Pattern, function and constraint of infant handling in wild Japanese macaques

In group-living primates, non-mother individuals frequently contact and handle an infant. Although previous studies showed that the function of this behaviour varies among and within species, few have considered a possibility the handlers' accessibility to an infant may be restricted by a social relationship between the handlers and infants' mothers. Considering this proximate constraint on the choice and handling of an infant, we aimed to clarify the function of infant handling in wild Japanese macaques (Macaca fuscata). We collected behavioural data on infant handling from 24 mother-infant pairs over three years on Kinkazan Island, Japan. We tested the predictions of three functional hypotheses: learning-to-mother, kin selection and reproductive competition among females. We found that handling by males rarely occurred. The proximity between mother and a potential handler positively affected the frequency of infant handling, and its effect interacted with the handler's parity, kinship and relative dominance relationship with the mother. Hence, the effects of handlers' proximity were prominent for nulliparous, related and relatively lower-ranked females relative to parous, unrelated and higher-ranked females. We also found that infants were handled positively in most cases. These results fit the predictions of the learning-to-mother hypothesis and kin selection hypothesis, indicating that the functions of infant handling varied with the attributes of the handlers. This study provides rare results clarifying the function of infant handling in wild primates, based on the fact that handler behaviour is constrained by accessibility to the mother.