Social Relationships in Male Chimpanzees: Form, Function, and Development

Understanding the evolution of cooperation is a central goal in animal behavior research. In several animal taxa, socioecological environments that promote frequent interaction and social tolerance have favored the evolution of strong, equitable, and enduring social bonds, which facilitate cooperation and confer fitness benefits. Among males, strong bonds are believed to have evolved in the context of long-term alliances that help individuals compete for dominance status and mating access, but it remains unclear in some species what factors predict the strength and quality of bonds and how sociality relates to adaptive outcomes. To fill these gaps, this dissertation presents three studies of male chimpanzees at Gombe National Park, Tanzania, addressing the form, function, and development of male social relationships. Maternal brothers who were close in age formed the strongest bonds, strong bonds were associated with more reciprocal grooming relationships, and the strength of bonds were stable for an average of two years, while lasting up to 13 years. For other males, similarity in age and rank had negligible effects on bond strength, suggesting that bond strength results from a more complex process than a simple accounting of basic characteristics. Additionally, these social bonds, identified using both association in small groups and grooming activity, showed positive relationships with changes in dominance. In combination with prior studies, these results suggest that having strong bonds is a valuable strategy for achieving higher rank and, ultimately, increased reproductive success. Lastly, immature males who associated more with particular adult males while growing up were more likely to both associate and groom with those same males after entering adulthood. By contrast, association rates among immature male peers were not correlated with bond strength as adults. These findings suggest that the persistence of social relationships beyond those between mothers and offspring are more likely to evolve in long-lived species where young males enter adult hierarchies comprised of stronger or more socially experienced competitors. Overall, these studies reinforce the notion that social bonds are a chimpanzee universal, fill in gaps about the relationship between sociality and fitness, and emphasize the utility of a prolonged immature period.