Male monkeys fight in between-group conflicts as protective parents and reluctant recruits

In many social species, group members cooperate to defend a communal home range. Fighting in between-group conflicts carries an opportunity cost, a risk of injury or death, and the possibility of exploitation by free-riding group members. As a result, it is rare that all group members fight in a given between-group conflict, and individual participation in range defence is often highly variable. Thus, to understand the patterns of behaviour observed at the group level, we must first understand the causes of within- and between-individual variability. Although sex differences have been well studied, our understanding of the relative importance of the various mechanisms promoting between-group aggression within a sex is limited. We observed the participation of 22 male vervet monkeys, Chlorocebus aethiops pygerythrus, in 126 between-group conflicts, and then partitioned aggressive acts according to the context in which they occurred. Using this approach, we found evidence that two mechanisms drive male between-group aggression and, therefore, that individual variability is in part driven by the multiple selective benefits of participation. First, males that were likely to have sired offspring tended to exhibit defensive aggression and were more active when infants were present in the group, suggesting they fight to defend probable offspring. Second, males were more likely to support females in initiating between-group aggression just prior to, and during, the mating season. Female vervet monkeys are able to exert female choice, and males that frequently supported female instigators tended to enjoy the highest mating success. These results indicate that males probably use between-group aggression to improve their reputation with choosy females and subsequently maximize their mating success. Our findings indicate that a greater understanding of the evolutionary mechanisms promoting cooperative home range defence can be gained if we consider the context in which acts of between-group aggression occur. (C) 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.