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.

Continued on Next Page >>


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