Sylvia Amsler

A chimpanzee patrol party composed of primarily males traveling in the cohesive single-file line typical of patrolsSylvia Amsler


- Chimpanzees kill their neighbors in order to acquire territory, new research shows.

- Chimps are our closest primate relatives, so the behavior could help to explain why humans sometimes conduct lethal raids.

- Human cooperation could also have origins in the primate inter-group competition.

Chimpanzees, our closest primate relatives, engage in war-like behavior to gain territory, new research finds.

The findings, published in the latest issue of Current Biology, explain why chimpanzees sometimes brutally kill their neighbors. The killings are most often done by patrolling packs of male chimps that are "quiet and move with stealth," according to lead author John Mitani of the University of Michigan.

To the victors go similar spoils of early human wars: land, often-improved security and strength, extra food and resources, and even better access to females.

"There are certainly valid parallels, and there is literature which discusses the territorial behavior of common chimpanzees in explaining the evolution of human warfare," co-author Sylvia Amsler told Discovery News.

"However, we are equally closely related to the other species of chimpanzee, the bonobo," added Amsler, a primate behavioral ecologist at the University of Arkansas at Little Rock. "The bonobo does not appear to engage in territorial behavior and lethal coalitionary aggression. It's also true that human warfare is extremely complex, with lethal raids being only one type of human aggression."

She, Mitani and colleague David Watts observed such chimpanzee aggression firsthand during a decade-long study of a large group of chimps living in Ngogo, Kibale National Park, Uganda. Over that period they witnessed 18 killings of individual chimps from other groups. Another three killings were deduced from evidence, bringing the total number of deaths to 21.

Amsler said the process usually begins when patrolling chimps move in a single file along the border of their territory, or outside of this region where the chimpanzee community typically lives. If the party encounters one from the neighboring group and the two are fairly evenly matched, "the patrollers will either call loudly as they retreat immediately back toward their home territory, or a brief, indecisive battle will ensue."

If, on the other hand, the patrolling party greatly outnumbers the strangers, its members will generally attack.

"Such attacks can be severe and fatal," she said. "In the case of an adult victim, the attacking males take turns beating and jumping on the victim. Attackers use their canines to bite and tear at the victim, so that any body parts that stick out, such as testes and ears, are often ripped off during an attack."

She added, "In the case of infanticides, the attackers beat the female, but not as severely as they beat adult males. The goal appears to be to separate the infant from its mother."

Once an infant is removed from its mother, the attacker "generally kills the infant fairly quickly either with a bite to the head or by biting the stomach and disemboweling it." The infant is then sometimes cannibalized, with the meat shared as the chimpanzees would do after killing red colobus monkey prey during hunts.

At the end of the "battle," the victorious chimpanzee group can then expand into the newly acquired territory.

Jennifer Williams of the Jane Goodall Institute's Center for Primate Studies and colleagues have conducted studies on chimpanzees living in Gombe. They've discovered that as a chimp's territory size increases, females weigh more and have shorter times between births.

"Therefore males seem to defend a feeding territory benefitting the entire community," said Amsler.

Other researchers, such as Samuel Bowles, have suggested that human cooperation may have its origins in intergroup competition. Amsler and her colleagues believe the new research might support this theory.

"Cooperation with other community members in territorial activities leads to benefits for all from resulting territorial gains," Amsler said. "Humans are an unusually cooperative species, and we suggest a new discussion about whether cooperation during coalitionary attacks by chimpanzees may provide insights into the evolution of human cooperation."