That is also the case when the animals are primates. The need for information about particular species might be less widely recognized, however, when the animals are monkeys or apes. Resemblance of those animals to humans in appearance and behavior encourages the assumption that they have identical needs and abilities. But that cannot be accepted uncritically. Nonhuman primates are a highly diverse group. They not only differ from humans in many respects, but also differ widely from each other. Their normal behavior, needs, and abilities differ from one taxonomic group to another.

Their diversity is easier to accept when one considers that there are more than 200 species of primates. In addition to humans, primates include apes, New World and Old World monkeys, and an assortment of other forms collectively referred to as prosimians, such as tarsiers, lemurs, aye ayes, and bushbabies. Nonhuman primates range in size from the diminutive mouse lemur, weighing only a few grams, to the gigantic mountain gorilla, weighing more than 180 kg. Species also differ in habitat, diet, activity patterns, use of space, reproductive physiology, growth rates, social relationships, and cognitive abilities. Few of the species are used extensively in research. Although more are maintained in zoos, some are rarely or never found in captivity. Information is lacking on the natural history, biology, and behavior of many species (Fleagle 1988; Smuts and others 1987).

All primates are placed taxonomically in a single group called an order. The order Primates is subdivided into the traditional taxonomic categories, such as suborders, infraorders, superfamilies, families, subfamilies, genera, and species. The informal scheme that follows is consistent with the accepted system of classifying primate species (for example, see Fleagle 1988; Napier and Napier 1967, 1985) and will be adequate for our purposes.

The prosimians include tarsiers, lemurs, sifakas, indris, aye ayes, lorises, pottos, and bushbabies or galagos (Fleagle 1988). Tree shrews, once considered in this group, have now been removed from the Primate order on the basis of structural considerations, most notably in the ear, and by early fossil differentiation. Most prosimians have pointed muzzles, a naked rhinarium (a moist patch of bare skin around the nose), and claws instead of nails on some fingers and toes. Many species of prosimians are nocturnal. Few institutions maintain them in captivity, but some lemurs, especially the ring-tailed lemur L. catta, and some species of Galago are found in exhibits.

The anthropoids include all other species in the Primate order. These animals conform to the popular view of what a monkey or ape should look like. A popular distinction between monkeys and apes is that monkeys have tails. It is true that apes are tailless, but some monkeys are also virtually tailless. A more basic distinction within the anthropoids is between New World and Old World forms. New World anthropoids are distinguished from Old World anthropoids by having three rather than two premolars (bicuspids) and by having a broader septum between the nostrils.

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