The name orangutan derives from the Malay and Indonesian phrase ‘orang-hutan’, meaning ‘person of the forest’.
Orangutans are highly intelligent with an ability to reason and think. These large, gentle red apes are one of our closest relatives, sharing 97% of our DNA.
They are part of the great ape family, so they don’t have a tail and tend to be larger and heavier than monkeys, even though they are both primates. Great apes also have a bigger brain and can use tools, such as sticks, to help them get food to eat or leaves to make a sunshade or umbrella.
Orangutans live in Asia and are only found on the islands of Borneo and Sumatra, living in lowland and hilly tropical rainforests.
There are three species of orangutan – the Tapanuli orangutan, the Sumatran orangutan, and the Bornean orangutan:
- Pongo tapanuliensis, or Tapanuli orangutan, was first described in 2017, besides others, by the current BOS Foundation’s deputy CEO, Dr Anton Nurcahyo. Pongo tapanuliensisis the most endangered of the great ape species in the world, with an estimated population of under 800 individuals. The Tapanuli orangutan can be found in northern Sumatra.
- Pongo abelii, or Sumatran orangutan, is endemic to northern Sumatra and has an estimated population of about 14,000 individuals.
- Pongo pygmaeus, or Bornean Orangutan, is classified into three subspecies:
- Pongo pygmaeus pygmaeus, or Northwest Bornean orangutan, is the most endangered of the Bornean orangutans, with an estimated population of 4520. This subspecies is endemic to northwest Kalimantan and Sarawak.
- Pongo pygmaeus morio, or Northeast Bornean orangutan, is found throughout Sabah and in East Kalimantan, with an estimated population of 14,630.
- Pongo pygmaeus wurmbii, or Central Bornean orangutan, is endemic to West and Central Kalimantan, with an estimated population of around 38,200.
You can only tell the difference between these three Bornean subspecies by DNA testing.
At BOS, we have all three Bornean subspecies in our care, so when returning them to the wild, we must ensure they go to their endemic areas in Indonesian Borneo, where we work.
Bornean Orangutan Sumatran Orangutan
The Bornean orangutans are larger than their Sumatran counterparts and have darker and shorter hair. The Tapanuli orangutans resemble Sumatran orangutans more than Bornean orangutans in body build and fur colour. However, they have frizzier hair, smaller heads and flatter faces.
Scientists estimate that there are fewer than 14,000 Sumatran orangutans and about 57,000 Bornean orangutans living in the wild today. In the IUCN Red List, both are listed as critically endangered.
Orangutans are ‘umbrella species’, which means they are pivotal in creating the necessary environment for the thousands of fauna and flora that make up the biodiversity of the South East Asian rainforest.
They are the largest arboreal mammals in the world, and their tree-swinging journeys help to spread seeds – in fact, some trees can only germinate when they have passed through the gut of an orangutan.
Orangutans are essentially solitary with the only permanent bond being between mother and infant. However, they do live in very loose clusters of related females. These occasionally meet in twos and sometimes more during periods of higher food-availability to allow bonds to be maintained, infants to play and learn together and novel behaviours to be shared.
Males disperse away from their natal range to prevent inbreeding and become sexually mature at around 15 years of age. Between 18 and 20 years old they grow much larger and develop secondary sexual characteristics of cheek flanges and a large throat sac, which they use to make booming ‘long calls’ to attract females and warn-off other males. Males compete to become dominant, although non-dominant males do father a large proportion of infants, particularly unflanged males aged 15 to 20 who have not attained full size and can forcefully mate females.
Female orangutans give birth to one infant at a time after eight and a half months of pregnancy, and they do not have another child until the first infant reaches seven years of age. This is the longest inter-birth interval known in the animal kingdom. It enables the mother to give full attention to her offspring as it learns the skills needed to survive on its own, including making nests, recognising different kinds of food and avoiding predators. Once the infant has learnt these skills, it still stays with its mother for protection and to make a mental map of the forest, particularly to learn where all the important food resources are, until it is time to move away.
Orangutans are highly intelligent creatures with advanced problem-solving skills. They can make use of things in their environment for tools and medicines. They display cultural behaviours, with different populations tackling the same problem in different ways. They learn from other orangutans and pass on their own skills when they meet, which is more frequent when food availability is high. In artificial circumstances where food-availability is unlimited, such as in captivity or on the BOS Foundation’s pre-release islands, they can quickly develop and share very advanced skills, such as fishing, swimming with the use of a float or tool-use to efficiently access foods.
Orangutans are predominately frugivorous, with over 100 types of fruit typically recorded in their diet from any one site. However, they have a very wide diet which includes flowers, leaves, the cambium layer of bark; the inner pith of rattans, pandans, gingers and palms; termites, ants and other invertebrates; honey, fungi and on very rare occasions have been observed to eat small mammals. They need to rely on these foods during periods when fruits are rare and also to develop large fat reserves to help them through extreme periods of food shortage, or ‘fruit-crunch’ periods. Orangutans are important dispersal agents of fruit seeds, either by passing seeds through their digestive tract or by carrying and discarding seeds as they move through the trees, thus playing a key role in forest ecology and regeneration.
Photo credits: Gorilla: Mila Zinkova; Chimpanzee: Thomas Lersch; Bonobo: USAID; Orangutan: Karen Stenner