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.
Flanged male Bornean Orangutan (Pongopygmaeus)
©David Lawson / WWF-UK
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.