Scientists have been skeptical of claims of mathematical abilities in animals ever since the case of Clever Hans about 100 years ago. The horse, which performed arithmetic and other intellectual tasks to delighted European audiences, was in reality simply taking subconscious cues from his trainer. Modern examples, such as Alex the African grey parrot, which could count up to six and knew sums and differences, are seen by some as special cases or the product of conditioning.

Recent studies, however, have uncovered new instances of a counting skill in different species, suggesting that mathematical abilities could be more fundamental in biology than previously thought. Under certain conditions, monkeys could sometimes outperform college students.

In a study published last summer in the Proceedings of the Royal Society B, Kevin C. Burns of Victoria University of Wellington in New Zealand and his colleagues burrowed holes in fallen logs and stored varying numbers of mealworms (beetle larvae) in these holes in full view of wild New Zealand robins at the Karori Wildlife Sanctuary. Not only did the robins flock first to the holes with the most mealworms, but if Burns tricked them, removing some of the insects when they weren’t looking, the robins spent twice as long scouring the hole for the missing mealworms. “They probably have some innate ability to discern between small numbers” as three and four, Burns thinks, but they also “use their number sense on a daily basis, and so through trial and error, they can train themselves to identify numbers up to 12.”

More recently, in the April issue of the same Royal Society journal, Rosa Rugani of the University of Trento in Italy and her team demonstrated arithmetic in newly hatched chickens. The scientists reared the chicks with five identical objects, and the newborns imprinted on these objects, considering them their parents. But when the scientists subtracted two or three of the original objects and left the remainders behind screens, the chicks went looking for the larger number of objects, sensing that Mom was more like a three and not a two. Rugani also varied the size of the objects to rule out the possibility the chicks were identifying groups based simply on the fact that larger numbers of items take up more space than smaller numbers.

For the past five years Jessica Cantlon of the University of Rochester has been conducting a series of experiments with rhesus monkeys that shows how their numerical skills can rival those of humans. The monkeys, she found, could choose the lesser of two sets of objects when they were the same in size, shape and color. And when size, shape and color were varied, the monkeys showed no change in accuracy or reaction time. One animal, rewarded with Kool-Aid, was 10 to 20 percent less accurate than college students but beat them in reaction time. “The monkey didn’t mind missing every once in a while,” Cantlon recounts. “It wants to get past the mistake and on to the next problem where it can get more Kool-Aid, whereas college students can’t shake their worry over guessing wrong.”

Elizabeth Brannon of Duke University has conducted similar experiments with rhesus monkeys, getting them to match the number of sounds they hear to the number of shapes they see, proving they can do math across different senses. She also tested the monkeys’ ability to do subtraction by covering a number of objects and then removing some of them. In all cases, the monkeys picked the correct remainder at a rate greater than chance. And although they might not grasp the deeper concept of zero as a number, the monkeys knew it was less than two or one, conclude Brannon and her colleagues in the May Journal of Experimental Psychology: General.

Although Brannon feels that animals do not have a linguistic sense of numbers—they aren’t counting “one, two, three” in their heads—they can do a rough sort of math by summing sets of objects without actually using numbers, and she believes that ability is innate. Brannon thinks that it might have evolved from the need for territorial animals “to access the different sizes of competing groups and for foraging animals to determine whether it is good to stay in one area given the amount of food retrieved versus the amount of time invested.”

Irene Pepperberg of the Massachusetts Institute of Technology, famous for her 30-year work with Alex the parrot, says that even bees can learn to discriminate among small quantities. “So some degree of ‘number sense’ seems to be able to be learned even in invertebrates, and such learning is unlikely without some underlying neural architecture on which it is based,” she remarks.

Understanding the biological basis of number sense in animals could have relevance to people. According to Brannon, it may suggest to childhood educators that math, usually taught after age four or five, could actually be introduced earlier into the curriculum.

Note: This article was originally printed with the title, "Animals by the Numbers."