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.

群体层面的合作往往会引发社会困境（social dilemma），此时联合行动往往难以达成。针对人类的理论模型与实验研究显示，社会激励——如对背叛者的惩罚、对合作者的奖励——可促进无亲缘关系个体组成的群体中的合作行为。本研究证实，这类社会激励过程可作用于非人类动物物种，并能有效推动公共物品（public good）的生产。我们借助黑长尾猴（Chlorocebus aethiops pygerythrus）群体间冲突的特征：这类冲突以多段带有间歇的群体间攻击行为为表现形式。在间歇期内，雌性会选择性地为参与过此前攻击回合的雄性梳理毛发，同时攻击未参与攻击的雄性群体成员。在后续的冲突回合中，曾遭受雌性攻击或获得梳理的雄性，其参与冲突的水平会高于其个人基线水平。因此，雌性对雄性的攻击与梳理行为，似乎均承担了社会激励的功能，可有效推动雄性参与群体间冲突。重要的是，雌性通过招募雄性能够获得显著收益，因为群体间冲突的获胜概率取决于活跃参与者的数量，以及该数量与对立群体参战者数量的相对比例。此外，雌性似乎最大化了招募雄性所带来的收益：她们仅在优质食物资源——这类资源是限制雌性适合度（fitness）的核心因素——面临威胁时，才会动用这类社会激励手段。