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Chimpanzee Third Party Behavior: Insights into the Evolution of Human Conflict Management Open Access

Conflict management is an important topic to the fields of animal behavior and human evolution, as mitigating the negative consequences of aggression is a crucial component to our ability to coexist and cooperate in social groups. Exploring conflict management behaviors performed by third parties can shed light on our understanding of the origins of prosocial behavior: given the risks of escalated or redirected aggression, the fitness benefits that third parties accrue which may drive them to participate in conflict management strategies remain unclear. The purpose of this dissertation is to investigate conflict management in wild chimpanzees (Pan troglodytes) by exploring the ontogeny, patterns and function of third party conflict management behaviors.Chapter 1 provides an overview of the evolution of group living and the costs of aggression that accompany life in a social group. I introduce conflict management behaviors as a strategy for mitigating the deleterious outcomes of aggression and promoting group stability. Next, I argue that chimpanzee are a compelling species in which to investigate patterns of third party conflict management given their elevated rates of within group aggression compared to humans, diverse repertoire of third party behaviors, fluid social groups and close genetic relationship to humans. I focus on two third party conflict management behaviors that may be prosocial in nature: third party affiliation in which the third party extends affiliative behaviors to the victim of the aggression, and policing in which a third party impartially intervenes in the conflict.Chapter 2 uses 26 years of data to investigate the emergence of third party affiliation in wild immature eastern chimpanzees (P.t. schweinfurthii) of Gombe National Park, Tanzania. I find that immatures between the ages of 1.5 - 12.0 years do not express third party affiliation as measured by grooming or play. These results contrast with findings from captive studies which found that chimpanzees as young as 6 years of age demonstrated third party affiliation, suggesting that variation in conflict management behaviors of young chimpanzees may be heavily influenced by social, ecological, and demographic factors. Chapter 3 investigates four hypotheses for the functional value of policing behavior in chimpanzees, a conflict management strategy whereby uninvolved third parties monitor and control conflicts between members of their social group. This study analyzes 19 years of long-term behavioral data from Gombe National Park, Tanzania, finding that while policing behavior occurs at relatively low levels (3.3% of all aggressive interactions are policed), individuals are more likely to police aggressive interactions involving maternal kin compared to those involving individuals not related through the matriline, so long as their relative social power is sufficient. Furthermore, mature males police altercations between individuals of all age-sex classes, and males of varying dominance rank act as policers. These results provide at least partial support for factors that influence policing including kinship, the assurance of sexual benefits and group stability.In order to expand upon Chapter 3, I examine the relationship among policing, aggression and measures of community cohesion in an explicit test of the Group Stability Hypothesis for the function of policing behavior in wild chimpanzees in Chapter 4. The Group Stability Hypothesis proposes that policing enhances group cohesion through the reduction of group-level rates of conflict. However, the results of this study found that in wild chimpanzees, community cohesion was better explained by socio-ecological factors, such as community size, season, or presence of estrous females, than either policing or aggression. In summary, this study provides no support for the Group Stability Hypothesis in terms of broad social patterns in wild chimpanzees.This dissertation provides evidence that the fission-fusion dynamics and broader socio-ecology of wild populations play an important role in the expression of third party conflict management behaviors in chimpanzees. Importantly, this lends support to the notion that conflict management strategies in chimpanzees are flexible, and may exist at high or low levels according to a population’s demography and grouping patterns. This suggests that the last common ancestor of the Pan-Homo clade likely possessed the foundations for conflict management, and that these behaviors could have evolved in complexity within the Homo lineage as a result of changes in population density and social structure.

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