"Do monkeys naturally represent numbers or is it just a lab circus act?" asks Duke University psychology professor Elizabeth Brannon, PhD.
A new study by Brannon and graduate student Jessica Cantlon, published in the January Journal of Experimental Psychology: Animal Behavior Processes (Vol. 33, No. 1) finds that it's the former: Monkeys do use number to distinguish between different stimuli, even when they have the option of using cues such as shape and color. The finding runs counter to past research suggesting that animals only use number when other options aren't available, says Hank Davis, PhD, a psychology professor at the University of Guelph, in Ontario, Canada.
If monkeys spontaneously notice number, that could mean that math has deeper evolutionary roots than many scientists previously thought, says Cantlon. The line of research could even shed light on how infants quantify the world, she says.
"Number is a really important concept to humans-we use it all over the place to balance budgets and in the marketplace," Cantlon says. "But it isn't something unique to humans, it seems like it's something that has been around for a long time, evolutionarily speaking."
While Cantlon and Brannon's participants-four adult macaques-were not wild animals, one of them had never before been used in a number study. The other three had participated in many experiments where they learned to match items by number.
For this experiment, all four animals first learned to match identical pictures. For example, the monkey might see an image of four red hearts on a touch-screen computer. After one second, the image would be replaced by two slides-one with the same four red hearts and one with three blue diamonds. If the monkey chose the slide identical to the sample slide, it was rewarded with a sip of Kool-Aid.
Once the animals got good at the easy matching task, the researchers gave them a tricky choice, where either of the two slides could be correct. For instance, after seeing a slide with four red hearts, the animals would have to choose between one with four red diamonds and one with two red hearts. If an animal chose the slide with the four diamonds, that would show that it based its decision on number, but if it chose the two hearts, that would suggest that it matched on the basis of shape, Brannon notes.
The three previously trained monkeys preferred to match by number much of the time, especially when there was a big difference in number between the first slide and one of the choices. If the difference was small-if, for instance, the first slide had four red hearts and one of the matching options had three red hearts-they would tend to rely on color or shape to pick a match.
In contrast to the trained monkeys, the untrained monkey usually matched by color or shape. But it too was swayed by the difference in number-more often matching shapes that were similar in number than those that were vastly different. The fact that all the monkeys were affected by the difference between the number of items in the two slides shows that they do take note of quantity, even if it doesn't always control their decision-making, says Brannon.
"All the monkeys were definitely influenced by number," she says. "Even the naive monkey showed the same kind of ratio dependence."
That interpretation conflicts with a 1983 study by Davis, published in Animal Learning and Behavior (Vol. 11, No. 1, pages 95-100). In that study, Davis and his colleague, John Memmott, PhD, also of the University of Guelph, identified which cues rats prefer to use when estimating when they would receive an electric shock. They found that the animals readily learned the pattern if they received a shock after a certain amount of elapsed time. For instance, if the rats always got a shock 15 minutes after being put in a Skinner box, they would lever-press for food for the first ten minutes, and then freeze in fear until they got shocked. Afterward they'd return to lever pressing.
In a second experimental condition, the animals couldn't predict when they'd get the shocks on the basis of time. They occurred at random. However, there would only be three shocks during the course of the experiment. It took many trials before the rats learned to count to three, and there's no reason to assume primates would behave any differently, Davis says.
"When their backs were to the wall, and they needed to use number in order to feel safe, they could do it," says Davis. "But you could almost see their minds reaching for other types of stimulus information."
Most animals prefer to use cues other than number, Davis posits.
"My guess is that counting really involves some high-level cognitive processes, and even in animals capable of these things, it simply takes more calories," says Davis. "I think even for humans there are certain types of numerical skills that are difficult."
As evidence, Davis notes that the number-naive monkey in Cantlon and Brannon's study probably started paying attention to number because of the training sessions and the early stages of the experiment itself, he notes.
"This is not a rhesus macaque they have caught on an island....It's lived its life in a lab and is now part of a second experiment in a number study," he notes. "In some deeper sense I don't see this as a number-naive animal."
However, one study, published in the October 2005 issue of Cognition (Vol. 97, No. 3, pages 315-325), has used wild monkeys on an island, and found that they do spontaneously pay attention to number. In the experiment, Yale University psychology graduate student Jonathan Flombaum, PhD, and his colleagues captured the attention of wild rhesus monkeys on a Puerto Rican island. They showed them a group of lemons and then put the fruit behind a screen. The researcher showed the monkeys another number of lemons, added the lemons to the group behind the screen and then lifted the screen. When the total number of lemons revealed was equal to the two groups of lemons added together, the monkeys tended not to look at it for very long-suggesting that they expected that result. But when the number of revealed lemons was much different than the two groups added together, the surprised monkeys gazed at the fruit for a few seconds more.
This shows that monkeys naturally keep track of number, and may even do something like addition, says study co-author Marc Hauser, PhD, a psychology professor at Harvard University.
"You don't have to train animals to pay attention to numbers; they do so spontaneously," he says.
It all depends on how you define number, counters Davis. Animals may naturally make a distinction between more versus less, he says, but they aren't using specific quantities to do it. For instance, a hawk might see that one field has more mice than another, but it doesn't know that one field has 367 mice while the other only has 215.
"This is relative numerosity, not absolute number," he says. "People would like to believe that number is a natural sense for nonhumans, but I don't believe it. I still think it is a last resort."