Orang-utans have a characteristic ape-like shape, shaggy reddish fur and grasping hands and feet. Their powerful arms are stronger and longer than their legs and can reach 2m in length, long enough to touch their ankles when they stand.
There are two different types of adult male orang-utan: flanged and unflanged. Flanged males have prominent cheek pads called flanges and a throat sac used to make loud verbalizations called 'long calls'. They also have a long coat of dark hair on their back.
The unflanged male looks like an adult female. Both reproduce and an unflanged male can change to a flanged male for reasons that are not yet fully understood. Orang-utans are the only primate in which this biological phenomenon occurs.
Male orang-utans can tip the scales at 90 kg, while females weigh between 30-50 kg.
Orang-utan means 'person of the forest' in the Malay language. They live in primary and secondary forests. Although they can occur up to 1,500m above sea level, most are found in lowland areas and prefer forests in river valleys or floodplains.
Orang-utans travel by moving from one tree to another, and usually avoid climbing down to the ground. But when they do, they move on 'all fours', placing their clenched fists on the ground.
Orang-utans make a nest of vegetation to sleep in at night, and rest in smaller nets during the day.
Adults are generally solitary, although temporary aggregations are occasionally formed. Males' large home ranges overlap with the ranges of several adult females. Adult males are generally hostile to one another, although they do not display territorially.
Orang-utans can live up to 50 years in the wild. Females first reproduce between 10-15 years of age. They give birth at most once every 5 years, and the interval between babies can be as long as 10 years.
Orang-utans usually give birth to a single young, or occasionally twins. Orang-utans stay with their mothers for the first 7-11 years of their life. An infant rides on its mother's body and sleeps in her nest until it is able to survive on its own.
The long time taken to reach sexual maturity, the long interbirth periods, and the fact that orang-utans normally give birth to just a single young mean that orang-utans have an extremely low reproductive rate.
This makes orang-utan populations highly vulnerable to excessive mortality, and means that populations take a long time to recover from population declines.
About 60% of the orang-utan's diet includes fruit, such as durians, jackfruit, lychees, mangosteens, mangoes and figs. The rest comprises young leaves and shoots, insects, soil, tree bark, woody lianas, and occasionally eggs and small vertebrates. They obtain water not only from fruit, but also from tree holes.
There are three species of orang-utan - the Bornean, Sumatran and Tapanuli - which differ a little in appearance and behavior. The Bornean and Sumatran species have shaggy reddish fur, Sumatran orangutans have longer facial hair.
Sumatran orang-utans are also reported to have closer social bonds than their Bornean cousins.
The Sumatran orang-utan is almost exclusively arboreal. Females virtually never travel on the ground and adult males do so only rarely. This is in contrast to Bornean orang-utans, especially adult males, which more often descend to the ground.
A third species of orangutan was announced in November, 2017. With no more than 800 individuals in existence, the Tapanuli orangutan is the most endangered of all great apes. This new third species lives in North Sumatra, but is genetically and behaviorally distinct from the two other species.
An international team of scientists described the new species in Current Biology. The Tapanuli orangutan (Pongo tapanuliensis) is distinguished from other orangutan populations based on morphological and genomic evidence. The new species is endemic to 475 square miles of upland forest in the Batang Toru Ecosystem of Sumatra and is believed to have been isolated from other orangutan populations for 10,000-20,000 years.