When population growth intensifies intergroup competition, female colobus monkeys free-ride less
In many social species, intergroup aggression is a cooperative activity that produces public goods such as a safe and stable social environment and a home range containing the resources required to survive and reproduce. In this study, we investigate temporal variation in intergroup aggression in a growing population of colobus monkeys to ask a novel question: "Who stepped-up to produce these public goods when the competitive landscape changed?". Both whole-group encounters and male incursions occurred more frequently as the population grew. Males and females were both more likely to participate in whole-group encounters when monopolizable food resources were available, indicating both sexes engaged in food defence. However, only females increasingly did so over time, suggesting that when intergroup competition intensified, it was females who increasingly invested in home range defence. Females were also more active in male incursions at high population densities, suggesting they also worked harder to maintain a safe and stable social environment over time. This is not to say that males were chronic free-riders when it came to maintaining public goods. Males consistently participated in the majority of intergroup interactions throughout the study period, indicating they may have lacked the capacity to invest more time and effort.