Patterns of participation and free riding in territorial conflicts among ringtailed lemurs (Lemur catta)

Cooperation in animal social groups may be limited by the threat of free riding, the potential for individuals to reap the benefits of other individual's actions without paying their share of the costs. Here we investigate the factors that influence individual contributions to group-level benefits by studying individual participation in territorial defense among female ringtailed lemurs (Lemur catta). To control for potentially confounding factors, particularly group size, we studied two semi-free-ranging groups at the Duke University Primate Center. First, we used a combination of experimental and observational methods to investigate the costs and benefits of territorial defense for individual lemurs. We found three indications of costs: physical contact occurred during inter-group encounters, participation in territorial defense was negatively correlated with ambient temperature, and rates of self-directed behaviors increased during encounters. Benefits were more difficult to quantify, but observational and experimental tests suggested that individuals shared the gains of territorial defense by foraging in defended territories. Thus, during experiments in which one of the groups was prevented from defending its territory, the free-ranging group made more frequent incursions into the other group's territory. Second, we examined variation in participation in territorial defense. Individuals varied significantly in their rates of aggression and genital marking during inter-group encounters. The extensive variation documented among individuals was partially accounted for by dominance rank, kinship and patterns of parental care. However, we found no evidence to suggest that participation was enforced through punishment (policing) or exchange of benefits involving grooming. In conclusion, this study provides further insights into cooperative behavior in mammalian social groups by revealing how the costs and benefits of territoriality influence patterns of individual participation in the context of shared (collective) goods.