Every once in a while groups of strong chimp males would form up and head north, toward the border between their territory and the land of the neighboring tribe. They'd move through the jungle silently and in a single-file line, with practically no eating, socializing, or masturbating allowed. They'd stealthily scavenge for signs of individuals from the other tribe, such as feces, abandoned termite-fishing tools, etc. When they found a member of the northern tribe off on his own, then they'd gang up on his ass and murder him, goddamn Sam Fisher-style.
Then they did it again. And again. It wasn't just random animal-on-animal savagery; when the scientists studied the pattern of the attacks, they found the chimps were at war.
You expect this kind of bullshit from apes.
During the decade they watched the area, scientists saw 18 of these attacks, mostly all along the northern border, wiping out more than 13 rival chimps from a tribe of 100 (you don't get kill ratios like than in most human wars). And each time, they moved the border north. They were fighting over land, and doing it in a very organized way.