Monkeys Can Subtract, Study Finds

Christine Dell'Amore in Chicago
National Geographic News
February 18, 2009
Add this to the growing list of reasons humans aren't so special, after all: Monkeys can subtract.

The discovery marks the first time a nonhuman species has been seen having "widespread success" with subtraction, scientists announced last Thursday.

Rhesus macaques placed in front of touch screens in a Duke University laboratory were able to subtract dots—not by counting them individually but by using a more instantaneous ability researchers call number sense.

In each session a monkey was presented with a number of dots.

Next, a large square would hide all the dots.

Then some of the dots would glide off the screen from "behind" the shape.

Only the big square, with the remaining dots "hidden" behind it, would be left on screen, as seen in this two-second video:

Finally, decision time: Two groups of dots would appear on screen—one of them the correct number of remaining dots—and the monkey would indicate its answer to the math problem by touching one of the groups (see picture of monkey making selection). Each correct answer was worth a serving of Kool-Aid.

(Related: "Lemur Logic May Provide Clues to Primate Intellect Evolution.")

Monkeys Subtract as Well as College Students?

In the vast majority of trials, the monkeys chose the right answer without counting, said psychologist Jessica Cantlon, who co-led the studies at Duke.

In fact, college students used as controls in the study had the same success rate as the macaques—each group choosing the correct answer in as little as a second, Cantlon said at an American Association for the Advancement of Science meeting in Chicago.

Such similarities "suggest that these abilities are part of a primitive system for reasoning about numbers that has been passed down for millions of years of evolutionary time," she said.

Though previous studies have shown a limited capability for subtraction among nonhuman animals, the new data are "the first evidence of widespread success," Cantlon said by email.

Past experiments by other researchers, for example, used only small numbers, she said. Previous studies also did not vary the dot sizes in the multiple choice section, so the larger number always looked physically bigger—possibly tipping off the animals to which set of dots represented the larger sum.

A 2007 study co-authored by Cantlon proved monkeys' success with addition—again on par with college students performing the same tasks.

Wild Subtraction

Animals' knack for numbers can boost survival in the wild, Cantlon told National Geographic News.

For instance, research has shown that apes can determine at a glance roughly how much food is present in an area and decide whether to stay and eat or to move on, she said.

On the 200th anniversary of Charles Darwin's birth, "these studies suggest we humans should keep our egos in check," said Edward Wasserman, an experimental psychologist at the University of Iowa.

"We are certainly not the only intelligent animals on Earth."

RELATED VIDEO: Chimp Memory Beats Humans'


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