Lethal Aggression in Pan (chimpanzees and bonobos)

This is old news, as old as the Gombe Chimpanzee War reported by Jane Goodall. But most folks still don't want to believe it. Nature is red in tooth and claw.

Nature

"Most victims were members of different communities from the attackers (n = 62 of 99 cases; 63%) and thus not likely to be close kin²⁶. This difference is particularly striking given that chimpanzees could potentially attack members of their own community on a daily basis, but rarely encounter members of other communities (for example, 1.9% of follow days at Kanyawara²⁷).

...

Several robust patterns emerge from these data. Killing was most common in eastern chimpanzees and least common among bonobos. Among chimpanzees, killings increased with more males and higher population density, whereas none of the three human impact variables had an obvious effect. Male chimpanzees killed more often than females, and killed mainly male victims; attackers most frequently killed unweaned infants; victims were mainly members of other communities (and thus unlikely to be close kin); and intercommunity killings typically occurred when attackers had an overwhelming numerical advantage.  

The most important predictors of violence were thus variables related to adaptive strategies: species; age–sex class of attackers and victims; community membership; numerical asymmetries; and demography. We conclude that patterns of lethal aggression in Pan show little correlation with human impacts, but are instead better explained by the adaptive hypothesis that killing is a means to eliminate rivals when the costs of killing are low."