Like humans, capuchin monkeys can determine probability, study finds

Science is continually trying to find an answer to the question, what makes humans unique? Is it our social interactions? Tools? Ability to reason? Time and again, what were thought to be unique human traits turn out to be shared in some form or another with other animals. 

Now, scientists from the University of Toronto have found that the ability to make decisions based on probabilities is not just found in humans.

You likely haven't thought a lot about it, but we can judge how good our chances are at something without really thinking. 

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Let's say you have preference for chocolate over mint candy. There are two bowls in front of you, both filled with chocolates and mints. But in one bowl there are more chocolates than mints. The second has more chocolates, but far more mints. Which would you choose to take a handful from?

"That requires not just looking at the bowl that has the most of the candy you want, but the highest proportion of the candy you want so that you have the highest chance of getting the candy you want," assistant professor Daphna Buchsbaum told CBC News.

In an experiment with 19 capuchin monkeys at the Living Research Centre at the Edinburgh Zoo in Scotland, researchers found that the monkeys were able to correctly choose from a jar that gave them the best chance of getting a peanut rather than a much less preferred monkey pellet.

Ability in humans

Buchsbaum explained that the classic view was that determining probability was a uniquely human ability and an ability that didn't develop until relatively late, around school age. However, new research has found that it may occur in infants. We do this intuitively rather than work out the complicated math in our heads.

Are there other species that share this ability with us?- Emma Tecwyn, University of Toronto

Though this ability has been found in apes, it's never been seen in a monkey species. The last time we shared a common ancestor with capuchins was 30 million years ago. 

"The question is, is it much more ancient than that? Are there other species that share this ability with us?" post-doctoral researcher Emma Tecwyn told CBC News.

Evolutionary benefits

The ability to determine probabilities may help the capuchins in social interactions. If, for example, one male capuchin knows that the dominant male in the group prefers a particular food, he knows that he can collect other food without rousing the ire of the dominant male. 

It could also help the monkeys predict behaviour in others, allowing them to reason about the mental state of a group member.

"I think there's a lot of different abilities that have been proposed as the one that definitively makes humans unique, but we're frequently finding that monkeys share those abilities," Buchsbaum said.

But don't worry: while primates may be able to use tools as well as reason, we're still a long way off from a Planet of the Apes scenario. Probably.