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

Intergroup relationships represent consequential extensions of a social group that not only shape within-group social relationships and organization, but provide a bridge to the formation of more complex social systems. These relationships form and change according to intergroup encounter (IGE) dynamics, which encompass when, where, why and how groups interact. 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. With the aim of expanding our knowledge about what contributes to variation in intergroup relationships, I investigated how social and ecological factors influenced IGE dynamics between two groups of wild-living bonobos (Pan paniscus). I found that rates of encounters tracked the seasonal abundance of a key food resource rather than the commonly-hypothesized factor of overall fruit abundance. I also demonstrated that infanticide risk and dispersal-related benefits for females – two previously untested factors – have credible influence on the decisions of groups to interact. Once two groups engaged in visual contact, I found that encounters were more likely to be aggressive when parties from the group largely responsible for aggression were smaller – a result that completely upends theoretical expectations about imbalances in power. Finally, I found preliminary evidence for intergroup dominance. By using new methodological approaches and exploring untested hypotheses, I provided fresh insights into what drives behavioural flexibility during IGEs in of one of our closest-living relatives. When synthesized alongside concurrent observations of range expansion and potential lethal aggression, these results offer tantalizing evidence for war-like tendencies in a species that often exhibits remarkable outgroup tolerance.