"Long before it's in the papers"
January 27, 2015


Chimps kill each other for territory, study finds

June 21, 2010
Courtesy of Cell Press
and World Science staff

Chimps, just like hu­mans, kill each oth­er for ter­ri­to­ry, re­search­ers have found.

“Chim­panzees kill each oth­er. They kill their neigh­bors. Up un­til now, we have not known why. Our ob­serva­t­ions in­di­cate that they do so to ex­pand their ter­ri­to­ries at the ex­pense of their vic­tims,” said John Mi­tani of the Uni­vers­ity of Mich­i­gan, a mem­ber of the re­search group.

The slay­ings usu­ally are com­mit­ted by small groups of males on pa­trol, said the sci­en­tists. But un­like in much hu­man warfare—where armies are some­times will­ing to at­tack oth­er, com­pa­rably sized armies—the chimp killers spe­cif­ic­ally seek out lone or badly out­num­bered vic­tims for an easy am­bush.

Com­mon chim­panzees are one of the two spe­cies with the clos­est ev­o­lu­tion­ary rela­t­ion­ship to peo­ple. Sci­en­tists be­lieve that traits shared by chimps and hu­mans were likely pos­sessed by the com­mon an­ces­tors of both. Thus some re­search­ers say the mo­tives and meth­ods of chimp vi­o­lence may shed light on how hu­man vi­o­lence evolved.

Yet Mi­tani and col­leagues ar­gue that stud­ies of chimp ag­gres­sion will re­veal more about why hu­mans so of­ten work to­geth­er than about why we do bat­tle. “Us­ing our re­sults to ad­dress an en­dur­ing ques­tion about why hu­mans are an un­usu­ally co­op­er­a­tive spe­cies may prove to be a more pro­duc­tive line of in­quiry,” the re­search­ers wrote. 

Their find­ings are re­ported in the June 22 is­sue of the re­search jour­nal Cur­rent Bi­ol­o­gy

Mi­tani and col­leagues stud­ied chimps liv­ing in Ngogo, Kibale Na­tional Park, Ugan­da, which have been un­der ob­serva­t­ion over a dec­ade. Dur­ing that time, a team di­rect­ed by Mi­tani and Da­vid Watts of Yale Uni­vers­ity doc­u­mented 21 killings by the Ngogo chimps of in­di­vid­u­als from oth­er groups. Eight­een of those killings were seen di­rect­ly, while the rest were de­duced from cir­cum­stan­tial ev­i­dence. 

The re­search­ers think up to 13 of the vic­tims be­longed to a sin­gle neigh­bor­ing group, rep­re­sent­ing an “ex­tremely high” rate of mor­tal­ity due to vi­o­lence among groups.

With some of their com­peti­tors out of the way, the Ngogo chim­panzees be­gan to use a large por­tion of new ter­ri­to­ry to the north­east of their pre­vi­ous range. “Be­cause the newly ac­quired ter­ri­to­ry cor­re­sponds to the ar­ea once oc­cu­pied by many of the vic­tims, we sug­gest that a caus­al link ex­ists” be­tween the vi­o­lence and the ter­ri­to­rial ex­pan­sion, Mi­tani said.

Mi­tani and his col­leagues think the new ter­ri­to­ry most likely ben­e­fits the chimps by af­ford­ing great­er ac­cess to food. It may al­so ul­ti­mately lead to great­er ac­cess to fe­males, but it’s too early to tell, the sci­en­tists said.

The at­tacks are trig­gered when bands of chim­panzees go out “on pa­trol” in­to the ter­ri­to­ry of a neigh­bor­ing chim­pan­zee com­mun­ity, the re­search­ers added. “Pa­trollers are qui­et and move with stealth,” Mi­tani said. “They pause fre­quently to scan the en­vi­ron­ment as they search for oth­er chim­panzees. At­tacks are typ­ic­ally made only when pa­trolling chim­panzees have overwhelming nu­mer­i­cal su­pe­ri­or­ity over their ad­ver­saries.”

The Ngogo chimps may have an un­usu­al ad­van­tage over their neigh­bors due to the im­pres­sive size of their com­mun­ity, which may ex­plain the sur­pris­ingly high lev­el of vi­o­lence, the re­search­ers say. There are more than 150 Ngogo chimps—a­bout three times the num­ber found in chimp com­mun­i­ties stud­ied else­where.

The study could shed light on how coop­era­t­ion evolved among hu­mans and re­lat­ed spe­cies, Mi­tani and col­leagues said. This is be­cause bru­tal as the chimp slay­ings may be, they il­lus­trate how an­i­mals coop­erate for the ben­e­fit of their group.

“Our ob­serva­t­ions in­di­cate that ter­ri­to­rial con­flict leads chim­panzees in some groups to cede land to mem­bers of oth­er groups as a con­se­quence of le­thal coali­tionary ag­gres­sion,” the re­search­ers wrote. “In the pro­cess, chim­panzees in com­mun­i­ties that gain ter­ri­to­ry ob­tain in­creased ac­cess to re­sources that are then avail­a­ble to oth­ers in the group.” 

The ev­o­lu­tion­ary ques­tion is wheth­er these ben­e­fits out­weigh the costs for in­di­vid­ual chimps; if not, it’s hard to ex­plain how such co­op­er­a­tive be­hav­ior could have evolved, Mi­tani not­ed.

* * *

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Chimps, just like humans, kill each other for territory, researchers have found. “Chimpanzees kill each other. They kill their neighbors. Up until now, we have not known why. Our observations indicate that they do so to expand their territories at the expense of their victims,” said John Mitani of the University of Michigan, a member of the research group. The slayings usually are committed by small groups of males on patrol, said the scientists. But unlike in many human wars—where armies are sometimes willing to attack other, comparably sized armies—the chimp killers specifically seek out lone or heavily outnumbered victims for an easy ambush. The findings are reported in the June 22 issue of the research journal Current Biology. Common chimpanzees are one of the two species with the closest evolutionary relationship to people. Scientists believe that traits shared by chimps and humans were likely possessed by our common ancestors. Thus some researchers say the motives and methods of chimp violence may shed light on how human violence evolved. Yet Mitani and colleagues argue that studies of chimp aggression will reveal more about why humans so often work together than about why we do battle. “Using our results to address an enduring question about why humans are an unusually cooperative species may prove to be a more productive line of inquiry,” the researchers wrote. Mitani and colleagues studied chimps living in Ngogo, Kibale National Park, Uganda, which have been under observation over a decade. During that time, the team directed by Mitani and David Watts of Yale University saw the Ngogo chimps kill 21 individuals from other groups. Eighteen of those killings were observed directly, while the rest deduced from circumstantial evidence. The researchers think up to 13 of the victims belonged to a single neighboring group, representing an “extremely high” rate of mortality due to intergroup violence. With some of their competitors out of the way, the Ngogo chimpanzees began to use a large portion of new territory to the northeast of their previous range. “Because the newly acquired territory corresponds to the area once occupied by many of the victims, we suggest that a causal link exists” between the violence and the territorial expansion, Mitani said. Mitani and his colleagues think the new territory most likely benefits the chimps by affording greater access to food. It may also ultimately lead to greater access to females, but it’s too early to tell, the scientists said. The attacks are triggered when bands of chimpanzees go out “on patrol” into the territory of a neighboring chimpanzee community, the researchers added. “Patrollers are quiet and move with stealth,” Mitani said. “They pause frequently to scan the environment as they search for other chimpanzees. Attacks are typically made only when patrolling chimpanzees have overwhelming numerical superiority over their adversaries.” The Ngogo chimps may have an unusual advantage over their neighbors due to the impressive size of their community, which may explain the surprisingly high level of violence, the researchers say. There are more than 150 Ngogo chimps—about three times the number found in chimp communities studied elsewhere. The study could shed light on how cooperation evolved among humans and related species, Mitani and colleagues said. This is because brutal as the chimp slayings may be, they illustrate how animals cooperate for the benefit of their group. “Our observations indicate that territorial conflict leads chimpanzees in some groups to cede land to members of other groups as a consequence of lethal coalitionary aggression,” the researchers wrote. “In the process, chimpanzees in communities that gain territory obtain increased access to resources that are then available to others in the group.” The evolutionary question is whether these benefits outweigh the costs for individual chimps; if not, it’s hard to explain how such cooperative behavior could have evolved, Mitani noted.