The results of an experiment on a pair of Rhesus monkeys shatter the assumption that counting is a trait unique to humans who develop the skill through language.

Christine McGourty explains the researchers' findings

Elizabeth Brannon and Herbert Terrace carried out tests at Columbia University on two monkeys called Rosencrantz and Macduff, after two Shakespearean characters.

First, the animals were taught to choose images on touch screens in correct ascending order, from one object through to four.

The images were all a different shape, colour and pattern to make sure the monkeys were not reacting to the visual characteristics of the forms rather than the quantity.

When they got the correct answer, they were rewarded with food.

"Accuracy substantially exceeded the level predicted by chance," the researchers said in an article in Science magazine.

Fast learners

Rosencrantz and Macduff were then asked to pick two images in a row. The first had to contain fewer objects than the second.

The number of objects was increased to a maximum of nine and the monkeys were tested on each of 36 pairs of possible combinations.

During the second part of the experiment, the monkeys were shown up to nine images.

The researchers watched the monkeys apply the rule they had learned earlier.

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

Susan Carey, of New York University, explained that the results "challenge the consensus that the capacity to represent numbers is solely the product of culture, learned as we learned language".