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Characteristics

Orangutans display significant sexual dimorphism; females typically stand 115 cm (45 in) tall and weigh around 37 kg (82 lb), while adult males stand 137 cm (54 in) tall and weigh 75 kg (165 lb). Compared to humans, they have proportionally long arms, a male orangutan having an arm span of about 2 m (6 ft 7 in), and short legs. They are covered in long reddish hair that starts out bright orange and darkens to maroon or chocolate with age, while the skin is grey-black. Though largely hairless, males' faces can develop some hair, giving them a beard.

Orangutans have small ears and noses; the ears are unlobed. The mean endocranial volume is 397 cm. The cranium is elevated relative to the face, which is incurved and prognathous. Compared to chimpanzees and gorillas, the brow ridge of an orangutan is underdeveloped. Females and juveniles have relatively circular skulls and thin faces while mature males have a prominent sagittal crest, large cheek pads or flanges, extensive throat pouches and long canines. The cheek pads are made mostly of fatty tissue and are supported by the musculature of the face. The throat pouches act as resonance chambers for making long calls.

Orangutan hands have four long fingers but a dramatically shorter opposable thumb for a strong grip on branches as they travel high in the trees. The resting configuration of the fingers is curved, creating a suspensory hook grip. With the thumb out of the way, the fingers (and hands) can grip securely around objects with a small diameter by resting the tops of the fingers against the inside of the palm, thus creating a double-locked grip. Their feet have four long toes and an opposable big toe, giving them hand-like dexterity. The hip joints also allow for their legs to rotate similarly to their arms and shoulders.

Orangutans move through the trees by both vertical climbing and suspension. Compared to other great apes, they infrequently descend to the ground where they are more cumbersome. Unlike gorillas and chimpanzees, orangutans are not true knuckle-walkers, instead bending their digits and walking on the sides of their hands and feet.

Compared to their relatives in Borneo, Sumatran orangutans are more slender with paler and longer hair and a longer face. Tapanuli orangutans resemble Sumatran orangutans more than Bornean orangutans in body build and hair colour. They have shaggier hair, smaller skulls, and flatter faces than the other two species.