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


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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