Intergroup Conflicts in Vervet Monkeys

Females and males each fight in intergroup conflicts for different reasons.

High-ranking females, who stand to gain the most by defending monopolizable resources, and females without infants, who are less averse to the risks associated with fighting, are most active in intergroup conflicts. Females are primarily interested in defending access to food resources, but also defend important areas of their home range. (Arseneau-Robar et al. 2017 Animal Behaviour)

Males who are likely to have sired infants in the group are very protective, responding reactively when the other group is aggressive such that offspring might be at risk. Males often support females who are trying to instigate a fight, but they primarily do so during the mating season when doing so is associated with higher mating success. (Arseneau et al. 2015 Animal Behaviour, Arseneau-Robar et al. 2016 Scientific Reports)

Using Social Incentives to Resolve Conflicts of Interest

Conflicts of interest often arise between likely sires, who are averse to the risks intergroup conflicts pose to infants, and females, who need access to high-quality food resources to successfully produce and raise offspring.

When conflicts of interest arise, females use both punishment and rewards to obtain higher levels of male support. Females direct aggression towards males who are not participating; punished males are more likely to participate aggressively afterwards, and these increased levels of participation are higher than would be expected given each male’s baseline levels of participation in intergroup conflicts. (Arseneau-Robar et al. 2016 Proceedings of the Royal Society B)

Conversely, females preferentially groom males who have supported them in the intergroup conflict and males who receive this reward maintain their high levels of participation. The observed levels of participation after being rewarded are higher than each male’s baseline levels.

Males who are likely sires also have strategies for achieving more ideal outcomes when conflicts of interest arise. They use punishment/coercion to prevent encounters from escalating into intergroup fights that could put offspring at risk. Likely sires direct aggression towards group members who (try to) instigate a fight, and those who are punished/coerced are less likely to keep on fighting than would be expected given their baseline participation levels. Likely sires are most likely to use this strategy when they themselves are wounded, and so might feel unable to protect offspring should the need arise. (Arseneau-Robar et al. 2018 Proceedings of the Royal Society B)

Conclusions

Our findings highlight the heterogeneity that exists within social groups, and the consequences this heterogeneity has for the evolution of cooperation in social groups. Interindividual variability in the costs and benefits of intergroup aggression causes conflicts of interest to arise among group members. This creates selective pressure for female strategies that increase the odds of winning an intergroup conflict, and male strategies which prevent intergroup encounters from escalating into high-risk fights. Because the number of active participants, relative to the number in the opposing group, determines whether the group wins or loses, using punishment and rewards to recruit males likely improves the odds females win access to fitness-limiting resources. (Arseneau-Robar et al. 2016 Proceedings of the Royal Society B)

Conversely, male punishment/coercion was highly effective in preventing intergroup conflicts from erupting when used when the two groups were near to one another but no fighting had yet occurred. When males used punishment/coercion in the middle of an ongoing intergroup conflict, this strategy successfully ended the fight in ~50% of cases and intergroup conflicts tended to end sooner than expected. (Arseneau-Robar et al. 2018 Proceedings of the Royal Society B)