Anatomy and Physiology
An orangutan has a large, bulky body, a thick neck, very long, strong arms, short, bowed legs, and no tail. It is mostly covered with long, reddish-brown hair and grey-black skin. Sumatran orangutans have more sparse and lighter-coloured coats. The orangutan has a large head with a prominent mouth area. Though largely hairless, their faces can develop some hair in males, giving them a moustache. Adult males have large cheek flaps to show their dominance to other males. The cheek flaps are made mostly of fatty tissue and are supported by the musculature of the face. Mature males' throat pouches allow them to make loud calls. The species display significant sexual dimorphism; females can grow to around 127 cm (4 ft 2 in) and weigh around 45.4 kg (100 lb), while flanged adult males can reach 175 cm (5 ft 9 in) in height and weigh over 118 kg (260 lb). A male orangutan has an arm span of about 2 m (6.6 ft).
Orangutan hands are similar to human hands; they have four long fingers and an opposable thumb. However, the joint and tendon arrangement in the orangutans' hands produces two adaptations that are significant for arboreal locomotion. The resting configuration of the fingers is curved, creating a suspensory hook grip. Additionally, without the use of the thumb, the fingers and hands can grip tightly around objects with a small diameter by resting the tops of the fingers against the inside of the palm, creating a double-locked grip.
Their feet have four long toes and an opposable big toe. Orangutans can grasp things with both their hands and their feet. Their fingers and toes are curved, allowing them to get a better grip on branches. Since their hips joint have the same flexibility as their shoulder and arm joints, orangutans have less restriction in the movements of their legs than humans have. Unlike gorillas and chimpanzees, orangutans are not true knuckle-walkers, and are instead fist-walkers.
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