Born Free Foundation - Keep Wildlife in the Wild

Facts

Asia’s only great ape, the orangutan, which means ‘person of the forest’, has distinctive red/rust-coloured fur. As a ‘keystone’ species they play an important ecological role in seed dispersal and forest regeneration through the fruits that they eat. Mature adult males can weigh as much as 90kg, and are twice as heavy as females. Their average lifespan in the wild is between 30-40 years but it can reach 45.

How are orangutans classified?

Living things can be organised into different groups.  Species that are alike are grouped together.  This is called classification.

Class:      Mammals

Order:      Primates

Family:     Hominidae

Species:               Pongo abelii (Sumatran orangutan)
                                Pongo pygmaeus (Bornean orangutan)

                                                Pongo pygmaeus ssp. Morio (Northeast Bornean Orangutan)

                                                Pongo pygmaeus ssp. Pygmaeus (Western Bornean Orangutan)

                                                Pongo pygmaeus ssp. Wurmbii (Southern Bornean Orangutan)

Distribution and Habitat

Orangutans have a limited distribution. They are found only in the rain forests on the islands of Borneo and Sumatra in Indonesia and Malaysia. The Bornean orangutan species is distributed across much of the island of Borneo in both the Malaysian and Indonesian territories while the Sumatran orangutan on the other hand is found in only six locations in northern Sumatra. The highest densities of orangutans occur in the swamp forests, where they are restricted to altitudes below 1,500 meters. Tripa Swamp is one of the last refuges for Sumatran orangutans but despite its importance, to both the local people and the orangutan, it is being cleared illegally at an alarming rate. In 2012, major fires destroyed a significant portion of the Tripa forest, and NGO groups estimated that half of the orangutan population may have been killed. Without decisive action, commitment and enforcement, Tripa may be completely destroyed within the next few years.

The orangutan is the largest arboreal (tree-living) mammal, spending over 90% of its life high up in the forest canopy. The male Bornean orangutan ventures to the ground more frequently than the female and the Sumatra orangutan, which virtually never ventures to the forest floor.  Males travel the furthest distances, migrating away once they are independent of their mothers, while the females tend to stay close and have close relationships with their offspring.

Diet

The orangutan is frugivorous and its diet is primarily composed of fruit.  However, it also eats leaves, seeds, bark and insects if fruit is scarce. Orangutans need to consume 12 kilograms of food each day in order to fulfil its nutritional requirements. Fruit availability in the Bornean forest is the dominating factor that affects the Bornean orangutan’s movements and behavior, including reproduction and sociability.

Adaptations

An adult orangutan's arm span can reach two meters and this enables it to swing from tree to tree more easily, a system of arboreal travel known as brachiation. Orangutans sleep in the tree canopy 15 to 100 feet above the ground, in nests which they build from folded branches and leaves on a daily basis.  If it is raining they have been known to add a ‘roof’ using the surrounding foliage. Orangutans use tools,   for example they use sticks to help them catch termites and collect honey. They have also been observed holding large leaves over their heads to shelter during rainstorms.

Male Bornean orangutan differ from male Sumatran orangutans by developing cheek flanges (cheek pads) when they become dominant. Accompanying the development of cheek pads is an overall growth spurt. The flanged mature males grow to twice the size of females, grow long dark hair on their back and develop a large throat sack to make loud long distance calls that can be heard a mile away. They do not tolerate the presence of other mature males and are aggressive towards them. Males may not develop flanges until years after they have reached sexual maturity and it is thought that it is the presence of a dominant cheek-padded male which may inhibit the full development of other males within its range. Dominant males are much more successful at attracting mates than subordinate males. In contrast the unflanged males do not have the additional sexual characteristics, they are the same size as females and they are not aggressive towards other males.

Ecology and Behaviour

Orangutans have a semi-solitary social system. It is thought that this has evolved in response to    the seasonal nature and the distribution of fruit, as well as to the absence of natural predators in the forest canopy. However, they are still social to an extent and young females, particularly those that are related, will travel and socialize together. Adult males and in particular dominant males are the most solitary, and only socialize with sexually receptive females for brief courtships.

With a lifespan of 30-40 years, females usually become sexually active at 12 years old. However, they usually do not give birth to their first offspring until they are 15-16 years old. The interval between births is, on average, eight years, which makes them extremely slow at reproducing.  As a result, a female will only give birth to three or four offspring during her lifetime. The gestation period for an orangutan is around eight and a half months. Infant orangutans are completely dependent on their mothers for the first two years of their life for both food and transportation. The baby is carried constantly for four months before it starts to explore its surroundings, but it will stay dependent upon its mother for up to 8 years. Young orangutans learn almost everything from their mothers and only live on their own or with other young orangutans when they are about 10 years old.

Adult orangutans are generally solitary, but female relatives maintain strong social bonds and will socialize together briefly when they encounter each other. Sumatran orangutans are more social, which has been attributed to the fact that the Sumatran dipterocarp forests have a greater fruit yield than Bornean rainforests. The forests of Sumatra undergo a phenomenon called mast fruiting every two to ten years. A very large number of trees fruit simultaneously, independent of any climate and seasonal changes and during this time when fruit availability and distribution is at its best, competition for food is at its lowest and orangutans are able to afford the costs of socializing.

Threats

The most serious threat to orangutans is the destruction of their forest habitat through deforestation. Over the last three decades it has been estimated that 80% of orangutan habitat has disappeared. Clear felling of pristine rain forest has been driven by the palm oil industry. The popularity of palm oil has increased in recent years and demand is still growing but with almost 90% of global oil production coming from Malaysia and Indonesia, reducing pressure on these forests has been challenging. In environmental terms, the establishment of oil palm plantations has been disastrous. Although degraded forest exists, companies have preferred to clear fell primary or mature secondary rain forest, because the revenue generated from selling timber, pays for the establishment of the plantation. Not only does the establishment of palm oil plantations destroy irreplaceable forest habitat, companies have been known to persecute orangutans directly. Hunting and the trade in orangutans are also adding to this ape’s decline.

If significant measures are not urgently taken to reduce habitat loss and poaching the Sumatran and Bornean orangutans face the threat of extinction in the near future. The population of the Sumatran orangutan has already decreased by over 50% since 1993 and it was estimated that there was only 7,300 remaining in 2004. While the estimate of Bornean orangutans produces a more positive picture on the surface, the steep decline in the population means the future is not bright for the orangutan.

Born Free Foundation
Broadlands Business Campus, Langhurstwood Road
, Horsham, RH12 4QP, UK - Charity Reg. No. 1070906


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