Natural order

By Alison Motluk MONKEYS are surprisingly numerate, say scientists in New York. They have found that monkeys trained to rank groups of up to four items in ascending orders could spontaneously do the same with larger numbers. The finding suggests that monkeys understand the sequence of numbers, undermining the view that language skills are required to understand the concept of numbers. Many scientists believe that numeracy and language ability are linked. Both require complex mental manipulations and scientists tend to think that animals that can’t speak can’t understand numbers either. Some have even suggested that numbers are a social construct. To test the idea, two psychologists at Columbia University, Elizabeth Brannon and Herbert Terrace, gave a pair of rhesus monkeys called Macduff and Rosencranz a touch-sensitive video monitor. They then flashed sets of four images on the screen. Each image contained a different number of elements—two circles, say, or three little flowers—to represent the numbers one to four. The researchers varied the elements in terms of brightness, size and arrangement to be sure the two monkeys were paying attention to the quantity and not something else, such as area. If the monkeys touched the boxes in ascending order from one to four, they were rewarded with a banana-flavoured treat. If they made a mistake, the screen went blank and they had to start all over again. After training them for about six weeks, the researchers put the monkeys through 150 new trials over five days, with brand new items—just to make sure they hadn’t been using memory tricks all along. “Now we were sure they could differentiate 1, 2, 3 and 4,” says Brannon. The big surprise came when the psychologists tested the monkeys with quantities they’d never seen before. When they were shown two new images containing between five and nine items, Rosencranz and Macduff put them in ascending order. They saw each pair only once and got no reward for getting it right. Yet they continued to respond with 75 per cent accuracy, Brannon and Terrace report in Science (vol 282, p 746). “They didn’t need training,” says Brannon. “They understood the ordinal relationship.” Brannon and Terrace think the animals must be somehow predisposed to process numbers. “Numbers must be important to them in order for them to show this kind of spontaneous behaviour,” says Brannon. Marc Hauser, a psychologist at Harvard University in Boston, says the study clearly shows that animals can have numeracy skills without language abilities. He believes the rhesus monkeys are better with numbers than one-year-old babies, who have begun to develop rudimentary language. He want to find out what happens in human brains that makes us able to develop the skill to such a complex level. “At some point the kids are going to leave the monkeys in the dust,” says Hauser. “What major cognitive event happens in a child that doesn’t happen in a monkey?” Brannon and Terrace now hope to discover what kind of mechanisms underlie the monkeys’ abilities. “Are they counting?” Brannon asks. “How are they representing the numbers?
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