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.当个体即便未付出合作参与成本，仍可从合作中获益时，集体行动该如何演化？一种颇具影响力的理论视角指出，集体行动困境（collective action problem）普遍存在，尤其在群体规模较大时，但当那些从集体公益中获益更多、或能以更低成本产出集体公益的个体，将其作为副产品提供给他人时，该困境或可得到解决。针对非人类动物中最引人注目的集体行动案例之一——雄性黑猩猩的领地边界巡逻——所开展的一项为期20年的研究，其得出的多项结果与上述理论观点相符。个体巡逻的概率会在以下情形下升高：(i) 因在群体中拥有较多后代而获益更多；(ii) 因自身较高的支配等级与优越的身体状况，承担的成本相对较低；(iii) 群体规模相对较小。不过另有多项发现更适用于群体扩张理论（group augmentation theory）的解释：该理论提出，若集体行动能够推动群体规模扩大，并带来繁殖成功率的长期提升，那么即便个体当下几乎无法即刻获益，也应当承担集体行动的短期成本。为支持该理论，研究结果显示：(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) 即便雄性在群体中没有后代或母系亲属，仍会频繁参与巡逻；(iii) 群体整体的巡逻投入并未随群体规模扩大而降低。我们认为，群体扩张理论在集体行动研究领域应当获得更多关注。