Mammals

Orangutans eat, sleep and travel in the trees. They are active during the day, and spend the majority of their time searching for and consuming food. Fruit such as figs, durians, jackfruit, lychees, mangosteens and mangos are eaten when available, along with smaller quantities of leaves, seeds, ants, termites and bark. Individuals tend to switch to eating these lower quality foods when fruit is scarce, rather than move to another area. In some areas over 400 different types of food have been recorded as part of the species’ diet. At night, individuals sleep in a nest made of twigs or leaves high up in the trees. They generally make a new nest each night, although old nests are occasionally re-used.

Unlike other diurnal primates – which are social and gregarious – orangutans lead a semi-solitary lifestyle. This is thought to be primarily due to the scattered distribution of their food. There is generally not enough fruit in an area at any one time to support a large group of orang-utans. Adult males occupy large, overlapping home ranges, which encompass those of several females. They spend almost all of their time alone, coming together with females only to mate. Although males are not territorial, they are generally hostile to one another. Confrontations are rare, as the male’s long-call is thought to serve to repel rivals, as well as advertise his availability to sexually receptive females. Female orangutans are slightly more social than the males. They maintain small, overlapping home ranges, and occasionally come together to feed when food is plentiful. In general, juvenile males and females display the most social behaviour, and are often observed feeding, playing and travelling together in groups. Social behaviour in Sumatran orangutans tends to coincide with the simultaneous (or “mast”) fruiting of fig trees, which does not occur in Borneo. The fact that Sumatran orangutans have more opportunity to come together to eat fruit that the Bornean species has enabled greater sociality among individuals. This in turn has enabled them to learn tool-using behaviour from one another. Orangutans play an important role in their forest habitat; they act as seed dispersers and help to open up the forest canopy which allows light to reach the forest floor so that the forest can regenerate.

The orangutan is the slowest breeding of all primates. Females reach sexual maturity at around 12-15 years of age, and generally give birth to a single young every 8-9 years thereafter, depending on the quality of the habitat. Infants are totally dependent on their mothers, and are carried constantly for the first two to three years of their lives. Weaning occurs at around 3.5 years, but the infant will remain close to its mother for at least another three years, learning the spatial and temporal patterns of fruiting in the forest. When the young disperse, females usually set up a home range adjacent to their mothers, while males tend to travel further afield. Life expectancy in the wild is thought to be more than 50 years. The incredibly long inter-birth period – the longest of any land-based mammal – means that a female orangutan can produce a maximum of four surviving young during her lifetime.

Inhabits lowland tropical rainforests and swamps up to 800 meters above sea level.

There are believed to be around 6,600 individuals surviving in just ten fragmented habitat units.

Decreasing. The population is believed to have declined by more than 80% in the last 75 years.

Classified as Critically Endangered (CR A2cd) on the 2010 IUCN Red List of Threatened Species.

The Sumatran orangutan is listed as one of the top 25 most endangered primates. The greatest threats to species are habitat loss and fragmentation resulting from logging, conversion of forest to agricultural land and oil palm plantations, and road construction. Habitat supporting around 1,000 orangutans was being lost each year in the Gunung Leuser Ecosystem alone during the late 1990s. This was largely due to legal logging concessions and conversion of lowland forests to palm oil estates, but illegal logging and encroachment was also taking place in some areas.

The rate of habitat loss decreased markedly in many areas during the Aceh civil conflict, as activities in the forests became unsafe, and as a result of a moratorium imposed on logging in the province by the Aceh government. However, a peace deal negotiated in 2005 led to political stability and many new applications to open up logging concessions and palm oil estates in orangutan habitat. Of major concern is the re-issuing of logging permits for large tracts of forest in Aceh and the controversial Ladia Galaska road project in the Leuser Ecosystem, which is poised to fragment two of the three largest remaining orangutan populations. The increased access provided by the road is also likely to result in illegal encroachment and logging, and poaching of threatened species.

Throughout their range orangutans are sometimes killed as pests along forest edges as they raid agricultural crops, and in the far south of their range they are occasionally still hunted as food. The pet trade, which was booming from 1900 to the 1960s, with a further surge in the 1980s, resulted in the death of up to 30,000 orang-utans. For every young orangutan that ends up in captivity there is a loss of about 50 from the wild population, due to the number that are directly killed before and the loss of a potential breeding animals. A small but significant pet trade in young Sumatran orangutans still persists.

Since 1975 orangutans have been listed under Appendix I of CITES, which affords them the highest degree of protection against international trade. They are also legally protected from hunting throughout their range. However, these regulations are difficult to enforce. By far the most significant populations, totalling some 5,600 animals, are found within the Leuser Ecosystem, a 26,000 km² conservation area that encompasses the smaller Gunung Leuser National Park (10,950 km²) and the 1,025 km² Singkil Swamps Wildlife Reserve. The Ecosystem and the national park within it form the only conservation area of note where viable wild populations of the Sumatran orangutan exist. However, the mountainous terrain of the National Park is not ideal habitat for the predominantly lowland orang-utan, and the majority are found outside the park boundaries.

Orangutans found in logging areas are often transported to sanctuaries created by the Indonesian government while orphaned youngsters may be “rehabilitated” for release at rehabilitation centres. However, these measures can only save a small number of animals and overcrowding and inbreeding in the relatively small, isolated reserves are likely.

Recommended conservation actions include: effective law enforcement and prosecution to stop hunting of orangutans for food and trade; mitigation of human-orangutan conflict in agricultural areas, including large-scale plantations; audits to assess the compliance of forestry concessions to their legal obligation to ensure orangutans are not hunted in concession areas; increased environmental awareness at a local level; and mechanisms for monitoring orangutan populations and forest cover.

UNEP/UNESCO GRASP Great Ape Survival Programme

Sumatran Orangutan Society

Orangutan Foundation   

Durrell Wildlife Conservation Trust     

IUCN/SSC Primate specialist Group

ZSL Oil Palm and Biodiversity Project

Mittermeier, R. A. et al. 2006. Primates in Peril: The World’s 25 Most Endangered Primates, 2004–2006. Primate Conservation (20): 1–28.

Singleton, I., Wich, S.A. & Griffiths, M. 2008. Pongo abelii. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>. Downloaded on 05 November 2009.

Wich, S. A. et al. 2008. Review: Distribution and conservation status of the orang-utan (Pongo spp.) on Borneo and Sumatra: how many remain? Oryx, 42(3), 329–339.