Monkeys match humans in arithmetic tests

Monkeys have shown they are almost as good as college students when it comes to simple arithmetic, according to a study demonstrating that a monkey's ability to perform mental addition can rival that of people.

The test was carried out on two female rhesus macaques, called Boxer and Feinstein, who were pitched against 14 American college students with an average age of 23. Both groups carried out the same tasks of mental addition and the results almost ended in an embarrassing draw.

The test consisted of choosing the correct sum of dots shown on two consecutive computer screens. A third screen showed two boxes, one with the correct number of dots formed from adding the two previous computer screens, and another box alongside it with an incorrect number of dots.

Scientists asked the college students to choose the box with the right number of dots without counting them individually, but just using an overall visual estimate. The monkeys, meanwhile, were given a reward if they chose the box with the correct sum.

In hundreds of trials involving more than 40 different addition problems, the college students were correct 94 per cent of the time, while the two macaque monkeys scored a respectable 76 per cent. But response times for monkeys and students choosing their answers were not that different 1,099 milliseconds for the monkeys, compared with 940 milliseconds for the students.

Intriguingly, both the students and the monkeys in the study, which is published in the journal Science, took longer to come to a decision when the two boxes were close to one another in the number of dots, indicating a common approach to solving the problem, said Jessica Cantlon, a graduate student at Duke University Medical Centre in North Carolina. "We know that animals can recognise quantities, but there is less evidence for their ability to carry out explicit mathematical tasks, such as addition. Our study shows that they can," Ms Cantlon said.

Of the confusion caused by similarly numerical boxes, she added: "If the correct sum was 11 and the box with the incorrect number held 12 dots, both monkeys and the college students took longer to answer and had more errors. We call this the ratio effect. What's remarkable is that both species suffered from the ratio effect at virtually the same rates."

This may indicate a common method of performing the task, and a shared evolutionary origin when both monkeys and humans had a common ancestor several million years ago.

Professor Elizabeth Brannon, of Duke University, said that human language makes a huge difference to the way we perform calculations because we can represent numbers as symbols that can be manipulated arithmetically.

"Much of adult humans' mathematical capacity lies in their ability to represent numerical concepts using symbolic language. A monkey can't tell the difference between 2,000 and 2,001 objects, for instance," Professor Brannon said.

"However, our work has shown that both humans and monkeys can mentally manipulate representations of numbers to generate approximate sums of individual objects."