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Not our fault chimps kill each other, study concludes

Sept. 19, 2014
Courtesy of Arizona State University
and World Science staff

Why do chim­panzees some­times kill each oth­er? Al­though some sci­en­tists have ar­gued that it’s due to dis­tur­bances caused by us hu­mans, a new study con­cludes it’s not our fault­—it’s just their way.

In oth­er words, the study says, we can blame ev­o­lu­tion. 

In the 1970s, re­ports of chim­pan­zee vi­o­lence from the an­thro­po­l­o­gist Jane Goodall drew glob­al at­ten­tion. Since then, many have com­pared chimp-on-chimp vi­o­lence to prim­i­tive war­fare. This view holds that their ag­gres­sion is an ev­o­lu­tion­ary strat­e­gy: they do it be­cause for at least some chimps of the past, it paid off, wheth­er through ac­cess to mates, food, ter­ri­to­ry or oth­erwise.

But oth­ers have ar­gued that le­thal chimp ag­gres­sion is the con­se­quence of hu­man ac­ti­vi­ties such as ar­ti­fi­cial feed­ing or hab­i­tat de­struc­tion. 

The study looked at pat­terns of ag­gres­sion among chimps and among their close rel­a­tives, bono­bos, both of which are al­so close ev­o­lu­tion­ary rel­a­tives of hu­mans. In fact, the de­bate over wheth­er chimps are vi­o­lent by na­ture has in­flu­enced the contro­versy over wheth­er we our­selves are that way. What com­pli­cates this al­ready dif­fi­cult ques­tion is that bono­bos are rel­a­tively peace­a­ble.

The new study "de­bunks the idea that le­thal ag­gres­sion among wild chim­panzees is an ab­er­rant be­hav­ior caused by hu­man dis­tur­bances," said Ar­i­zo­na State Uni­vers­ity an­thro­po­l­o­gist Ian Gilby. Gilby is codi­rec­tor of the long-term Gombe chim­pan­zee da­tabase for the Jane Goodall In­sti­tute Re­search Cen­ter and a co-author of the new find­ings, pub­lished in the jour­nal Na­ture on Sept. 18.

The proj­ect com­piled da­ta gath­ered over half a cen­tu­ry from 18 chimp and four bonobo com­mun­i­ties. Af­ter an­a­lyz­ing 152 le­thal at­tacks, the re­search­ers con­clud­ed that chimps kill each oth­er re­gard­less of the lev­el of hu­man in­flu­ence in their en­vi­ron­ment. Bono­bos, on the oth­er hand, weren’t seen to kill each oth­er at all.

Jo­an Silk, a sci­ent­ist at the uni­vers­ity who stud­ies pri­mate be­hav­ioral ecol­o­gy, ev­o­lu­tion and so­cial­ity, wrote a com­men­tary on the study for the same is­sue of Na­ture. In the article, she dis­cusses how the chimp de­bate bears on contro­versy about the ori­gins and prev­a­lence of hu­man war­fare. She ar­gues that peo­ple of­ten dis­tort the im­age of chimps to make them seem nic­er than they are: we at­trib­ute their mor­ally ap­peal­ing traits, such as coop­era­t­ion, to ev­o­lu­tion, while blam­ing their less sa­vory traits on ar­ti­fi­cial fac­tors. 

Some sci­en­tists who ar­gued that chimps were nat­u­rally vi­o­lent have faced a back­lash—as if they were sug­gest­ing that hu­mans are the same way, a per­haps more dis­turb­ing idea. What the study sug­gests, Silk wrote, is simply that for chimps, that "there are some cir­cum­stances in which the ben­e­fits of le­thal ag­gres­sion ex­ceed the costs… noth­ing more.” It does­n’t mean peo­ple are fat­ed to be war­like, she added.

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