Quantity counts for baboons

Peanut experiment shows that monkeys roughly compare numbers of nuts using logic similar to humans

monkey portrait

COUNTER INTELLIGENCE  An olive baboon named Pearl, shown here, and one of her troopmates could keep tabs on approximate quantities of peanuts placed one at a time in containers. Researchers say this counting-like logic, which humans use to count exact amounts, comes naturally to monkeys.

Adam Fenster/University of Rochester Photography

Monkeys can’t count. But they can mentally keep track of and compare approximate quantities that increase one item at a time. That shows that monkeys use a kind of reasoning that also underlies human counting, researchers report May 7 in Psychological Science.

In a series of trials, two baboons watched from behind a barrier as one to eight peanuts were placed one by one into a container. Researchers then began singly dropping varying numbers of peanuts into a second container. The animals continually updated and compared that second inexact amount to the first quantity, enabling them to choose the larger cache of nuts as a snack an average of 68 percent of the time, psychologist Jessica Cantlon of the University of Rochester in New York and colleagues report.

If the number of peanuts in the second container reached about the same amount as in the first container, both monkeys spontaneously moved in front of the second container — even before it had been fully stocked. Seventy percent of such moves, on average, occurred when the second container ended up holding the most peanuts.

NUTTY MOVES From behind a barrier, a baboon named Pearl situates herself in front of a container into which an experimenter drops peanuts one at a time. Next, the experimenter places peanuts one at a time into a second container. Pearl moves in front of it when she decides that it holds about as many peanuts as the first container does. Pearl then gets to eat peanuts from the second container. Allison Barnard

Monkeys and many other nonhuman animals can detect differences in approximate numbers of objects. But until now, there was no direct evidence that any creatures other than humans can keep tabs on the relative values of separate quantities.

“What the baboons are doing is counting-like but only an approximation, whereas true human counting is exact,” Cantlon says. Still, she adds, item-by-item comparisons are critical for human counting.

Cantlon’s group studied two olive baboons housed at Rochester’s Seneca Park Zoo. Neither animal had any previous lab training in discriminating quantities.

Their behavior indicates that the baboons repeatedly revised the approximate amount of peanuts in the second container to compare it with their memory of the first container’s peanut stash, says psychologist Véronique Izard of Université Paris Descartes.

The results support a long-standing idea that estimating approximate quantities lies at the root of human counting, remarks behavioral neuroscientist Randy Gallistel of Rutgers University in New Brunswick, N.J.

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