Once widespread throughout the forests of Asia, orangutans are now found on just two islands, Sumatra and Borneo.
There are two genetically distinct species: the Sumatran orangutan (Pongo abelii) and the Bornean orangutan (Pongo pygmaeus). The two species look slightly different: Sumatran orangutans have lighter hair and a longer beard than their Bornean relatives, and Sumatran males have narrower cheekpads. Both species are highly endangered due to habitat loss and poaching. 100 years ago there were thought to be 315,000 orangutans in the wild. There are now less than 14,600 left in Sumatra, and less than 54,000 in Borneo. It is thought that Sumatran orangutans may be the first Great Apes to become extinct unless we help to protect them.
Orangutans also differ from the other Great Apes in that they do not live in family groups. The largest family unit is a female and two offspring, and males and females usually meet up only to breed.
Orangutans are highly intelligent and gentle animals. They use tools in the wild and have excellent memories, making mental maps of their forest home in order to find fruiting trees throughout the seasons.
Males who are not yet fully mature tend to associate with other females, particularly adolescent females, but are not usually aggressive. Flanged adult male orangutans are the most solitary of all orangutans. Their participation in social groups is usually limited to temporary sexual encounters with receptive females.
This semi-solitary social system may have evolved as a result of a ripe fruit diet, scattered food distribution, and the unpredictability of available food. Orangutans spend up to 60% of their time foraging and eating.
There are two types of mature male orangutans: flanged and unflanged males. A flanged male has big cheek pads on the sides of his face and a large throat sack under his chin. An unflanged male has neither of these and his body is usually smaller. Unflanged males are sexually mature and fully able to father offspring. Females, however, seem to prefer to mate with the flanged males.
Male to male competition for access to females is a major factor in orangutan society. Flanged adult males use their pendulous throat sacs as a resonating chamber for the ‘long call’, parts of which sound like a loud roar. Sometimes the sound of a long call can carry for almost a mile.
The flanged male orangutan’s long call seems to play an important role in repelling male rivals and advertising availability to females. We still don’t really know when and why a male undergoes the transformation from unflanged to flanged, or even if every male undergoes this transformation.
For the first two years of a young orangutan’s life, the baby is completely dependent on its mother for food and transportation. A baby orangutan clings to its mother’s stomach, side, or back while she moves through the trees, and feeds on her breast milk. Orangutan offspring will sometimes be carried until they are 5 years old and be breast-fed until they are 8 years old. Even when young orangutans are too old to be carried and fed by their mother, they may still remain close to her, traveling with her, eating, and resting in the same trees, until they are about 10 years old.
Orangutans live for around 45 years in the wild, and a female will usually have no more than 3 offspring in her lifetime. This means that orangutan populations grow very slowly, and take a long time to recover from habitat disturbance and hunting.
Such prolonged association between mother and offspring is rare among mammals. Probably only humans have a more intensive relationship with their mothers. Primatologists believe that orangutans have such long childhoods because there is so much that they need to learn before they can live alone successfully. Young orangutans learn almost everything from their mothers, including where to find food, what to eat and how to eat it, and how to build a proper sleeping nest.
Although mostly arboreal, males in Borneo occasionally travel on the ground to move between stands of trees. In Sumatra this rarely happens and may be linked to sharing their habitat with the Sumatran tiger. While females stay near their mothers’ home ranges during the course of their lifetimes, males may migrate long distances away from their mother’s home range.
Almost every night orangutans construct a new sleeping nest from branches, usually 15 to 100 feet up in a tree. Sometimes orangutans will make a mid-day nest for napping. Occasionally, they will also reuse an old nest, adding new branches.
Orangutans have been observed making simple tools to scratch themselves. They also use leafy branches to shelter themselves from rain and sun, and sometimes even drape large leaves over themselves. They have also been observed using branches as tools during insect foraging, honey collection, and protection against stinging insects. In Sumatra, wild orangutans use tools to extract seeds from hard shelled species of fruit.
In parts of Borneo, orangutans use handfuls of leaves as napkins to wipe their chins while orangutans in parts of Sumatra use leaves as gloves, helping them handle spiny fruits and branches, or as seat cushions in spiny trees.