Abstract: Among group-living animals, social systems are expected to evolve via selection on individuals to maximize the benefits of group membership while minimizing the costs imposed by group mates. When costs of group membership increase relative to benefits, this can result in dramatic, permanent changes to group composition via group fission events, or more subtle changes to social structure. At the same time, research in animal behavior is increasingly focused on the ways that the social environment influences individual behavior and fitness. I will discuss work using long-term data on chimpanzees in Gombe National Park, Tanzania, analyzing the precursors of unique community fission event, as well as longitudinal changes to social network structure, to better understand the costs and benefits of group membership among males. Additionally, I will discuss the consequences of social structure on aggression between males as it relates to in-group biases in chimpanzees.