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 characterised 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 maximise 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催生社会困境，在此类情境中集体行动往往难以达成。针对人类群体的理论模型与实验研究显示，诸如惩罚背叛者、奖励合作者这类社会激励手段，能够推动无亲缘关系个体组成的群体实现合作。本研究证实，此类社会激励机制可在非人动物类群中发挥作用，并可被用于有效促进公共物品（public good）的生产。我们依托黑长尾猴（Chlorocebus aethiops pygerythrus）的群体间争斗特征开展研究：该物种的群体间攻击行为以多个攻击回合为单元，回合之间存在间歇期。在攻击间隙阶段，雌性个体会有选择性地为参与过前一轮攻击回合的雄性梳理毛发，却会攻击未参与该轮攻击的雄性群体同伴。在后续的争斗回合中，无论此前遭受过雌性攻击还是梳理的雄性，其参与争斗的活跃程度均显著高于其个体基线水平。由此可见，雌雄个体间的攻击与梳理行为，均发挥了社会激励的功能，可有效推动雄性参与群体间争斗。尤为关键的是，雌性个体通过招募雄性可获得显著收益：群体间争斗的获胜概率，取决于本方活跃参战者的数量相较于敌方参战者数量的占比。进一步分析显示，雌性个体最大化了招募雄性所带来的收益——它们仅在与高质量食物资源相关的情境中启用社会激励手段，而此类食物资源正是限制雌性繁殖适合度的核心限制性资源，此时该资源正处于争夺状态。