The Collective Action Problem but Not Numerical Superiority Explains Success in Intergroup Encounters in Verreaux’s Sifaka (Propithecus verreauxi): Implications for Individual Participation and Free-Riding

Abstract

Socioecological theory predicts that larger groups are able to gain better access to resources because of their numerical advantage in intergroup contests. Nevertheless, the numerical superiority hypothesis has received mixed support, perhaps due to the collective action problem and sex differences in strategies and payoffs. We tested the numerical superiority and the collective action problem hypotheses using 141 intergroup encounters in a 7-year dataset on 5 groups of Verreaux’s sifaka (Propithecus verreauxi) at Ankoatsifaka Research Station in Kirindy Mitea National Park in western Madagascar. Contrary to the numerical superiority hypothesis, larger groups were not more likely to supplant smaller groups, nor was success determined by participation of a larger number of one sex. Consistent with a collective action problem, group size negatively affected participation. Notably, the sexes differed in which factors predicted individual participation. Male participation, which may serve to defend mating opportunities with resident females and signal competitive ability to neighboring groups, was greater than female participation, though we did not detect any strong predictor of male participation. Female participation was more likely during the lactation season, when food resources were scarce, than during the mating season. Contrary to previous studies, which argued that mothers with infants avoid intergroup conflicts because of the risk of infanticide, our study suggests that maternal energetic stress due to lactation and food scarcity increases the benefits of defending resources and instead drives mothers to participate in intergroup encounters. Thus, individual reproductive payoffs provide the private incentives needed to induce collective action.