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

Abstract

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.

Keywords: n-player games; public good; punishment; reward; social dilemma; social incentives.

Figure 1.

Typical levels of participation in intergroup fights, calculated as the proportion of aggressive episodes that each male participated in during intergroup fights in which no social incentives (FM-agg or FM-gr) were observed. Each dot represents one male in the population (n = 20).

Figure 2.

The proportion of aggressive episodes that targeted males participated in before (light) versus after (dark) receiving aggression from female group members (a) at the population level and (b) at the individual level (note: each dot represents the proportion of aggressive episodes participated in for one male in the population (n = 9 males); means are portrayed by the white dotted line and medians by the dark line). (c) The proportion of aggressive episodes targeted males participated in after being aggressed by a female group member, compared to their base-line level of participation (i.e. proportion of episodes participated in during intergroup fights where social incentives were observed, but they were not the male targeted). Significance levels denoted by *p < 0.05 and ***p < 0.001.

Figure 3.

The proportion of aggressive episodes that targeted males participated in before (light) versus after (dark) receiving grooming from female group members (a) at the population level and (b) at the individual level (note: each dot represents the proportion of aggressive episodes participated in for one male in the population (n = 11 males); means are portrayed by the white dotted line and medians by the dark line). (c) The proportion of aggressive episodes targeted males participated in after being groomed by a female group member compared to their base-line level of participation (i.e. proportion of episodes participated in during intergroup fights where social incentives were observed, but they were not the male targeted). Significance levels denoted by *p < 0.05 and ***p < 0.001.