Apes only gamble on a sure thing

DUKE (US) — When chimpanzees and bonobos, human’s closest living primate relatives, aren’t certain of the odds or don’t have confidence in their judgments, they’re more likely to  play it safe, rather than take a risk.

In studies conducted at the Tchimpounga Chimpanzee Sanctuary in Republic of Congo and Lola ya Bonobo Sanctuary in Democratic Republic of Congo, 16 chimps and 14 bonobos were asked to make choices between two bowls of treats.

Though no dice or chips are involved, this kind of experiment is considered a “gambling game,” says Alexandra Rosati, graduate student at Duke University.

The apes could choose between a safe bet that would always provide a food they liked a little, some peanuts, or a variable bet in which the payout could be either a highly-preferred big piece of banana or less-desirable cucumber slice.

The apes “gambled” by making a choice of one bowl or the other. They knew the odds before they chose because the experimenter showed them bowls with two potential outcomes, but then gave them only one. Depending on the contents of the bowls, there could be a 100 percent chance of receiving a banana, a 50 percent chance, or a 0 percent chance.

The apes readily distinguished between the different probabilities of winning: they gambled a lot when there was a 100 percent chance, less when there was a 50 percent chance, and only rarely when there was no chance.

In some trials, however, the experimenter didn’t remove a lid from the bowl, so the apes couldn’t assess the likelihood of winning a banana.

The odds from the covered bowl were identical to those from the risky option: a 50 percent chance of getting the much sought-after banana.

Apes of both species were less likely to choose this ambiguous option, but were willing to take the chance on the covered bowl when they knew the only alternative was food they didn’t like (the cucumber), or no food at all.

But, like humans, they showed “ambiguity aversion”—preferring to gamble more when they knew the odds than when they didn’t.

Details appear in the journal Biology Letters.

Given some of the other differences between chimps and bonobos, Rosati and Brian Hare, assistant professor of evolutionary anthropology, had expected to find the bonobos to be more averse to ambiguity, but that didn’t turn out to be the case.

Looking for the foundations of economic decision-making, researchers have been doing studies like this on a variety of species, but these two apes are the two species most closely related to humans, Rosati says.

“These results suggest that understanding how animals forage may be more complex than previously thought.”

Decision-making matters to animals in the wild who have to make choices about which resources to pursue on the fly, without knowing if their choices will pay off.

“It may be that different decision mechanisms come into play when animals are faced with choices when they have incomplete knowledge,” Rosati says.

“These results also suggest that some of our human economic biases may be evolutionarily ancient, predating modern markets: chimpanzees and bonobos act just like us when faced with a primate slot machine.”

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