Data from: Group augmentation, collective action, and territorial boundary patrols by male chimpanzees

How can collective action evolve when individuals benefit from cooperation regardless of whether they pay its participation costs? According to one influential perspective, collective action problems are common, especially when groups are large, but may be solved when individuals who have more to gain from the collective good or can produce it at low costs provide it to others as a byproduct. Several results from a 20-y study of one of the most striking examples of collective action in nonhuman animals, territorial boundary patrolling by male chimpanzees, are consistent with these ideas. Individuals were more likely to patrol when (i) they had more to gain because they had many offspring in the group; (ii) they incurred relatively low costs because of their high dominance rank and superior physical condition; and (iii) the group size was relatively small. However, several other findings were better explained by group augmentation theory, which proposes that individuals should bear the short-term costs of collective action even when they have little to gain immediately if such action leads to increases in group size and long-term increases in reproductive success. In support of this theory, (i) individual patrolling effort was higher and less variable than participation in intergroup aggression in other primate species; (ii) males often patrolled when they had no offspring or maternal relatives in the group; and (iii) the aggregate patrolling effort of the group did not decrease with group size. We propose that group augmentation theory deserves more consideration in research on collective action.

当个体无需付出参与成本即可从合作行为中获益时，集体行动该如何演化？根据一种颇具影响力的学术视角，集体行动困境普遍存在，尤其在群体规模较大的场景中，但当部分个体从集体利益中获益更多，或以更低成本产出集体利益，并将其作为副产品惠及其他个体时，此类困境或可得到破解。一项针对非人类动物最具代表性的集体行动案例——雄性黑猩猩的领地边界巡逻——开展的为期20年的研究，所得出的多项结论与上述观点一致。当满足以下条件时，个体参与巡逻的概率会更高：(i) 自身获益更多，即群体中拥有更多后代；(ii) 承担的成本相对更低，得益于其较高的统治等级与优越的身体状况；(iii) 群体规模相对偏小。不过，另有若干研究发现更能用群体扩增理论（group augmentation theory）加以阐释。该理论主张，即便个体当下从集体行动中获益寥寥，但若此类行动能够推动群体规模扩大，并长期提升繁殖成功率，那么个体理应承担集体行动的短期成本。为佐证这一理论，现有研究显示：(i) 相较于其他灵长类物种的群体间攻击参与行为，个体的巡逻投入更高且变异程度更低；(ii) 即便群体中不存在自身的后代或母系亲属，雄性个体也常会参与巡逻；(iii) 群体的整体巡逻投入并未随群体规模增大而出现下降。我们认为，在集体行动相关研究领域，群体扩增理论值得获得更多的关注与探讨。