International Journal of Comparative Psychology

Primates, particularly females, tend to be attracted to infants that are not their own and are often highly motivated to touch and handle them. However, species vary markedly in forms of handling and extents to which handling constitutes direct care (e.g., carrying and nursing), other affiliative behaviors, or aggression/ abuse. Here we review infant handling among primates from ultimate and proximate perspectives, focusing on a promising, but understudied hypothesized benefit—that handling enhances social bonds. We pay special attention to macaques and baboons, because infant handling in most of these species poses a special challenge in that it involves little actual care, and hence may be shaped by different and as yet unclear selective pressures from typical alloparental care. Costs, benefits, and hypothesized functions appear to vary across species based on: a) individuals’ roles (mother, handler, and infant), b) each of their characteristics, c) relationships between handlers and mothers, and d) the social context within the group. As a result, observed patterns of handling appear to be complex outcomes of the interaction of different, sometimes conflicting interests. The most promising hypotheses based on short/ medium term benefits appear to vary with breeding system, reproductive biology, socioecological factors, and life history characteristics. Explanations based on life history variables or long-term evolutionary processes related to cooperation appear to have broader applications, but nevertheless fail to explain infant handling in all its manifestations. We end by calling for more quantitative comparative and longitudinal research to further elucidate our understanding of this puzzling behavior.