Chimpanzees as a Model for Evaluating the Evolution of Human Hyper-Sociality

Much of humanity’s success across the globe has been attributed to our hyper-sociality and associated traits, such as a proclivity to form social bonds with non-kin, lethal intergroup aggression, and complex social cognition. This dissertation uses chimpanzees, one of our closest living relatives which share key aspects of our social organization and behavior that are uncommon for mammals, as a model for examining forces driving the evolution of this hyper-sociality. Study 1 explores differences in common methods of quantifying and comparing social tie strengths, including the assumptions and implications of these metrics, particularly as they pertain to links to fitness outcomes. A case study using real-world data from Ngogo reveals that although the overall impact of these choices may appear small, the differences are often concentrated in predictable subsets of the data. Study 2 explores why activities related to intergroup conflicts in chimpanzees are overwhelmingly male-biased, as they are in humans. Data over a 27-year period from Ngogo reveals that females are sensitive to the costs imposed by their reproductive biology, which largely deter them from participating in intergroup conflicts given potentially limited benefits. However, under certain conditions, such as times of territorial instability, the balance of costs and benefits can shift to overcome these deterrents. Moreover, substantial female participation in patrols and intergroup aggression, including intergroup killing, was associated with one group seizing considerable territory from another after a permanent community fission, suggesting that females may play an important role in chimpanzee intergroup conflicts, despite a predominant overall male-bias. Study 3 challenges a key premise underlying the “cooperative eye hypothesis,” which posits that humans are unique in having visible white sclera, and that this trait co-evolved with our complex social cognition by facilitating the ability to direct and attend to each other’s gaze direction. Examination of 230 chimpanzees from Ngogo reveals that almost 1 in 6 chimpanzees had full or partial white sclera in at least one eye. Longitudinal comparisons from a subset of individuals suggest that most chimpanzees are born with white or light sclera that tends to quickly darken with age, but not for all individuals.