Monkeys can do maths. Researchers have taught three rhesus monkeys to do simple addition using the numbers 1 through 25. The feat represents the best evidence yet for primates鈥� mathematical abilities.
To test how well the monkeys can represent and manipulate numbers in their brains, the team, led by neurobiologist at Harvard Medical School in Boston, had their subjects perform thousands of calculations with a system of written symbols, including the numerals 1 through 9 and letters of the alphabet.
First, the monkeys were taught to recognize the symbols for each of the numbers 鈥� when they correctly linked a symbol to its corresponding number, they were rewarded with the same number of drops of water or juice. Then they were faced with a series of choices on a computer screen: A single symbol on one side versus a combination of two symbols on the other. The monkeys were rewarded as before, which should have motivated them to pick the larger value.
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The monkeys chose the larger number between 70 and 90 per cent of the time. They were even able to repeat the success after learning a second, totally new set of symbols.
Past studies have hinted at primates鈥� quantitative abilities but have been limited to numbers less than 10, or were unable to prove that the animals were performing addition. This latest work provides clear evidence that monkeys can indeed perform basic arithmetic.
Hard to figure
鈥淭he data in my opinion are quite convincing, because it is really unlikely that the monkeys remembered each combination of symbols,鈥� says at the Institute of Cognitive Sciences and Technologies in Rome. 鈥淢aybe previous studies underestimated the addition capabilities of monkeys.鈥�
The monkeys made more mistakes on problems involving numbers that were close in value 鈥� 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鈥檇 be less likely to spot the difference between a flock of 22 and a flock of 24.
What we don鈥檛 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.
One of the study鈥檚 peer reviewers criticized the team on this point, arguing that the monkeys may have been biased towards a linear scale, since they were asked to do linear calculations. Teaching the monkeys to multiply should shed additional light on the issue, says Livingstone, but it may be a while before they can retrain the monkeys. 鈥淪o far, it has been two months and they鈥檙e still adding,鈥� she says.
Journal reference: Proceedings of the National Academy of Sciences,
