Orangutans are wholly dependent on trees for their existence. They are perfectly adapted to life in the forest - they sleep in nests, feed predominantly on fruit and travel with ease through the forest canopy, rarely descending to the forest floor.
Orangutans are almost unique among primate species. All other apes and monkeys are social and gregarious, whilst the orangutan is semi-solitary — the largest group being a mother and two offspring. Females may spend up to 25% of their time with other orangutans. In contrast, male orangutans will spend less than 9% of their time in association with other orangutans. Sumatran orangutans are more social and this social behaviour usually coincides with the simultaneous fruiting of the fig tree, which doesn’t occur in Borneo.
Reproduction and Parenting
Courtship lasts between 3-10 days and it is the female who, not wanting to share her food source, initiates the final separation. The male has no role in the raising of his offspring. It is thought that this solitary lifestyle evolved due to erratic fruiting, leading to competition for food. With a predominantly frugivorous diet, containing relatively few calories for such a large body size, the orangutan needs to forage for 60% of the day, with the other 40% spent sleeping and resting.
Orangutan Population at Risk
Orangutans are the slowest breeding of all primates and, at eight years, have the longest inter-birth interval of any land-based mammal. A female orangutan will normally have her first infant between the ages of 12 and 15. Offspring are dependent on their mothers for at least five years and with a life expectancy of 45 years plus, females will normally have no more than three offspring. With these factors combined, the orangutan population, especially small fragmented populations, are at considerable risk. They don’t have the capacity to recover from disasters that may strike a population. A slight rise in the adult female mortality rate by just 1-2% could drive a local population to extinction.