Chimpanzees (Pan troglodytes) are capable of extreme violence. They engage in inter-group, sometimes lethal, aggression that provides the winners with an opportunity to enlarge their territory, increase their food supply and, potentially, attract more mates. Lethal violence between adult males also occurs within groups but this is rare; to date, only four cases (three observed and one inferred) have been recorded despite decades of observation. In consequence, the reasons for within-group lethal violence in chimpanzees remain unclear. Such aggression may be rare due to the importance of coalitions between males during inter-group encounters; cooperation between males is also thought to be key in the defense or advancement of social rank within the group. Previous accounts of within-group lethal violence concern victims who were low-ranking males; here we provide the first account of the killing of an incumbent alpha male by a coalition of adult males from the same community. We found no clear evidence that the alpha male's position was under threat during the months before the lethal attack: the male dominance hierarchy was highly stable, with low rates of male–male aggression, and there were no significant changes in social interactions (i.e. grooming and aggression) between the alpha male and the other adult males. Two of the four attackers were former alpha males and were the individuals with whom the victim appeared, in the period preceding his death, to be most strongly affiliated: his most frequent grooming partners and those with whom he spent most time in proximity. The lethal attack triggered a period of instability in the male hierarchy and was likely an opportunistic attempt to seize alpha status by the third-ranking male.
Nevertheless, within-community coalitional (or gang) attacks have been reported from both the wild and captivity [de Waal, 1986; Fawcett & Muhumuza, 2000; Goodall, 1992; Mjungu, 2010; Nishida, 1996; Nishida et al., 1995; Watts, 2004] although only four cases of lethal violence between adult males of the same community of wild chimpanzees have been documented despite decades of detailed observation [Sonso community: Fawcett & Muhumuza, 2000; Mahale M-group: Nishida, 1996, 2012; Ngogo community: Watts, 2004; Mitumba community: Mjungu, 2010; Wrangham et al., 2006]. Of these, one (in Mahale M-group) was inferred from circumstantial evidence while three were observed directly. In both the Sonso and Ngogo communities, the victim was a low-ranking young adult male, while in Mahale's M-group and in Gombe's Mitumba community, the victim was a deposed alpha male who was certainly (M-group) or probably (Mitumba) low ranking at the time of the attack. In addition, a gang attack on a deposed alpha male of Gombe's Kasakela community could have been fatal without human intervention [through antibiotic treatment: Goodall, 1992].
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