Kate Wong reports on chimpanzee and bonobo presentations at the AAPA meetings: "Why chimpanzees kill".
As for the bonobos, [Michael Wilson's] study bolsters the claim that they are less aggressive than chimpanzees: there were no clear-cut homicides in any of the bonobo communities. Another presentation given at the meeting provided a possible clue to the apparent absence of male aggression among these apes: Victoria Wobber of Harvard University and her colleagues studied testosterone levels in chimpanzees and bonobos from infancy to adulthood and found that whereas chimpanzee testosterone levels surged during adolescence (particularly among males), bonobo testosterone production remained consistent over the course of development.
The relationship between the number of local males and intergroup aggression must point to some interesting population dynamics, since the local sex ratio varies stochastically over time.