Persistent Infant Handling Despite an Infant’s Negative Reaction by Female Japanese Macaques in Yakushima (Macaca fuscata yakui): Exploring its Function, Process, and Relationship to Social Tolerance

Females other than the mother often handle infant primates. In some cases, this infant handling can affect the infant negatively and persist after that (excessive infant handling [EIH]). We tested two hypothesized functions (reproductive competition and social bridging) and three hypothesized mechanisms (social exchange, inexperience, and accessibility) for EIH to investigate why and how handlers perform EIH. We recorded infant handling bouts (n = 1446) involving 12 wild, 0- to 16-week-old Japanese macaques inhabiting Yakushima Island (Macaca fuscata yakui) through all-occurrence sampling. We found that aggression tended not to become EIH and that adults performed EIH more than subadults and juveniles. We also found that handlers groomed mothers less before EIH than before non-EIH. Handlers performed EIH, rather than non-EIH, toward infants who were out of contact with mothers and those of closer-ranking, higher-ranking, and unfamiliar females. These results suggest that EIH is not performed for reproductive competition but for social bridging with higher-value social partners. Social bridges with such individuals may be important in despotic species, such as that of the Japanese macaque. Our findings also suggest that EIH does not occur through proximate mechanisms of social exchange and inexperience with infants but partially through accessibility of infants. Handlers reduce the cost of EIH by choosing accessible infants (those who were out of contact with mothers and those of closer-ranking females), but at the same time, maternal tolerance allows the handlers to choose inaccessible infants (those of higher-ranking and unfamiliar females). Our study provides clear evidence of social tolerance in a despotic system and insights into natal attraction that negatively affects infants in wild primates.