Head and body length: 1.25-1.50 m
Arm span: 2.25 m
Weight: Male: 50-90 kg
Female: 30-50 kg
The orangutan is the only great ape found outside Africa, and the largest arboreal mammal in the world. It has a coarse, shaggy reddish-brown coat, and long, powerful arms which can reach up to 2 m in length. Both the thumbs and the large toes are opposable, allowing the ape to grasp branches with both its hands and feet. The orangutan is highly sexually dimorphic, with males growing to about twice the size of females. When sexually mature, males develop flanges on either side of their faces, known as ‘cheekpads’. These are deposits of subcutaneous fat bound by connective tissue. Adult males also develop throat sacs, which can be inflated to add resonance to the male’s loud ‘long-call’.
The two species of orangutan differ slightly in appearance and behaviour. Sumatran orangutans tend to be lighter in colour and have longer fur around their faces than the Bornean species. Bornean orangutans have wider faces, particularly the males, which have wider cheek pads than their Sumatran relatives.
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 orangutans. Adult males of both species 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. Social behaviour has been observed more frequently in the Sumatran species. In particular, it coincides with the simultaneous (or “mast”) fruiting of fig trees, which does not occur in Borneo. Juvenile males and females of both species display the most social behaviour, and are often observed feeding, playing and travelling together in groups. 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 mammal species. Females reach sexual maturity at around 10-15 years of age, and generally give birth to a single young every 6-8 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 45-50 years. The incredibly long inter-birth period means that a female orangutan can produce a maximum of four surviving young during her lifetime.
Known from primary and secondary forest, typically in lowland dipterocarp, freshwater and peat swamp forests. Orangutans are rarely found above 1000 m.
Orangutan population sizes are difficult to estimate with precision. The most recent estimates of total surviving numbers for the Sumatran and Bornean orangutans are around 7,300 and 57,000 respectively.
Densities and population sizes are in decline across the species' range, and forest continues to be lost at a rapid rate.
The Bornean orangutan is classified as Endangered (EN A2cd) on the 2006 IUCN Red List of Threatened Species. The Sumatran orangutan is listed as Critically Endangered (CR A2bcd).
Orangutan populations have crashed during the past 200 years, as a result of deforestation of their habitat. Today the species continues to be threatened by habitat loss, degradation and fragmentation as a result of legal and illegal logging, forest fires, illegal gold mining, and conversion to agriculture, in particular oil palm plantations. The species’ highly arboreal lifestyle makes them particularly vulnerable to habitat fragmentation, as they rarely travel long distances on the ground. More than 80% of orangutan habitat has been altered or destroyed by these factors. If this alarming rate of habitat loss continues then the orangutan may become extinct in the wild in as little as 10 years.
Individuals that have lost their natural habitat sometimes enter oil palm estates, where they are often shot as pests. Mothers with babies are particularly vulnerable. They are often killed and the infants sold into the illegal pet trade. It is estimated that at least three orangutans are killed for each one that is captured and transported. The species is also hunted for meat in some areas.
With such a low reproductive rate, the orangutan is particularly vulnerable to these threats. Small, fragmented populations are at considerable risk. Many of the remaining populations, particularly in Sumatra number fewer than 250 individuals. These small, isolated populations do not have the capacity to recover from population declines. A slight rise in female mortality rate of just 1-2% can drive a local population to extinction.
Orangutans are protected by law in Malaysia and Indonesia, and listed on Appendix I of CITES, which prohibits international trade. However, weak law enforcement in both countries means that the species continues to be at risk from the above threats. Populations occur in a number of national parks and other protected areas, although illegal logging is still a problem in many of these areas. The Orangutan Conservation Forum is a consortium of groups working to stop illegal logging and hunting by patrolling protected areas, increasing sustainable economic alternatives for communities surrounding critical orangutan habitat, raising awareness, and releasing ex-captive orangutans into suitable protected habitat. There are currently three orangutan rehabilitation centres in Kalimantan, one in Sabah, one in Sarawak and one in Sumatra.
Increasing the conservation awareness of orangutans to every organisation involved in the issues facing orangutans in Sumatra, including plantation owners and their workers, villages in the remaining forest, educationalists, government ministers and general public.
Orang-utan nest surveys in Murung Raya region as part of the Barito River Initiative for Nature Conservation and Communities (BRINCC) expedition.
Areas of important orangutan habitat are in urgent need of some form of legal protection. Existing protected areas need more effective management and better protection from poachers. Rehabilitation of degraded areas of forests and the creation of additional forested corridors to link protected areas would also be beneficial. Recent studies have shown that orangutans can survive in selectively logged areas of forest, albeit at low densities. This indicates that selective logging may be able to continue in certain areas of orangutan habitat, providing it is managed appropriately. Awareness and education programmes for local communities, and the promotion of alternative sources of income should help to relieve the pressure of illegal logging and poaching on the two species. Finally, long term research and monitoring of important populations should continues to learn more about the species and its habitat requirements. Research stations can provide a presence that would deter outsiders from entering.
Madelaine is actively involved in Great ape conservation and has been involved in the production of many wildlife documentaries
Research expedition to the scientifically unstudied Murung Raya region in Central Kalimantan, Borneo.