Intergroup violence
Compared to all species, chimpanzees show high levels of territoriality however levels of intergroup violence vary across chimpanzee populations \cite{G_mez_2016}. Inter-group conflict is regular and expressed mostly as vocal exchanges, which can escalate to chasing and attacking the out-group, occasionally ending in lethal violence \cite{Watts_2001}. Some chimpanzee communities also conduct lethal raids on other groups, killing males and juveniles \cite{G_mez_2016}.
Population density is correlated with the frequency of inter-group conflict in chimpanzee species. West African chimpanzees (Pan troglodytes verus) are the most sparsely populated, and fewer intergroup killings are observed in this sub-species compared to all others we have data on \cite{Wessling_2020}. Notably for 2 studied west African populations (at Bompusa and Bossou), no intergroup killing has been observed at all, suggested to be due to comparatively large home ranges and reduced border patrols (\cite{Lemoine_2022}). In stark comparison to east African chimpanzee species (Pan troglodytes schweinfurthii), where the ecology of their habitats can support much population densities and interaction rates increase accordingly \cite{Wilson_2014}.
The intensity of intergroup conflict also varies between species and sites, notably the lethality of intergroup encounters. Killing is most common in eastern chimpanzees and least common in bonobos, and the most common victims are adult males and unweaned infants. Previous studies have shown lethality of intergroup encounters generally increases with population density, and the number of adult males in the attacking community \cite{G_mez_2016,Southern_2021,Wilson_2014}.