Biological, social and ecological correlates of social play in immature Bwindi mountain gorillas, Gorilla beringei beringei

Social play is a widespread behaviour in mammals, yet quantifying the proximate and ultimate costs and benefits of such play for immature individuals can be challenging. Here we assess how social play in Bwindi mountain gorillas varies with biological (age, sex, weaning status, mother parity), social (partner availability) and ecological factors (fruit consumption), while considering three nonmutually exclusive hypotheses for the function of play: the motor training, social relationship and energetic constraints hypotheses. We examine variation in social play in a data set representing over 6000 h of focal sampling of 33 immature Bwindi mountain gorillas (aged 0.6–8.0 years) spanning 18 years. We found that male and female mountain gorillas have different ontogenies of social play. Male gorillas played more than females, adhering to our prediction that males would be under stronger selection pressure than females to enhance development of social and motor skills. The rate of play per individual was also correlated with the number of other immature individuals present, suggesting that the development of certain skills may be linked to social opportunities for play. Furthermore, play rate was positively correlated with fruit consumption and weaning status, but not with a mother's parity, suggesting energetic constraints of play. Variation in social play in gorillas is consistent with predictions for both motor and social development functions and suggests that social play is under energetic constraints. Our results demonstrate that sex differences in immature play reflect sex differences in competition and affiliation as adults, suggesting that the costs and benefits of social play are not equal between the sexes.