Monkeys Master Basic Addition Algorithm

January 18, 2008

Monkeys and people apparently share some of the same basic ways of doing mathematics. Although it has already been suggested that monkeys can do simple arithmetic, our cousins on the evolutionary tree also demonstrate a humanlike mathematical ability, say Jessica Cantlon of Duke University and neuroscientist Elizabeth Brannon.

The scientists trained two monkeys to watch two groups of dots quickly displayed on a monitor. When two new sets of dots appeared, the monkeys had to pick the one set of dots that represented the sum of the earlier group, which always added up to less than 20. Cantlon and Brannon varied the sizes and positions of the dots so that when people — in this case, college students — took the same test, they could not use estimates of surface area or pattern recognition to come up with the right answer.

Both groups needed only about a second to formulate an answer. The surprising result was that "when the two [possible answer] numbers get larger and closer together, the probability of confusing them increases," Cantlon told Nature. "That was the case in both humans and monkeys."

The monkeys, who performed thousands of trial runs before being tested, were rewarded with Kool-Aid. The college students received $10 for their participation.

"Monkeys can mentally add the numerical values of two sets of objects and choose a visual array that roughly corresponds to the arithmetic sum of these two sets," Cantlon and Brennan concluded. "Furthermore, monkeys' performance during these calculations adheres to the same pattern as humans tested on the same nonverbal addition task."

"Our data demonstrate that nonverbal arithmetic is not unique to humans but is instead part of an evolutionarily primitive system for mathematical thinking shared by monkeys," they said.

A subsequent step will involve untrained animals. "This will be important," said Marc Hauser of Harvard University, "especially since the human-animal comparison often breaks down because the former are tested without training and the latter with training."

The results appeared in the article "Basic Math in Monkeys and College Students" in PLoS Biology.

Source: Nature, Dec. 18, 2007.