Monkeys seem to have a way with numbers.

A team of researchers trained three Rhesus monkeys to associate 26 distinct symbols consisting of numerals and selective letters with 0–25 drops of water or juice as a reward. The researchers then tested how the monkeys combined – or added – the symbols to get the reward.

Here's how Harvard Medical School neurobiologist Margaret Livingstone, who led the team, described the experiment: In their cages the monkeys were provided with touch screens. On one part of the screen, a symbol would pop up, and on the other side two symbols inside a circle were shown. For example, the numeral 7 would flash on one side of the screen and the other end would have 9 and 8. If the monkeys touched the left side of the screen they would be rewarded with seven drops of water or juice, whereas if they went for the circle, they would be rewarded with the sum of the numbers – 17 in this example.

After running hundreds of tests, the researchers noted that monkeys would go for the higher values more than half the time "indicating that they were performing a calculation, not just memorizing the value of each combination," according to the paper.

When the team examined the results of the experiment is greater detail, they noticed that the monkeys "tended to underestimate a sum compared with a single symbol when the two were close in value — sometimes choosing, for example, a 13 over the sum of eight and six. The underestimation was systematic: when adding two numbers, the monkeys always paid attention to the larger of the two, and then added only a fraction of the smaller number to it," according to an article published in Science Now.

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This indicates that there is a certain way quantity is represented in their brains, Dr. Livingstone says. But in this experiment “[w]hat they’re doing is paying more attention to the big number than the little one,” Livingstone told Science Now.

The researchers' findings are detailed in the current issue of the Proceedings of the National Academy of Sciences.