The family Cebidae includes 11 genera: Alouatta (howler monkeys), Aotus (night or owl monkeys), Ateles (spider monkeys), Brachyteles (woolly spider monkeys), Cacajao (uacaris), Callicebus (titi monkeys), Cebus (capuchin monkeys), Chiropotes (bearded sakis), Lagothrix (woolly monkeys), Pithecia (sakis), and Saimiri (squirrel monkeys). The most comprehensive references available on the natural history of these genera are the two volumes in the series Ecology and Behavior of Neotropical Primates (Coimbra-Filho and Mittermeier 1981; Kinzey 1997; Mittermeier and others 1988). The taxonomy of these genera is still being revised; according to one authoritative source (Mittermeier and others 1988), revisions are likely to increase the number of recognized species and subspecies.

The squirrel monkeys are the most common cebids in laboratory environments. Opinions vary about the number of species, and Hershkovitz (1984) has argued for the recognition of four species. On the basis of the pattern created by pigmentation and the white hair around the eyes, squirrel monkeys have been described as gothic arch (Saimiri sciureus, S. oerstedii, and S. ustus) or roman arch (S. boliviensis) (Hershkovitz 1984). Variation in head and body color from yellow-orange to black to gray-green is noted among species and subspecies. Species have pronounced differences in susceptibility to experimentally induced diseases and social behavior. Differences in the number of acrocentric autosomes are noted in karyo-types among species and subspecies of squirrel monkeys. The "squirrel monkey" is clearly several kinds of monkey.

Aotus (the night or owl monkey, also sometimes called the dourocouli) is found in many laboratories and should be recognized as a group of nine species (Hershkovitz 1983). Differences in dipliod chromosome numbers among karyo-types exist, and one who ignores these differences cannot readily form breeding pairs.

Species of Cebus are often in exhibits and increasingly often in laboratories. Various common names have been applied to these "organ-grinder monkeys." Most are called capuchins with an inconsistent use of modifiers, such as wedge-capped or weeper (C. olivaceus), white-faced (C. capucinus ), and brown and