A new study of wild chimpanzees shows that the biggest predictor of territorial boundary patrols is the number of males in the group

Laura Bailey:

The more males in the group, the more often they will patrol their territory.

Chimpanzees will sometimes attack and kill their neighbors during the rarely observed boundary patrols, said John Mitani, professor of anthropology at University of Michigan and co-author of the paper "Correlates of Territorial Boundary Patrol Behavior in Wild Chimpanzees," with David Watts of Yale University.

Scientists have known for about 25 years that the patrols and fatal attacks occur, the question has been what accounts for the varying number and frequency of these patrols and attacks.

Researchers hypothesized that five variables might impact the number of patrols: food availability, hunting activity, the presence of estrous females, intruder pressure, and male party size.

During boundary patrols, a group of males will rise without warning, form a single file line and silently depart the group, Mitani said. The behavior is markedly different from normal feeding parties, which are loud and scattered.

"What they are doing is actually seeking signs if not contact with members of other groups," Mitani said. "If the patrollers outnumber them, then they will launch an attack." During the attacks, the chimps beat and often kill their neighbors.

The groups are generally all male, but on rare occasions females—typically infertile—will join the patrol, Mitani said. The patrols and attacks are an important part of the chimp society, he said.

"They take up about two hours out of a 12-hour work day," Mitani said. "That is not trivial exercise in terms of energy expended."

Chimpanzees live in fission-fusion societies. This means that like humans living in a town, chimps form cliques and aren't all together in one place at the same time. But on patrol days, researchers found that a larger number of males gathered together than on non-patrol days. The addition of one male to the group increased the odds of a patrol by 17 percent.

Mitani and Watts observed a community of about 150 chimps in Ngogo, Kibale National Park, Uganda and collected 24 month of data compiled over five years. The Ngogo community is significantly larger than two other well-studied chimpanzee communities in Gombe and Taï, but the males in all three communities patrolled with equal frequency on a per capita basis. However, the chimps in Ngogo patrolled about twice as often as the other communities, due solely to the large number of males.

"The take home of all of this is that male numbers seem to matter, they find strength in numbers in doing this behavior, and they find strength in making these attacks," Mitani said.

This may explain why cities are often more violent than small towns since in a city you have larger numbers of males living in close proximity to each other.