Maternal gregariousness and female audience effects mediate mother–infant proximity in wild chimpanzees

In animal species with parental care, maintaining offspring–carer proximity is an important adaptation protecting offspring from threats such as predation and conspecific aggression, but doing so may limit other social opportunities. Investigating factors impacting mother–infant proximity can, therefore, provide insights into the evolution of maternal responses towards multidimensional threats. Here, we examine the social factors impacting mother–offspring proximity in two populations of wild chimpanzees with differing levels of infanticidal threats, eastern chimpanzees, Pan troglodytes schweinfurthii , in Budongo Forest, Uganda and western chimpanzees, P. t. verus , in Taï Forest, Ivory Coast. We assessed whether (1) the number of males and females in fission–fusion subgroups predicts proximity levels between mothers and their youngest infants, (2) whether it is mediated by maternal gregariousness and (3) whether this relationship differs in the two populations. In both populations and independent of maternal gregariousness, we found no clear relationship between mother–infant proximity and the number of males in the party. However, in Budongo, where an infanticidal threat is high, mother–infant proximity was mediated by both maternal gregariousness and the number of other females present. Less gregarious mothers were closer to their youngest offspring in parties with large numbers of females, while the opposite pattern applied to highly gregarious mothers. In Taï, more gregarious females were more often in proximity with their offspring. Our results demonstrate that the immediate social environment, maternal social phenotype and overall community-specific threats can all influence maternal response to varying exposure to threats. The consequences of exposure to this environment on offspring’s social development merit further investigation. • Close mother–infant proximity in chimpanzees protects offspring from threats. • Mother–infant proximity can be affected by gregariousness and party size. • In Budongo, female party size and gregariousness predicted mother–infant proximity. • In both populations, male party size did not predict mother–infant proximity. • These population-level differences highlight chimpanzee’s behavioural flexibility.