Monkeys Do Math Like Humans
A rhesus macaque pictured at the NIH Animal Center in Poolesville, MD.
Credit: Science/Joshua Moglia

Monkeys can perform mental addition in a manner remarkably similar to college students, a new study shows.

The researchers stressed that monkeys will not pass college math tests anytime soon. Nevertheless, the finding promises to shed light on the ancient origins of math in humanity and our distant relatives.

Humans possess a sophisticated repertoire of mathematical capabilities unmatched anywhere else in the animal kingdom. Still, there is increasing evidence that at least some of these abilities are shared with other animals. For instance, many animals can figure out which of two sets of dots is larger or smaller.

Too easy

To see how far back more advanced capabilities such as addition might go, researchers at Duke University in Durham, N.C., focused on somewhat distant relatives of humans—rhesus monkeys. While the ancestors of chimpanzees—humanity's closest living relatives—diverged from us about 6 million years ago, humans and rhesus monkeys parted ways roughly 25 million years ago. In comparison, the age of dinosaurs only ended roughly 65 million years ago.

The scientists tested two monkeys and 14 college students on a math task where they had to add two sets of dots together. They were each shown one set of dots on a computer touchscreen for a half-second, and then another set a half-second later. They were then shown two separate clusters of dots at the same time, one of which was the correct sum of the first two sets. The monkeys were rewarded with Kool-Aid for choosing the right answers.

"When I first began training the monkeys on the addition task, I thought I would have to wait for many weeks before they understood the task," said researcher Jessica Cantlon, a cognitive neuroscientist at Duke. "We started the monkeys out on an easy version of the addition task, and the plan was to increase the difficulty of the problems gradually over time."

However, when Cantlon looked at the data from the first sessions, "it turned out that the monkeys were already performing the easy problems very well, and so I had to scramble to program the more difficult version of the task," she recalled.

Adding in their heads

The college students unsurprisingly did better than the monkeys, scoring right 94 percent on average compared to the monkey's 76 percent.

Still, like humans, the more similar in size the two given choices were, the more the monkeys had a hard time picking the right answer. This suggests monkeys and humans were adding up numbers in their heads in a similar way. Supporting this notion is the fact that both humans and monkeys found it harder to pick the right choice the larger the numbers got.

"People might think that we are implying that monkeys can take out a paper and pencil and pass a college math exam, but we're not," Cantlon stressed. Instead, she noted they found that when monkeys and humans were made to do addition in their heads, "they use the same mental estimation process."

"This research gives us a glimpse into where our sophisticated human minds came from," Cantlon told LiveScience. This is important "for both figuring out what makes our human minds evolutionarily similar to those of other species but also for figuring out what makes humans so unique."

Why they do it

Cantlon and her colleagues suggest math could help monkeys and other animals choose larger amounts of food or gauge the size of a rival group.

"Although we can never travel back in time to know exactly why or how this arithmetic ability evolved in humans, social battles might have something to do with it," she said. "Finding the perfect spot in the forest to stop and forage might also have something to do with it."

The researchers now want to learn more about what this primitive math system in monkeys is capable of "and whether it is the evolutionary basis of human mathematical thinking," Cantlon said. "We are also interested in whether this primitive mathematical system forms the basis of mathematical development in human children."

Psychologist Michael Beran at Georgia State University, who did not participate in this study, said, "it would be exciting to expand this kind of test to other arithmetic operations such as subtraction, for example, to see if monkeys perform similarly to humans in that case as well. I suspect that subtraction would be a harder operation for animals to understand and deal with, but little research has been done. My own research with subtraction did show that chimpanzees had much more difficulty in making correct responses compared to tasks that used addition of items."

Cantlon and her colleague Elizabeth Brannon reported their findings online Dec. 17 in the journal PLoS Biology.