Orangutans
..................................................................................................................................

Introduction | Habitat | Behaviour | Development

Development
Orangutans have the slowest life history among great apes. Wild female orangutans give birth to their first infant around the age of 15. Their gestation period is around 8 months and their interbirth interval, which is the time between successive births, is very long. For Bornean orangutans this is estimated to be around 6-7 years and for Sumatran orangutans 8-9 years. However, between the different research sites there is also some variation observed. For example, the interbirth interval at the following sites is:

Ketambe (Sumatra): 9.3 years
Suaq (Sumatra): 8.2 years
Sebangau (Borneo): 7.7 years
Tanjung Puting (Borneo): 7.7 years
Gunung Palung (Borneo): 7.0 years
Kinabatanang (Borneo): 6.5
Kutai (Borneo): 6.1

An infant is dependent of its mother until it has reached the age of weaning. The first years after birth, an infant stays cling (hangs on) to its mother, drinking its mother’s milk and gradually learning to eat its mother’s diet. At the age of 2 the infant starts to move independently from its mother, but it takes up to 5 years on Borneo and almost 6 on Sumatra, for them to spend more than 50% of their time over 2 meters away from their mother. Infants also sleep in their mothers nest. Data on when infants sleep in their own nest is scarce, but in Tuanan, Borneo, the earliest an infant slept in its own nest was at the age of 4.5 years. But after this occasion, the infant spend most of its nights in her mother’s nest again. In Sumatra the youngest orangutan seen sleeping in its own nest was an infant of 7 years old. After this “dependent” period the young orangutans reach the juvenile stage. For Sumatran orangutans this stage starts around the age of 7 and for Bornean orangutans around 6 years. Juvenile orangutans are capable of surviving on their own. Adult females give birth to their first infant between 13 and 15 years old and maximally they give birth to four infants during all their life. Orangutans' average lifespan in the wild is of 59 years.

Adult males give no parental care and have almost no contact with their offspring. There are two morphs of adult orangutan males, that is, males of different morphs look considerably different. This differences concern body size, the development of face flanges and a throatpoach (that facilitates the production of long calls) and the growth of a long hair coat in the back of the body and a beard. Usually males of different morphs have been called sub-adult and adult, however, this distinction is misleading since "sub-adult" males are sexually mature and can sire orangutan infants. Accordingly, the most correct distinction usually used is unflanged and flanged males. Although the specific reasons that sustain unflanged males from becoming flanged remain unknown, it is known that it is not related to age and that the presence of a flanged male in the area usually inhibits other males from becoming flanged. In captivity, when a flanged male is taken from a particular group of orangutans, the young male of that group rapidly grows flanges.