Behavioral development and socialization of infants in a free-ranging group of howling monkeys (Alouatta palliata).

A 22-month field study was carried out on free-ranging mantled howlers in Costa Rica. Six female and 5 male infants were observed systematically from birth until they died, left the group, or the study ended. Interaction patterns, activity patterns, and proximity data were analyzed from 703 h of focal observations and 753 h of ad lib observations. Developmental trends in weaning and nonmother care were associated with mothers' feeding patterns, suggesting an increase in maternal feeding efficiency. As howler immigration patterns resulted in groups of adults of low relatedness, analyses based on social bonding or kin selection were inappropriate, and socialization patterns instead appeared to prepare howler infants to respond predictably in an adult world. Females, which were more sociable as adults, were also more sociable as infants, initiating interactions and reacting positively. They also exhibited less weaning stress than males. Males, which were forced out of the group sooner, remained solitary longer, and primarily interacted with adult females as adults, were forced to be independent sooner, reacted negatively to interactions, and ceased interacting with adult males by 3 months of age. Ecological constraints on development could not be determined from this study, although there was no evidence for developmental trends being influenced by predator stress.