yas (the sacred baboon), which is also found on the Arabian peninsula. Macaca (macaques) and Papio (baboons) are generally considered closely related, and Papio is also closely related to Theropithecus (geladas) and Mandrillus (drills and mandrills). Cercocebus (mangabeys) is considered intermediate between these genera (Macaca and Papio) and other cercopithecines, and some would divide the mangabeys into two genera according to the degree of similarity to Macaca and Papio or to Cercopithecus (guenons). The guenons include many of the colorful forest species and the vervet or green monkey (Cercopithecus aethiops ). Miopithecus (talapoins) and Erythrocebus (patas monkeys) are closely related. Allenopithecus (Allen's swamp monkey) is poorly known.

The colobines are found in both Africa and Asia; greater diversity occurs in Asia. African forms are sometimes placed in the single genus Colobus, but some authorities prefer to recognize several distinct genera. All agree that Asia has multiple genera of colobines, including Rhinopithecus (snub-nosed langurs—one species is known as the Chinese golden monkey) and Nasalis (proboscis monkey). Pygathrix (douc langurs) and Simias (Mentawi island langurs) are also generally recognized; the remaining langurs are either all grouped as Presbytis or divided into several related genera.

Old World monkeys all develop ischial callosities (specialized calluses used in sitting) prenatally and have the same dental formula as apes and humans: two incisors, a canine, two premolars or bicuspids, and three molars in each quadrant. Sexual size dimorphism is pronounced in many of the larger species. Other morphological characteristics—such as tail length, the presence of female sexual swellings, and distinct natal coat colors in infants—are extremely variable.

Old World monkeys include many forms that spend substantial portions of the day on the ground. Geladas and some baboons can live in areas where there are few trees, and they retire to rocky cliffs at night. Patas monkeys are also highly adapted to life on the ground. Most other forms, however, are never far from trees. Even macaques, which some describe as semiterrestrial, spend most of the day in elevated locations and seek the refuge of trees at night.

The colobines have enlarged salivary glands and a sacculated stomach and can digest mature leaves. They nevertheless prefer a diet of fruit, flowers, seeds, and buds and consume both young and mature leaves (Napier and Napier 1985; Struhsaker 1975). Cercopithecines are much more omnivorous, and some macaques will eat virtually anything that humans find edible, in addition to many items that humans pass by. This flexibility means that some macaques and baboons live close to humans in the wild state and will raid crops, steal from markets, and seek handouts from humans.

The most commonly seen monkey in captivity is Macaca mulatta (rhesus monkey), sometimes called the Indian monkey, although its distribution extends from Pakistan through India, Bangladesh, Burma, and Thailand into northern China (Napier and Napier 1985). Until recently, this was the laboratory monkey.

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