monkey, any of a large and varied group of mammals of the primate order. The term monkey includes all primates that do not belong to the categories human, ape, or prosimian; however, monkeys do have certain common features. All are excellent climbers, and most are primarily arboreal. Nearly all live in tropical or subtropical climates. Unlike most of the prosimians, or lower primates, they are almost all day-active animals. Their faces are usually flat and rather human in appearance, their eyes point forward, and they have stereoscopic color vision. Their hands and feet are highly developed for grasping; the big toes and, where present, the thumbs are opposable. Nearly all have flat nails. Monkeys habitually sit in an erect posture. Unlike the apes, most cannot swing arm-over-arm (the spider monkey is an exception) but move about in trees by running along the branches on all fours; their skeletal structure is similar to that of other four-footed animals. Monkeys live in troops of up to several hundred individuals and travel about in search of food, having no permanent shelter. As in apes and humans, the female has a monthly reproductive cycle, and mating may occur at any time, but in some species mating is seasonal. Usually only one infant is born at a time; it is cared for by the mother for a long period. There are two large groups, or superfamilies, of monkeys: Old World monkeys (Cercopithecoidea) and New World monkeys (Ceboidea).
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