constant burden of the infant. If such signs occur, the infant might have to be removed to prevent injury.
Male squirrel monkeys display an unusual pattern of seasonal "fatting." Males can gain 20–30% in body weight and become noticeably bulkier in the shoulders and upper torso as the breeding season approaches. Not all males show this change—it is most commonly observed in the Roman Arch species—and it is not required for fertility. Males gradually lose the weight during the course of the breeding season.
Cognition in squirrel monkeys, capuchins, and some of the other New World and Old World genera has been reviewed by Rumbaugh (1967) and Fragaszy (1985). Cebids seem generally similar to rhesus monkeys in several cognitive capacities, although they appear to be somewhat less able in conceptual and relational learning tasks. There is, however, wide latitude in their activity levels and responsiveness to the physical and social environment. Capuchins, for example, show greater manipulative ability than rhesus monkeys and are the prototype of the active monkey for which provision of opportunities for productive activity is essential to well-being. When not locomoting, they are most often busy with their hands (Fragaszy and Adams-Curtis 1991). When no other opportunities are present, their attention is directed to surfaces in the cage or nearby objects, such as locks. This activity can be safely redirected by providing them with such objects and materials as wood, soft plastic, straw, and small containers (Fragaszy and Westergaard 1985; Visalberghi 1988). They will spend much time shredding and destroying disposable objects. They also retain interest in objects that require dexterous probing or scraping (Fragaszy and Visalberghi 1989; Westergaard and Fragaszy 1987; Westergaard and Suomi 1993).
Other cebids are less manipulative than capuchins; Aotus and Callicebus perhaps are least so. Monitoring other members of the social group appears to be more important to these animals than physically interacting with the inanimate environment.
New World monkeys (cebids and callitrichids) in general are less likely to be aggressive toward humans than the more common species of Old World monkeys. They often exhibit curiosity toward humans and do not respond aggressively when prolonged direct eye contact is made. They will often approach familiar people and readily accept food or objects offered by hand. That can be useful when supplementing the diet or medication of particular animals (Abee 1985). It might be necessary to get a timid animal away from other group members for such supplementation.