Exploration and play were observed in eight troops of howler monkeys (Alouatta palliata) containing 56 young and 95 adults. The behavior patterns of seven age classes are described. There was a clear trend for animals to spend less time in nonsocial exploration with increasing age, and for the frequency of social play to follow an inverted “U” pattern, peaking in the early juvenile stage at 15–30 min per day. The howlers' specialization for eating large quantities of difficult to digest plant matter affected their play: as the young became increasingly dependent on plant food around the infants stage, there was a shift in their playtimes to allow for the energy demanding activity of digestion. This and other aspects of howler behavior are related to a reinforcement theory of exploration and play.