Factors influencing intergroup encounter dynamics in a wild bonobo population : the untold story of risk, reward and the roots of warfare

Abstract

Intergroup relationships represent consequential extensions of a social group that not only feedback to shape within-group social relationships and organization, but provide a bridge to the formation of more complex social systems. They range in nature from highly affiliative to lethally agonistic, and can be incredibly variable both within and between animal species. Humans stand out in this regard due to our extreme capacities for both intergroup cooperation that extends beyond kin, and intergroup hostility that extends to large-scale warfare. In a world where news headlines are increasingly dominated by stories of violent conflict, understanding what promotes intergroup tolerance vs. aggression seems especially relevant to understanding the world around us. Relationships between groups form and change according to intergroup encounter (IGE) dynamics, which encompass when, where, why and how groups interact. These dynamics are influenced by local socioecological conditions; asymmetries in groups’ fighting abilities and/or interaction payoffs; and/or social or genetic ties between groups. In turn, they influence access to resources, which can have short-term energetic consequences, as well as long-term impacts on individual survival and reproductive success. Studies of these dynamics provide valuable insights into the selective forces that structure animal sociality, as well as what incentivizes tolerance and aggression towards outgroup members. To that end, chimpanzees and bonobos are often presented as analogous, dichotomous models of intergroup aggression and tolerance. In short, chimpanzees teach us about the roots of our warlike tendencies while bonobos teach us where our peaceable sides come from. Recent studies in both Pan species have started to blur this dichotomy, revealing that both species exhibit behavioural flexibility during intergroup encounters that may parallel our own. To better understand the proximate causes of this behavioural flexibility, I integrated socioecological theory with real-world observational data to model IGE dynamics in a population of wild bonobos that exhibits unique behavioural extremes. For my first model, I investigated how different metrics of fruit abundance influence encounter frequency, and showed that rates of encounters track the seasonal abundance of a key food resource rather than overall fruit abundance. These results provide a more nuanced understanding of how fruit abundance influences encounter frequency than previous studies. As part of this investigation, I devised a new method for making spatially-explicit predictions about fruit productivity that can be applied across multiple spatial scales. This investigation also showcased the importance of using consumer-centric methods when modelling how fruit abundance influences animal behaviour. For my second model, I adopted a novel magnet-model approach to investigate how social factors influenced the decision of two parties to interact once they were within earshot. By testing hypotheses that have never been tested in bonobos, I demonstrated that infanticide risk and dispersal-related benefits for females have credible influence on the decisions of two groups to interact. For my third model, I investigated how socioecological factors influence the probability of an aggressive encounter. I found that encounters were more likely to be aggressive when parties from the bonobo group largely responsible for aggression were smaller – a result that completely upends theoretical expectations about imbalances in power. Finally, for my fourth model, I investigated whether winner-loser effects could explain patterns of ‘wins’ and ‘non-wins’ between two groups. I found preliminary evidence for a winner effect and intergroup dominance. Collectively, results from this thesis provide new insights into what drives behavioural flexibility in one of our closest-living relatives. When synthesized alongside concurrent observations of range expansion and potential lethal aggression, they offer tantalizing evidence for war-like tendencies in a species that often exhibits remarkable outgroup tolerance. This truly shatters the dichotomy of the warrior chimpanzee and peace-loving bonobo. Overall, the work presented here lays the foundation for further investigations about how demography interacts with varying socioecological conditions to facilitate increased intergroup aggression.