Monkey see, monkey count.

Rhesus monkeys may not perform advanced calculus or even basic division, but a new study suggests that they can distinguish between varying numbers of items, from one to nine, and correctly order them from the smallest to the largest.

These results offer the strongest evidence to date that non-human primates wield numerical knowledge, say psychologist Herbert S. Terrace and psychology graduate student Elizabeth M. Brannon, both of Columbia University.

Since the monkeys make no obvious attempts to count on their fingers and maintain a humble silence in the face of researchers' questions, it remains difficult to pin down the mental tactics they use in determining that, for example, five diamonds represent a lesser quantity than six rectangles.

The animals apparently have learned some type of numerical rule for ordering amounts from one to nine, Brannon theorizes. "We don't have direct evidence yet, but it seems likely that these monkeys, and other nonhuman primates, can count," she says.

Terrace and Brannon presented two male rhesus monkeys with a series of 35 displays, each consisting of four images, on a touch-sensitive computer screen. Each image portrayed a different object in numbers ranging from one to four. Images were arranged randomly, from left to right. They might show, for example, two bananas, one triangle, four apples, and three hearts.

The size, surface area, shape, and color of objects were also changed randomly from one display to the next.

The monkeys rapidly learned to touch pictures in ascending numerical order when they received food pellets for correct answers. After errors, the screen turned blank, and a few seconds later, a new set of pictures appeared.

A subsequent series of displays presented arrays of four different objects shown in quantities ranging from five to nine. The monkeys ordered these novel amounts as accurately as they had learned to order the smaller numbers of items, the investigators report in the Oct. 23 SCIENCE. In a final trial, the rhesus duo usually distinguished smaller from larger amounts, ranging from one to nine, in pairs of images.

The Columbia scientists' findings bolster the view that people and many other animals possess a nonverbal brain system for reasoning about small quantities (SN: 7/11/98, p. 27).