University of Michigan scientists say the biggest predictor of territorial boundary patrols among wild chimpanzees is the number of males in the group.
Chimpanzees will sometimes attack and kill their neighbors during rarely observed boundary patrols, said John Mitani, professor of anthropology and co-author of the study with David Watts of Yale University.
Scientists have known for about 25 years that boundary patrol fatal attacks occur, the question has been what accounts for the varying number and frequency of such patrols and attacks.
During boundary patrols, a group of males will rise without warning, form a single file line and silently depart the group, Mitani said.
"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, but the patrols are apparently an important part of chimp society.
"They take up about two hours out of a 12-hour work day," Mitani said. "That is not trivial exercise..."
Added Mitani: "I think it's difficult to make any general conclusions about what this says about human behavior."