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Cebus monkey were less mature at birth than squirrel monkeys in terms of body wieght and brain weight in proportion to adult weight. Postnatally, cebus were slower to develop locomotion, perceptual-motor capacities, and object permanence. The differences in rate of development between the species increased with age. The findings indicate that the cebus have a longer period of infancy before becoming skilled at locomotion and manipulation. In view of the cebus monkey's recognized skill as a manipulator and problem solver, the long period of infancy suggests that cebus have evolved further in the direction of higher primates. [ABSTRACT FROM AUTHOR]