Lethal intergroup aggression leads to territorial expansion in wild chimpanzees

SummaryChimpanzees make lethal coalitionary attacks on members of other groups [1]. This behavior generates considerable attention because it resembles lethal intergroup raiding in humans [2]. Similarities are nevertheless difficult to evaluate because the function of lethal intergroup aggression by chimpanzees remains unclear. One prominent hypothesis suggests that chimpanzees attack neighbors to expand their territories and to gain access to more food [2]. Two cases apparently support this hypothesis, but neither furnishes definitive evidence. Chimpanzees in the Kasekela community at Gombe National Park took over the territory of the neighboring Kahama community after a series of lethal attacks [3]. Understanding these events is complicated because the Kahama community had recently formed by fissioning from the Kasekela group and members of both communities had been provisioned with food. In a second example from the Mahale Mountains, the M group chimpanzees acquired part of the territory of the adjacent K group after all of the adult males in the latter disappeared [4]. Although fatal attacks were suspected from observations of intergroup aggression, they were not witnessed, and as a consequence, this case also fails to furnish conclusive evidence. Here we present data collected over 10 years from an unusually large chimpanzee community at Ngogo, Kibale National Park, Uganda. During this time, we observed the Ngogo chimpanzees kill or fatally wound 18 individuals from other groups; we inferred three additional cases of lethal intergroup aggression based on circumstantial evidence (see Supplemental Information). Most victims were caught in the same region and likely belonged to the same neighboring group. A causal link between lethal intergroup aggression and territorial expansion can be made now that the Ngogo chimpanzees use the area once occupied by some of their victims