Every day, John Mitani or a colleague is up at sunrise to check on the action among the chimpanzees at Ngogo, in Uganda’s Kibale National Park. Most days the male chimps behave a lot like frat boys, making a lot of noise or beating each other up. But once every 10 to 14 days, they do something more adult and cooperative: they wage war.

A band of males, up to 20 or so, will assemble in single file and move to the edge of their territory. They fall into unusual silence as they penetrate deep into the area controlled by the neighboring group. They tensely scan the treetops and startle at every noise. “It’s quite clear that they are looking for individuals of the other community,” Dr. Mitani says.

When the enemy is encountered, the patrol’s reaction depends on its assessment of the opposing force. If they seem to be outnumbered, members of the patrol will break file and bolt back to home territory. But if a single chimp has wandered into their path, they will attack. Enemy males will be held down, then bitten and battered to death. Females are usually let go, but their babies will be eaten.

These killings have a purpose, but one that did not emerge until after Ngogo chimps’ patrols had been tracked and cataloged for 10 years. The Ngogo group has about 150 chimps and is particularly large, about three times the usual size. And its size makes it unusually aggressive. Its males directed most of their patrols against a chimp group that lived in a region to the northeast of their territory. Last year, the Ngogo chimps stopped patrolling the region and annexed it outright, increasing their home territory by 22 percent, Dr. Mitani said in a report being published Tuesday in Current Biology with his colleagues David P. Watts of Yale University and Sylvia J. Amsler of the University of Arkansas at Little Rock. Dr. Mitani is at the University of Michigan.