Kevin Langergraber: Arizona State University

Many animals carry out activities together because the benefits derived from collective action exceed those that can be achieved individually. But 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-year study of one of the most striking examples of collective action in non-human animals, territorial boundary patrolling by male chimpanzees, are consistent with these ideas: individual participation varied positively with both paternity success, i.e., the benefits, and dominance rank, i.e., a proxy for costs, and negatively with group size. Collective action theory nevertheless could not explain all our results because individual patrolling effort was higher and less variable than participation in intergroup aggression in other primate species, and males often patrolled when they had no offspring or maternal relatives in the group to protect. In addition, the aggregate patrolling effort of the group did not decrease with group size. These results are 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 ultimately long-term reproductive success.