maroon shade. They have shorter hair and shorter faces than their Sumatran relatives. Both male and female orangutans can have long beards (Courtenay et al. 1988; Rowe 1996). Young infants have paler faces but as they age, their skin changes to dark brown or almost black skin. Males have large, pendulous throat pouches and large cheek pads called flanges. Males and females are different in size – the males weigh in, on average, at around 87 kg (192 lb) and measuring 970 mm (3.18 ft) and females weigh, on average, 37 kg (81.6 lb) and measuring 780 mm (2.56 ft) (Markham & Groves 1990; Rowe 1996).
Economic change, the change in the global environments and the extensive land change and land management by humans has meant forest areas have reduced considerably. Humans and associated land change are progressively increasing the likelihood of forest species' extinction. Orangutan populations have suffered considerably due to the decline of good quality habiat - loosing well over 80% of their habitat in the last 20 years. It has been estimated one-third of the wild population died during the fires of 1997-98.
Now even the remaining habitat on two islands of Sumatra and Borneo is threatened. This loss of habitat is the result of economic pressures, the human need and desire for community and industrial development. Over 200 million people now live in Indonesia, and the needs of these people, most who own small areas of land becoming increasingly urban and to do with infrastructural development.
Context within OuTrop
OuTrop have studied orang-utans in Sabangau since our founders first visited the site in 1995. See our orang-utan research page here. We have been monitoring orang-utan density for thirteen years, and currently estimate the Sabangau catchment to support approx. 6,900 individuals, which is 12.5% of the remaining global population. Our monitoring data show substantial declines between 2000 and 2001, probably as a result of illegal logging in the forest in preceding years.
However, the work of the CIMTROP Patrol team, the designation of the National Park, and practical conservation projects from OuTrop have substantially reduced disturbance in Sabangau, and the orang-utan population now appears to be increasing slowly. Meet the OuTrop orang-utans here!
The orang-utan behaviour project, our flagship ape study, involves following and recording the behaviour of wild orang-utans in their natural habitat. We know and study more than 50 individuals around the Natural Laboratory. Our long-term objective is to understand orang-utan behaviour and ecology in the highly-specialised tropical peat-swamp forest, and how this is impacted by human activities. This includes research topics relating to (i) feeding behaviour and energetics, (ii) health and self-medication, and (iii) social networks, relatedness, communication and dispersal. This allows us to compare behaviour in peat-swamp habitat and other forest types, improving our understanding of orang-utan evolution and conservation. Our results are informing orang-utan conservation and management plans, including identifying the effects of logging damage on populations and behaviour; assessing area requirements for the protection of viable populations, and informing reintroduction programmes.