True apes vary in size from the 3–ft (90–cm), 15–lb (6.8–kg) gibbon to the 6–ft (1.8–m), 450–lb (200–kg) gorilla. All apes are forest dwellers and most spend at least some of the time in trees. Except for adult gorillas, they can run along branches on all fours; they are also able to move about by brachiation, or arm-over-arm swinging. Gibbons (including siamangs) are particularly adept at this type of locomotion; the heavier orangutans prefer to grasp a neighboring tree and pull itself across to it. Gorillas and chimpanzees are the most terrestrial of the apes, normally traveling on all fours by leaning on the knuckles of their forelimbs with the fingers of their hands curled under (knuckle-walking); orangutans ball their fingers into fists during the short periods they walk. Most apes are able to walk on two feet for short distances.
The skeleton of an ape is quite similar to that of a human in the structure of the chest and shoulders. Apes have broad, flat chests and arms capable of reaching up and backward from the shoulder; this construction is associated with brachiation. The pelvis, on the other hand, is more like that of a monkey, designed for walking on all fours, hence the use of knuckle-walking for ground locomotion. The arms of an ape are longer than the legs. The hands are similar to human hands, but with fingers and thumb of more equal length; the feet are handlike, grasping structures. Apes have neither tails nor the cheek pouches found in Old World monkeys; gibbons are the only apes that have the buttock callosities found in Old World monkeys. Like other anthropoid primates, the eyes are highly developed, with stereoscopic color vision. The brains of great apes are different from Old World monkeys and some structures are reminiscent of the uniquely elaborate features of the human cortex, rendering these primates capable of fairly advanced reasoning. Chimpanzees, bonobos, orangutans, and gorillas have been observed using objects as tools in the wild.
Estimates of the amount of identical genetic material (DNA) in chimpanzees and humans range from 94.6% to 99.4%. This marked similarity, and additional evidence, have led primatologists to suggest that the taxonomy of the apes should include two families: Hylobatidae (gibbons and siamangs) and Hominidae (orangutans, gorillas, and chimpanzees and humans). Apes are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Primates.
See S. Montgomery, Walking with the Great Apes (1991).
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