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No enforcement, no trade—not for chimps

May 28, 2009
World Science staff

Sci­en­tists have man­aged to teach chimps to trade a prim­i­tive “cur­ren­cy,” con­vert­i­ble in­to snacks, among each oth­er.

But don’t ex­pect an ape stock ex­change to open an­y­time soon. The chimp com­merce broke down amid soured trans­actions soon af­ter hu­man ex­pe­ri­menters stopped ref­er­ee­ing them—sug­gest­ing the fur­ry pri­ma­tes still have some ev­o­lu­tion­ary hur­dles to cross be­fore en­ter­ing the world of busi­ness.

"Sher­man," one of the chimps involved in the trad­ing ex­peri­ments. (Pho­to by Mi­chael Be­ran)


A range of stud­ies have sug­gested that apes have the pre­req­ui­sites to en­a­ble them to set up a bar­ter econ­o­my, but can’t make the leap to such a sys­tem on their own, the re­search­ers said. 

“Lack of trust,” trouble com­mu­ni­cat­ing, or dif­fi­cul­ty with men­tal score­keep­ing might have prompted the break­down in chimp trade ab­sent en­force­ment, the re­search­ers, Sar­ah F. Bros­nan and Mi­chael J. Be­ran of Geor­gia State Un­ivers­ity, spec­u­lat­ed.

Work­ing with three chimps, Bros­nan and Be­ran set out to study why an­i­mals don’t bar­ter, where­as some form of trade is com­mon in vir­tu­ally eve­ry hu­man so­ci­e­ty.

“Although oth­er spe­cies lack lan­guage, they do com­mu­ni­cate and of­ten en­gage in be­hav­ior that ap­pears re­lat­ed to trade, such as food shar­ing and the ex­change of ser­vices,” wrote Bros­nan and Be­ran in re­port­ing their find­ings in the May is­sue of the Jour­nal of Com­par­a­tive Psy­chol­o­gy

Our clos­est ev­o­lu­tion­ary cousins, chimps have been trained in the past to trade sim­ple to­kens with hu­mans for snacks. Chimps in the wild have al­so been doc­u­mented to trade ser­vic­es with each oth­er—such as groom­ing and sup­port in fights—and even some­times ser­vic­es for food. But not goods for goods. 

So al­though re­cent eco­nom­ic trends might seem to sug­gest oth­erwise, the low­er pri­ma­tes do not by na­ture get in­volved in com­mer­cial mar­kets. “Ex­ten­sive bar­ter may rep­re­sent a rel­a­tively re­cent ev­o­lu­tion­ary de­vel­op­ment” in the line­age lead­ing to hu­mans, Bros­nan and Be­ran wrote.

Tokens tra­ded be­tween the chimps. (Pho­to by Mi­chael Be­ran)


Af­ter lengthy train­ing, the re­search­ers placed pairs of chimps in sep­a­rate en­clo­sures and gave each to­kens that could be turned in to an ex­pe­ri­menter for treats. How­ev­er, in half the cases, the chimps had to ex­change the to­kens with each oth­er be­fore the cur­ren­cy could be con­vert­ed to food; this dis­tinc­tion was taught to chimps us­ing a lan­guage and sym­bol sys­tem that they had learn­ed since in­fan­cy. The to­kens them­selves bore some of these sym­bols. 

At first, an ex­pe­ri­menter “en­forced” the trade in­ter­ac­tions by en­sur­ing that apes had to re­cip­ro­cate any trade be­fore re­ceiv­ing their ex­tra treats. But chimps were none­the­less free to ref­use any deal. Chimps were also given a few “wrong” tokens that wouldn’t pro­duce food when exchanged, to make sure they under­stood the point of the ex­change rather than just blind­ly trading any­thing to please their hand­lers.

As long as the hu­man ref­er­ee stayed in the pic­ture, chimps usu­ally made the “cor­rect” ex­changes that brought them treats, ac­cord­ing to the re­search­ers. But when the hu­man en­forc­er was gone, trades started “go­ing sour,” Bros­nan ex­plained in an e­mail: chimps did­n’t al­ways re­turn to­kens prof­fered by their peers.

Af­ter the lack of en­force­ment be­came ap­par­ent, Bros­nan said, the an­i­mals “quit trad­ing as well.” Bros­nan and Be­ran spec­u­lat­ed that trade in ser­vic­es, as op­posed to goods, might per­sist in the wild be­cause ser­vic­es are hard to steal.


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Scientists have managed to teach chimps to trade a primitive “currency,” convertible into snacks, among each other. But don’t expect an ape stock exchange to open anytime soon. The chimp commerce broke down amid soured “deals” soon after human experimenters stopped refereeing the transactions—suggesting the furry primates still have some evolutionary hurdles to cross before entering the world of business. A range of studies have suggested that apes have the prerequisites to enable them to set up a barter economy, but can’t make the leap to such a system on their own, the researchers said. “Lack of trust,” difficulty communicating, or difficulty with mental scorekeeping might have prompted the breakdown in chimp commerce absent human enforcement, the researchers, Sarah F. Brosnan and Michael J. Beran of Georgia State University, speculated. Working with three chimps, Brosnan and Beran set out to study why animals don’t barter, whereas some form of trade is common in virtually every human society. “Although other species lack language, they do communicate and often engage in behavior that appears related to trade, such as food sharing and the exchange of services,” wrote Brosnan and Beran in reporting their findings in the May issue of the Journal of Comparative Psychology. Our closest evolutionary cousins, chimps have been trained in the past to trade simple tokens with humans. Chimps in the wild have also been documented to trade services with each other—such as grooming and support in fights—and even sometimes services for food. But not goods for goods. So although recent economic trends might seem to suggest otherwise, the lower primates do not by nature get involved in commercial markets. “Extensive barter may represent a relatively recent evolutionary development” in the lineage leading to humans, Brosnan and Beran wrote. After lengthy training, the researchers placed pairs of chimps in separate enclosures and gave each tokens that could be turned in to an experimenter for treats. However, in half the cases, the chimps had to exchange the tokens with each other before the currency could be converted to food; this distinction was taught to chimps using a language and symbol system that they had learned since infancy. The tokens themselves bore some of these symbols. At first, an experimenter “enforced” the trade interactions by ensuring that apes had to reciprocate any trade before receiving their extra treats. But chimps were nonetheless free to refuse any deal. As long as the human referee stayed in the picture, chimps usually made the “correct” exchanges that brought them treats, according to the researchers. But when the human enforcer was gone, trades started “going sour,” Brosnan explained in an email: chimps didn’t always return the tokens proffered by their peers. After the lack of enforcement became apparent, Brosnan recounted in an email, the animals “quit trading as well.” Brosnan and Beran speculated that trade in services, as opposed to goods, might persist in the wild because services are hard to steal.