...These data provide grounds for evaluating Wrangham's preliminary conclusion that “current evidence supports the hypothesis that selection has favored a hunt and kill propensity in chimpanzees and humans, and that coalitional killing has a long history in the evolution of both species” (ref. 6, p. 1).
The crux of Wrangham's explanation of the evolution of coalitionary killing is that fitness is closely correlated with access to food resources and that territorial enlargement thus enhances fitness. In short, “fitness is correlated with territory size” (other factors being equal) (ref. 6, p. 15). Unprovoked attacks on members of a neighboring community thus convey a selective advantage, provided that the costs to the attackers are low. The mechanism by which aggression is rewarded is intercommunity dominance. “If raiding leads to the wounding or death of a neighboring male, the neighboring community's competitive ability is substantially reduced” (ref. 6, p. 15). The dominant community can thus freely encroach on the territory of its neighbor whenever food resources within its own territory are in short supply. The dominant community also may have an advantage in recruiting reproductive females. However, the capacity to translate additional females into increased fitness would be contingent on the commensurate expansion of food resources.† Thus, although intercommunity dominance “tends to lead to increased fitness of the killers through improved access to resources such as food, females, or safety” (ref. 6, p. 12), territorial gain is the critical ingredient for the realization of this fitness enhancement....