Infant handling by female mountain gorillas: Establishing its frequency, function, and (ir)relevance for life history evolution

Infant handling increases grooming towards mothers in wild geladas (Theropithecus gelada)

Infant handling - involving affiliative behavior from non-mothers to infants - is a phenomenon that is variably present in Old World monkeys and can be granted by mothers to obtain social services, such as grooming. Here we investigated for the first time whether infant handling could influence grooming exchange in wild geladas. We gathered data on the population of Kundi highland (Ethiopia) in 2019/2020. Via sampling on 15 focal mothers from eight different One-Male Units, we video-recorded 55 grooming sessions between focal mothers and non-focal females (mothers or non-mothers). We also recorded the possible occurrence of infant handling performed by non-focal females. We found that grooming sessions were longer between mother and non-mothers and in the presence than in the absence of infant handling. Hence, our results show that infant handling can influence the grooming exchange between wild gelada females. Because grooming is used to establish and reinforce social bonds in primates, infant handling may act as a 'social bridge' in a female bonded society. From an evolutionary perspective, infant handling strategies might represent the stepping stone to more complex forms of infant care, such as allomaternal care and cooperative breeding.