Infant handling in bonobos (Pan paniscus): Exploring functional hypotheses and the relationship to oxytocin.

Infant handling describes interactions between infants and non-maternal group members and is widespread across mammalian taxa. The expression of infant handling behaviors, defined as any affiliative or agonistic interaction between a group member and an infant, varies considerably among primate species. Several functional hypotheses may explain the adaptive value of infant handling including the Kin Selection hypothesis, which describes handling as a mechanism through which indirect fitness is increased and predicts a bias in handling behaviors directed toward related (genetic) infants; the Alliance Formation hypothesis, which describes handling as a social commodity and predicts females with infants will support handlers during conflict; and the Learning-to-Mother hypothesis, which describes handling as a mechanism through which handlers learn species-specific maternal behaviors and predicts that handling will occur most frequently in immature and nulliparous females. Using behavioral observation and data on urinary oxytocin, a neuropeptide hormone known to modulate maternal care and social bonds in mammals, the purpose of this study was to describe the pattern of infant handling in bonobos (Pan paniscus) and to explore proposed functional hypotheses. Data show that related infant-handler dyads occurred significantly more frequently than unrelated infant-handler dyads during some of the study period and that handling was positively correlated with support during conflict. Data also showed that immature and nulliparous females handled infants significantly more than other age-sex categories and exhibited higher post handling oxytocin values than other age-sex class. The trends identified in this data set provide insight into the role oxytocin may play in facilitating care-giving behaviors in young female bonobos and help to narrow the focus of future research efforts, particularly those associated with the Kin Selection, Alliance Formation, and Learning-to-Mother functional hypotheses.