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Decisionmaking by great apes found unexpectedly complex

Dec. 30, 2011
Courtesy of the Max Planck Society
and World Science staff

Chimps, orangutans, go­ril­las and bono­bos cal­i­brate their de­ci­sions by tak­ing in­to ac­count the pos­si­ble re­wards and the role of chance, ac­cord­ing to a new stu­dy.

The re­search re­veals that these close ev­o­lu­tion­ary rel­a­tives of hu­mans, known as great apes, make choices in a way that's savvier than pre­vi­ously thought and that could shed light on how hu­man de­ci­sion­mak­ing evolved, the sci­en­tists claim.

“Our study adds to the grow­ing ev­i­dence that the men­tal life of the oth­er great apes is much more soph­is­t­icated than is of­ten as­sumed,“ said Dan­iel Haun of the Max Planck In­sti­tutes for Psy­cho­lin­guist­ics and Ev­o­lu­tion­ary An­thro­po­l­ogy in Ger­ma­ny, who led the stu­dy. The find­ings were pub­lished Dec. 21 in the on­line re­search jour­nal P­LoS One

The find­ings sug­gest that when choos­ing be­tween op­tions in ex­pe­ri­ments, the an­i­mals weighed fac­tors in­clud­ing the rel­a­tive re­wards avail­a­ble and the like­li­hood of mak­ing cor­rect guesses. Ex­pe­ri­menters pre­sented the apes with choices be­tween two ba­nana pieces: a smaller one, which was al­ways re­liably in the same place, and a larg­er one, which was hid­den un­der one of mul­ti­ple cups. The catch was that if they opted for the big piece and guessed the wrong cup, they would get noth­ing. 

The re­search­ers found that the apes' choices were gov­erned by their un­cer­tain­ty and their knowl­edge about the like­li­hood of suc­cess in the risky choice. The low­er their chance of guess­ing cor­rectly, the more of­ten they went for the small piece.

The re­search­ers al­so found that the apes opted for the larg­er piece – and risked get­ting noth­ing – no less than half of the time. This risky decision-mak­ing in­creased to nearly 100 per­cent when the size dif­fer­ence be­tween the two ba­nana pieces was larg­est. The sci­en­tists said chimps and orangutans were more prone to make risky choices, for un­known rea­sons, but all four spe­cies took chance in­to ac­count in a pre­dict­a­ble way.

“Based on our find­ings, we pro­pose that decision-mak­ing in the great apes pro­vides a prom­is­ing con­text for the in­ter­preta­t­ion of decision-mak­ing in hu­mans, the fifth great ape spe­cies,“ the re­search­ers wrote.


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Chimps, orangutans, gorillas and bonobos calibrate their decisions by taking into account the possible rewards and the role of chance, according to a new study. The research reveals that these close evolutionary relatives of humans, known as great apes, make choices in a way that's savvier than previously thought and that could shed light on how human decisionmaking evolved, the scientists claim. “Our study adds to the growing evidence that the mental life of the other great apes is much more sophisticated than is often assumed,“ said Daniel Haun of the Max Planck Institutes for Psycholinguistics and Evolutionary Anthropology in Germany, who led the study. The findings were published Dec. 21 in the online research journal PLoS One. The findings suggest that when choosing between options in experiments, the animals weighed factors including the relative rewards available and the likelihood of making correct guesses. Experimenters presented the apes with choices between two banana pieces: a smaller one, which was always reliably in the same place, and a larger one, which was hidden under one of multiple cups. The catch was that if they opted for the big piece and guessed the wrong cup, they would get nothing. The researchers found that the apes' choices were governed by their uncertainty and their knowledge about the likelihood of success in the risky choice. The lower their chance of guessing correctly, the more often they went for the small piece. The researchers also found that the apes opted for the larger piece – and risked getting nothing – no less than half of the time. This risky decision-making increased to nearly 100% when the size difference between the two banana pieces was largest. The scientists said chimps and orangutans were more prone to make risky choices, for unknown reasons, but all four species took chance into account in a predictable way. “Based on our findings, we propose that decision-making in the great apes provides a promising context for the interpretation of decision-making in humans, the fifth great ape species,“ the researchers wrote.