While conquests and invasions form a major part of human history, it's now clear that we aren't the only species that murders each other for land. A short correspondence in Current Biology details the first conclusive evidence that chimpanzees can gain extra territory after killing members of a neighboring group.

The Ngogo group of chimpanzees in Uganda's Kibale National Park has been watched for 10 years, and is larger than many other well-studied groups. Beginning in 1999, several members of the Ngogo group were repeatedly observed leaving their territory and embarking on "boundary patrols" in an area inhabited by another group of chimpanzees. During these patrols, members remained quiet, traveled in a single file line, and refrained from foraging or socializing. Between 1999 and 2008, the Ngogo chimpanzees attacked and killed 13 chimpanzees (four adult males and nine younger animals) in the neighboring group's territory during boundary patrols.

By 2009, the Ngoro chimpanzees began to regularly feed, rest, and socialize in the northeastern area, where the patrols had previously taken place. Meanwhile, the neighboring group of chimpanzees was no longer observed in the vicinity. Over the next five months, the Ngoro chimpanzees spent 32.3 percent of their time in this newly-acquired area, using it exactly as they did the rest of their territory. The range expansion increased the size of their territory by 22.3 percent.

Previously, two groups of chimpanzees in Tanzania have been suspected of similar conquests, but there was not enough evidence to identify a causal link between intergroup aggression and territory expansion. The authors provide two hypotheses for this lethal behavior: the chimpanzees probably sought out new land either to procure more mates or to increase their food supply. More evidence is required to tease out the particular forces that drive this type of range expansion.

The authors are careful to warn that these observations cannot be used to shed light on many forms of human human violence. Human war is an incredibly complex phenomenon stemming from many sources, and may not have as strong an evolutionary basis as lethal aggression among other primates.

Current Biology, 2010. DOI: doi:10.1016/j.cub.2010.04.021 (About DOIs).