Pigeons can learn abstract numerical rules, a skill that scientists had believed only primates possessed. Although the birds may not be able to do higher math, their ability to reason numerically is likely something that a wide variety of species can do, too, researchers say.
Many species, from honeybees to elephants, can discriminate between quantities of items, sounds, or smells, and represent numbers mentally. But only primates (all species, from lemurs to chimpanzees) were known to be able to reason numerically. For example, scientists showed in 1998 that rhesus monkeys can grasp the concept of "ordinal number." That is, given two sets containing from one to nine objects, they can determine that, say, a set with one thing should be placed before a set with two things, and so on. Since then, "there have been nice, consistent findings of this ability across all primate species," says Damian Scarf, a comparative psychologist at the University of Otago in Dunedin, New Zealand, and lead author of the new pigeon study. "But it's always been a question if this is unique to primates."
To find out, Scarf and his colleagues decided to give the same test to three pigeons. Scarf spent a year training the pigeons to order three sets containing one to three objects, such as a set including one yellow rectangle, two red ovals, and three yellow bars. The sets would appear on a computer screen, and the birds would have to peck at them in the correct, ascending sequence to get a reward of food. "They had to learn that it was the number of items that mattered, not the color or shape," says Scarf.
The pigeons were then asked to place two sets containing between one and nine items in the correct, ascending sequence to see if they understood the basic principle behind ordinal numbers. In their training sessions, the birds had only learned first, second, and third. But they didn't falter when presented with new numbers of shapes, such as five ovals or seven rectangles. The pigeons' scores were far above chance, says Scarf.
"I thought it was amazing that monkeys could do this, so we should be even more impressed that pigeons can, too!" says Elizabeth Brannon, a cognitive neuroscientist at Duke University in Durham, North Carolina, and the lead author on the original rhesus monkey study. The disparate creatures may be relying on the same neural mechanism to perform the task, she speculates. "These [new] findings suggest that, despite completely different brain organization and hundreds of millions of years of evolutionary divergence, pigeons and monkeys solve this problem in a similar way," says Brannon.
Scarf and his co-authors suggest that other species may demonstrate similar skills once they are tested. And colleagues agree. "The ability to represent and use numerosity is [probably] widespread among many animal species," says Michael Beran, a comparative psychologist at Georgia State University in Atlanta. Moreover, he says, the study suggests that other creatures may possess the "foundational mechanisms" that enable humans to reason so well with numbers and that "perhaps even advanced mathematical abilities may be found in other animals."