Chimpanzee boundary patrols systematically murder rivals and expand territory.

Evidence

In the forests of Uganda, primatologists have been studying a particularly large troop of chimpanzees at Ngogo for about two decades. As of 1999, this group of 150 or so individuals controlled a territory of 29 km2 (11.2 mi2 ). As in other chimpanzee groups, adult males go out on “boundary patrols.” During these nighttime expeditions, but unlike their other movements, males neither socialize nor feed, but silently travel in single file through regions separating their territory from that occupied by adjacent chimpanzee troops. There, moving along the border, they sometimes make targeted incursions into the territories of other troops. Over nine years, 114 of these patrols attacked and killed 21 members of other chimpanzee troops. Thirteen of these 21 murders occurred during incursions into the northeastern corner of their territory and thus targeted one particular troop. While the exact size of this other group is uncertain, this many kills implies that about three-quarters of the chimpanzees in the other group can expect to be murdered by a patrol before dying of old age at 50. In 2009, Ngogo chimpanzees, including females and infants, began regularly entering this new territory and acting as they do at the core of their own territory. It appears that their systematic raids over at least a decade drove the other troop back, allowing this large group to effectively expand its territory by 22.3%. — Joseph Henrich, The Secret of Our Success, 548