A recently concluded experiment shows that rhesus monkeys are capable of doing simple addition using numbers 1 through 25. But more interesting than that is the observation that they also make the same mistakes as us.

To test whether monkeys can represent and manipulate numbers in their head, neurobiologist Margaret Livingstone of Harvard Medical School and her colleagues set up a rather interesting experiment.

Prior to this, however, the monkeys learned to associate the Arabic numbers 0 through 9 and 15 select letters with the values zero through 25. This was done by having the monkeys choose larger numbers as a means to acquire greater quantities of a food reward.

But for the new experiment, the monkeys had to work a bit harder for it. They had to perform addition in order to correctly choose the larger reward. Specifically, they were given a choice between performing a sum calculation and a single symbol rather than just two single symbols. Eventually, they learned how add the two symbols and compare the sum to a third, single symbol.

To rule out the possibility that they were simply memorizing combinations of symbols, the researchers taught the monkeys an entirely new set of symbols. They were still successful, calculating previously unseen sets of combinations.

The monkeys weren't perfect, however. And in fact, they committed an error often exhibited by humans. Aviva Rutkin from New Scientist explains:

The monkeys made more mistakes on problems involving numbers that were close in value ā a fact which might ultimately prove more interesting than their success at small numbers.

Neuroscientists already know that the human brain is better at distinguishing between two low numbers than two high ones. For example, you could easily tell the difference between two and four birds sitting in a tree, but you'd be less likely to spot the difference between a flock of 22 and a flock of 24.

What we don't know is why. Some think it is because the brain encodes numbers logarithmically, so that we perceive the distance between two small numbers as greater than that between two large ones. Others argue that the brain encodes numbers linearly, as on a number line, but that our concept of a number becomes less distinct as the value increases.

As Rutkin points out, the monkeys were biased towards a linear scale. More insight is likely to emerge if and when monkeys are asked to perform tasks involving multiplication.