"What Do Animals Think about Numbers?" by Marc D. Hauser, in American Scientist (Mar.–Apr. 2000), P.O. Box 13975, Research Triangle Park, N.C. 27709–3975.

More than 1,000 rhesus monkeys live on the Puerto Rican island of Cayo Santiago. Hauser, a psychology professor at Harvard University and the author of Wild Minds (2000), gave some of the wild monkeys there an arithmetic test. He and his students conspicuously placed two bright purple eggplants behind a screen but when they removed the screen the monkeys might behold one, two, or three eggplants. Just as human infants had done in similar tests, the monkeys tended to look longer when one or three eggplants appeared instead of the expected two.

From those and other experiments, Hauser says, it appears that wild rhesus monkeys, like human infants, can distinguish among one, two, three, and many objects. Other research, moreover, has shown that with training, monkeys and other animals can develop more sophisticated numerical abilities. Pigeons and rats, for instance, have learned to peck or press a button 24 times, no more, no less, to obtain a food pellet. Recent experiments by Columbia University psychologists demonstrated that captive rhesus monkeys can grasp the ordinal relations among the numbers one to nine and indicate the proper numerical order for various quantities of different images. "The rhesus monkeys’ performance was excellent—but only after receiving hundreds of training trials," notes Hauser.

Though the situations that animals confront in the wild may call for limited numerical abilities—chimpanzees, for instance, insist on "strength in numbers" (at least three adult males) before they’ll attack an intruding chimp from another pack—they apparently do not require the numerical precision and skills found in humans. This prompts Hauser to ask: "What kind of evolutionary or ecological pressures would have favored the numerical competence found in Homo sapiens?" His admittedly speculative answer: When trading appeared on the scene, precision became necessary to ensure a fair exchange. "Selection favored those individuals capable of enumeration and combinatorial computation with symbols." And thus, he says, was the groundwork laid for algebra, calculus, and set theory.