Researchers counting on animals for clues to math

Many people would argue that the human capacity to manipulate numbers is one of the abilities that sets us apart from other animals. But research, especially over the past several decades, indicates that animals--from rats to birds to chimpanzees--have some basic sensitivity to quantity.

And, some researchers contend, the neural system that animals use to recognize two rather than three pieces of fruit is the same mechanism infants use to make similar discriminations.

A theory held by some is that humans and other animals share a basic neural system called an "accumulator" that can clearly distinguish numbers of objects less than three or four but that cannot reliably discriminate between bigger numbers. This accumulator is active in animals and, perhaps, in human infants, the theory contends. Higher-order number abilities require the collaboration of other, more highly developed brain systems found only in humans.

The accumulator, thought to be housed in the brain's inferior parietal cortex, can be thought of as a graduated cylinder that you pour water into to represent each item you're counting, says Psychologist Randy Gallistel, PhD, of the University of California, Los Angeles. The final amount in the cylinder represents the end-count. But the amount of water poured into the accumulator varies a bit each time, so the final count may not be very precise. And the bigger the number, the less precise it is. Also, the closer together in number two groups of objects are, the more difficult it is for the accumulator to tell the difference--just as it might be hard to tell that two cylinders of water have different amounts in them the closer the heights are.

In fact, animals are better able to recognize five objects than 12 objects. And they have an easier time telling apart 25 and 10 than 25 and 20, says Gallistel, a proponent of the accumulator model. Other researchers, including Northwestern University psychologist Karen Fuson, PhD, argue that counting per se is a culturally created phenomenon that only applies to the learning of a symbolic or verbal list that can be put in one-to-one correspondence with objects.

"Animals appear to have a wired-in capability to do number discrimination and to more or less process number," says Fuson. "But I'm not sure I would say that most of them are counting."

Research has shown that some animals can reason arithmetically. Most recently, in a study reported in Science (Vol. 282, p. 746­749), Columbia University's Herbert Terrace, PhD, and graduate student Elizabeth Brannon taught two monkeys to serially order displays of geometric stimuli depicting different numbers of objects, ranging in number from one to four.

The colors, sizes and shapes of the geometric elements were random, and the monkeys had to touch them in ascending order. Once the monkeys learned the task, the researchers tested them with a new group of stimuli depicting between five and nine geometric elements--a task the monkeys had never done. From the first try, they successfully ordered the stimuli in ascending order based on the number of objects.

"Their study is an unequivocal demonstration that monkeys have the numbers ordered," says Gallistel.

Other researchers have taught animals equally intriguing tasks, from University of Arizona psychologist Irene Pepperberg's African gray parrot Alex, who can say how many blue keys are scattered on a plate, to Ohio State University Sally Boysen's chimpanzee Sheba, who can add two Arabic numerals and pick out their sum.

These data in animals are often used to bolster arguments that human infants are born with an ability to discriminate discrete numbers of items--if animals have the ability, it must be an evolutionarily derived and therefore innate trait, researchers argue.

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