(Page 291 from Shadows of Forgotten Ancestors by Carl Sagan and Ann Druyan.)

There's one aspect of chimpanzee territoriality not shared by other apes-all of whom are divided into territorial, xenophobic groups, with a little exogamy thrown in: Unlike encounters within the group, where bluff and intimidation play major roles and only rarely does anyone get seriously hurt, when two chimp groups interact there can be real violence. No main force combat has ever been observed. They prefer guerrilla tactics. One group will pick off the members of the other in ones and twos until there's no longer a viable force left to defend the adjacent territory. Chimpanzee groups are constantly skirmishing to see if it's possible to annex more turf. If the penalty for failure in combat is death for the males and alien sexual bondage for the females, the males soon find themselves caught up in a powerful selection for military skills. Genes for those skills must have been racing through the tropical forests, by exogamous mating, until nearly all chimps had them. If they didn't, they died.

Moreover, the skills that make you good on patrol and good in skirmishes also make you good in the hunt. If your combat skills are honed, you can also supply your friends, loved ones, and concubines - to say nothing of yourself-with more of that delicious red meat. Except for the part about the good eating, being a male chimp is a little like being in the army....