white or white-fronted (C. albifrons). Cebus apella (the tufted or brown capuchin) is perhaps the most commonly seen.

The other genera of Cebidae are less commonly seen in captivity, although woolly monkeys (Lagothrix) enjoyed an unfortunate early popularity in the pet trade, and spider monkeys (Ateles) are often seen in exhibits.

Genera in the Cebidae share only a few features of ecology, social organization, and life history. All are primarily arboreal, and all except Aotus are diurnal. The basic locomotor pattern for all genera is quadrupedal walking, but capuchins, howler monkeys, spider monkeys, woolly spider monkeys, and woolly monkeys have prehensile tails, which are used in various degrees to support the body. All those can hang suspended by the tail alone, and the last four are considered "semibrachiators," meaning that they often suspend themselves from supports using only their hands and tail. Most cebids are agile climbers and capable of substantial leaps. With the exception of the uacaris, whose tails are only one-third as long as their bodies, the tail is usually longer than the body in cebids. Spider, woolly, howler, and woolly spider monkeys have a naked "fingertip" at the end of the tail, and they use the tail not only in brachiation but also to manipulate objects not within hand reach.

The genera differ markedly in appearance, from the striking bald head and distinctive coat color of the red or white uacaris to the spiky hair of the spider monkey and black uacaris. The adults range in size from Saimiri, at about 750 g (1.6 lb), to the Lagothrix, at nearly 10 kg (22 lb). Cebids generally show far less sexual dimorphism than Old World monkeys, and males are often only slightly larger than females. In contrast with other primate groups, in which sexual dimorphism increases with body size, the largest cebids are relatively monomorphic in size. However, the two sexes in sakis and some species of howler monkeys are of different color, even though of similar size; a naive observer might think that the two sexes were of different species because the color difference appears even in infancy.

Life history characteristics also vary among genera (Napier and Napier 1967, 1985). Infants nurse for from as little as 3 or 4 months (Saimiri , Callicebus, Pithecia, and possibly Chiropotes) to 2 years or longer (Ateles and Lagothrix). Average interbirth intervals range from 1 year to 2.5 years (or longer). Maximal life span (in captivity) varies from about 25 years for squirrel monkeys and titis to nearly 50 years for capuchins (and probably spider and woolly monkeys as well).

All genera except Callicebus, Pithecia, and Aotus (which are monogamous) live in mixed-sex groups; modal group size and organization vary widely among species. Diets also vary widely; some species are nearly completely folivorous (eating a diet of leaves), some are frugivores, and some are omnivores. Many species are found in association with other primate genera whose ecological niches overlap; this is especially common for Saimiri with Cebus apella.

Differences among genera in ecology and behavior are enormous, as should be expected by the length of evolutionary time that they have been radiating into



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