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

Abstract

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.

Keywords: Pan troglodytes; chimpanzees; collective action; cooperation; territoriality.

Fig. 1.

Individual variation in patrol participation, calculated as the percentage of patrols a male participated in out of the patrols that occurred while he was alive and of patrolling age (≥13 y). Each bar represents the percentage of patrols in which an individual male participated. Values are ordered from left to right along the x axis from low to high; the number of patrols that occurred while the male was alive and of patrolling age is indicated below each bar. Light gray bars indicate the 10 males who were alive and of patrolling age for all 284 patrols.

Fig. S1.

Variation in patrol size (the number of males aged ≥13 y participating in a patrol). n = 284 patrols.

Fig. S2.

Age and patrolling participation, calculated as the number of patrols in which a male participated relative to the number that occurred while he was alive and of patrolling age (≥13 y). We calculated patrol participation and age (on January 1) for each male on a yearly basis. Data points represent the means of these yearly values; error bars represent SDs.

Fig. 2.

The relationship between the aggregate patrolling effort (the number of patrols per day in each year of the study) and group size (the number of males aged ≥13 y in the group on January 1 of each year). n = 20 y.

Fig. 3.

The relationship between the aggregate patrolling effort (patrol size: the number of males in the patrol) and group size (number of males aged ≥13 y in the group on the day the patrol occurred). n = 284 patrols.