Experiments show monkeys are good at counting numbers up to 9

NEW YORK (CNN) -- According to a new study being published in the journal Science, monkeys can grasp the concept of numbers from one to nine, eroding the scientific consensus that counting is a phenomenon limited to humans.

Two rhesus monkeys named Rosencrantz and Macduff were taught to count at a Columbia University lab using touch-sensitive screens of images that each contained from one to four objects.

First, the monkeys were taught to touch the screens in order, from the image with one object to the image with four objects.

"A monkey learns by trial and error," said Elizabeth M. Brannon, a graduate student and author of the study. If the monkey touched all four pictures in the right order, he got a banana treat. If the monkey made an error, the screen blacked out.

The monkeys were then shown new images with between five and nine objects. When instructed to touch two images in a row, the first showing fewer objects than the second, the monkeys were able to use numbers five through nine.

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To make sure the animals were not memorizing pictures, Brannon and psychology professor Herbert Terrace used a series of 35 different images that varied in color, size and position.

"In the No. 1 picture, we might use a very large circle, while in the No. 4 picture we use very tiny circles," said Brannon. "This assures that the monkey is not just monitoring the size of the element."

Brannon and Terrace are not sure that monkeys count the same way humans do but are certain they understand the relationships between numbers. For example, Rosencrantz and Macduff were able to learn that seven is greater than six.

Both hope their research can be expanded to learn the process by which infants learn to count.

"Though monkeys do not recognize the word 'two' or the symbol '2,' they share with humans the ability to master simple arithmetic on at least the level of a 2-year-old child," Brannon said.

CNN Correspondent Ann Kellan, The Associated Press and Reuters contributed to this report.