Female monkeys use both the carrot and the stick to promote male participation in intergroup fights

Group-level cooperation often poses a social dilemma in which joint action may be difficult to achieve. Theoretical models and experimental work on humans show that social incentives, such as punishment of defectors and rewarding of cooperators, can promote cooperation in groups of unrelated individuals. Here, we demonstrate that these processes can operate in a non-human animal species, and be used to effectively promote the production of a public good. We took advantage of the fact that intergroup fights in vervet monkeys (Chlorocebus aethiops pygerythrus) are characterized by episodes of intergroup aggression with pauses in-between. During pauses, females selectively groomed males that had participated in the previous aggressive episode, but aggressed male group members that had not. In subsequent (i.e. future) episodes, males who had received either aggression or grooming participated above their personal base-line level. Therefore, female–male aggression and grooming both appear to function as social incentives that effectively promote male participation in intergroup fights. Importantly, females stood to gain much from recruiting males as the probability of winning intergroup fights was dependent on the number of active participants, relative to the number of fighters in the opposing group. Furthermore, females appear to maximize the benefits gained from recruiting males as they primarily used social incentives where and when high-quality food resources, which are the resources primarily limiting to female fitness, were at stake

Intra- and interindividual differences in the costs and benefits of intergroup aggression in female vervet monkeys

In social species, fighting in intergroup conflicts is one of the riskiest cooperative activities group members engage in, particularly for individuals of the smaller sex. In a number of species, female group members are significantly smaller than males, so the costs associated with intergroup aggression outweigh the potential benefits and females avoid participating. Studies conducted on species in which females are active participants have consistently found that they fight to defend access to food resources and that high-ranking females tend to be more active than low-rankers. However, additional factors may modulate the costs and benefits of participation, creating differences between individuals and variability within individuals over time. In this study, we investigated costs and benefits that potentially affect female vervet monkey, Chlorocebus aethiops pygerythrus, participation in intergroup conflicts. We observed the participation of 35 females in three groups, during 115 intergroup conflicts. Our findings suggest that female vervet monkeys defend access to valuable food resources, as well as to areas that are intensely used in the long term; however, rank also influenced female participation indicating that the potential benefits gained from resource defence vary with one's priority of access to these resources. We found that females were more likely to participate aggressively when they did not have an infant, and when they received more male support throughout the intergroup conflict, suggesting these factors influence the perceived risk, or costs, of intergroup aggression. Because we observed considerable temporal variability in both the proportion of female group members with infants and the number and identity of male group members (i.e. amount of male support provided), the relative fighting ability of neighbouring groups will inevitably fluctuate. Thus, our findings may help explain the lack of stable intergroup dominance relationships observed in many studies of intergroup conflict

Data from: Female monkeys use both the carrot and the stick to promote male participation in intergroup fights

Group-level cooperation often poses a social dilemma in which joint action may be difficult to achieve. Theoretical models and experimental work on humans show that social incentives, such as punishment of defectors and rewarding of cooperators, can promote cooperation in groups of unrelated individuals. Here, we demonstrate that these processes can operate in a non-human animal species, and be used to effectively promote the production of a public good. We took advantage of the fact that intergroup fights in vervet monkeys (Chlorocebus aethiops pygerythrus) are characterized by episodes of intergroup aggression with pauses in-between. During pauses, females selectively groomed males that had participated in the previous aggressive episode, but aggressed male group members that had not. In subsequent (i.e. future) episodes, males who had received either aggression or grooming participated above their personal base-line level. Therefore, female–male aggression and grooming both appear to function as social incentives that effectively promote male participation in intergroup fights. Importantly, females stood to gain much from recruiting males as the probability of winning intergroup fights was dependent on the number of active participants, relative to the number of fighters in the opposing group. Furthermore, females appear to maximize the benefits gained from recruiting males as they primarily used social incentives where and when high-quality food resources, which are the resources primarily limiting to female fitness, were at stake

Arseneau-Robar_Methods_Results_ESM from Female monkeys use both the carrot and the stick to promote male participation in intergroup fights

Female monkeys use both the carrot and the stick to promote male participation in intergroup fights: Spatio-Temporal Variability in the Occurrence of Female-Male Aggression and Female-Male Groomin