Group size, infant development and social networks in free-ranging rhesus monkeys

The hypothesis that demographic processes are important in determining social structure among non-human primates through their effects on the development of individual social relationships was tested. Changes in maternal behaviour, infants' social milieux and infants' social networks were examined as a function of changes in group size and composition among free-ranging rhesus monkeys, Macaca mulatta, on Cayo Santiago, Puerto Rico. Data were analysed for 10 group-years collected between 1974 and 1990 representing a single social group during periods of rapid expansion and fissioning, two daughter groups following fissioning and an unrelated group that did not undergo fissioning. As group size expanded, infants found themselves near (<5 m) larger numbers of group members and smaller proportions of close kin. Mothers spent more time near their infants and sought more proximity with them. Infants developed social networks that were more highly kin biased. When the groups fissioned, these trends reversed. Significant correlations were found between group size, mother-infant interaction and infant social networks across all group-years. Individual variation in the degree to which infant social networks were kin biased was related both directly to the infant's social milieu and indirectly to its relationship with its mother. Given the long-term nature of the mother's influence on social networks, these results suggest that demographic influences on developmental processes can lead to progressive changes in social structure in the absence of resource scarcity. (C) 1997 The Association for the Study of Animal Behaviour.