ANN ARBOR, Mich. -- A new study of wild chimpanzees shows that thebiggest predictor of territorial boundary patrols is the number ofmales in the group. The more males in the group, the more often theywill patrol their territory.
Chimpanzees will sometimes attack and kill their neighbors duringthe rarely observed boundary patrols, said John Mitani, professor ofanthropology at University of Michigan and co-author of the paper"Correlates of Territorial Boundary Patrol Behavior in WildChimpanzees," with David Watts of Yale University.
Scientists have known for about 25 years that the patrols and fatalattacks occur, the question has been what accounts for the varyingnumber and frequency of these patrols and attacks.
Researchers hypothesized that five variables might impact the number ofpatrols: food availability, hunting activity, the presence of estrousfemales, 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. Thebehavior is markedly different from normal feeding parties, which areloud and scattered.
"What they are doing is actually seeking signs if not contact withmembers of other groups," Mitani said. "If the patrollers outnumberthem, then they will launch an attack." During the attacks, the chimpsbeat and often kill their neighbors.
The groups are generally all male, but on rare occasionsfemales---typically infertile---will join the patrol, Mitani said. Thepatrols and attacks are an important part of the chimp society, hesaid.
"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 likehumans living in a town, chimps form cliques and aren't all together inone place at the same time. But on patrol days, researchers found thata larger number of males gathered together than on non-patrol days. Theaddition of one male to the group increased the odds of a patrol by 17percent.
Mitani and Watts observed a community of about 150 chimps in Ngogo,Kibale National Park, Uganda and collected 24 month of data compiledover five years. The Ngogo community is significantly larger than twoother well-studied chimpanzee communities in Gombe andTaï, but the males in all three communities patrolled with equal frequencyon a per capita basis. However, the chimps in Ngogo patrolled abouttwice as often as the other communities, due solely to the large numberof males.
"The take home of all of this is that male numbers seem to matter, theyfind strength in numbers in doing this behavior, and they find strengthin making these attacks," Mitani said.
Chimps are our closest living relatives, and it's tempting to drawanalogies between human and chimp behavior, especially because it'svery rare for mammals to seek out and attack neighbors in this way. ButMitani said the situation is much more complicated than that.
"I think it is difficult to make any general conclusions about what this says about human behavior," he said.
For information on Mitani, visit: http://sitemaker.umich.edu/mitani
For information on anthropology at U-M, visit: http://www.lsa.umich.edu/anthro/
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