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.

Patrol dataPatrol #: A unique identifier for each of 284 patrols. Date = Date each patrol occurred. Male = For each patrol, each male who was present in the community and >=13 years when the patrol ocurred is listed in his own row. Patrol participation = whether the male in that row participated in the patrol (1) or not (0). Age squared = the male's age in years at the time of the patrol, squared. Rank: the male's standardized dominance rank in the year the patrol occurred. Paternity success = at the time of the patrol, the sum of the relatedness values (0.5) to the male's offspring living in the community. Maternal relatedness = at the time of the patrol, the sum of the relatedness values to the male's close maternal relatives. See methods in supplementary info of original paper for details.