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ANIMAL BEHAVIOUR
A comprehensive analysis of lethal coalitionary aggression in chimpanzees convincingly demonstrates that such aggression is an adaptive behaviour, not one that has emerged in response to human impacts. See Letter p.414
In 2013, there were 33 armed state-level conflicts around the world1. Many of these had persisted for decades, killed thousands of people and thwarted international peace- keeping efforts. War is certainly a contempo- rary fixture, but has it always been one? There is vigorous disagreement over the answer to this question. Some argue that warfare has been a pervasive feature throughout human history and has had important effects on human nature2, whereas others contend that war is rare in foraging groups3, the kinds of societies that we lived in for most of our evo- lutionary history. Debates about the origins and prevalence of human warfare are echoed in the question of whether lethal coalition- ary aggression in chimpanzees has evolved through natural selection or whether it is a non-adaptive consequence of human distur- bance. In this issue, Wilson et al.4 (page 414) argue persuasively on the side of adaptation.
Many species of non-human primates have hostile relationships with members of neigh- bouring groups, and some species collectively defend the boundaries of their territories. But intergroup encounters rarely lead to seri- ous injuries or deaths, perhaps because the risks of escalated aggression usually do not outweigh the benefits of killing opponents. Lethal coalitionary attacks on individuals from neighbouring communities have been docu- mented only in chimpanzees. The first report of such killings was published 35 years ago5, but the debate about their adaptive significance continues.
One point of view is that natural selection has favoured the evolution of lethal coalition- ary intergroup aggression in chimpanzees as a means...