1 Monkey + 1 Monkey = Rhesus Arithmetic

I’ve never understood all those anti-evolution kooks who think that being related to apes and monkeys—sharing an ancestor with them—is an unspeakable insult. After all, chimps alone have been found to use tools, logic and semantics, and to demonstrate compassion and empathy greater than what some humans have shown themselves capable of. Now scientists have shown that rhesus monkeys can add.

Even a monkey can look at arrays of dots and determine which of two sets has more dots, which is not surprising: being able to take a quick scan of two bunches of bananas and decide which is worth climbing a tree for comes in handy in the jungle. But scientists didn’t know whether monkeys or other nonhuman animals can do mental arithmetic, although there had been hints. In 2005 scientists reported that when rhesus monkeys watched as two groups of four lemons were placed behind a screen, they looked longer when the screen was lowered to reveal an incorrect four lemons (“but wait!”, you could imagine them thinking; “4 + 4 = 8, not 4”) than when there were eight lemons. (Long looking time means the monkeys detected something amiss.) The most intriguing test of arithmetic in an animal was one conducted in 1989 on a chimpanzee who had been trained to recognize Arabic numerals; he could choose the right one when he had to add up sets of oranges, as long as the total was less than 4.

In the current study, Elizabeth Brannon of Duke University and graduate student Jessica F. Cantlon had two female rhesus monkeys (as well as 14 college students, which we’ll get to in a minute) look at two sets of dots on a touch screen monitor. The sets appeared half a second apart. Once the monkeys had had a good look at the sets, they saw two choices: an array with a number of dots equal to the sum of the sets and an array whose dots did not equal that sum. The arithmetic problems went up to sums of 16, and the questions used every permutation for that sum (for 8, for instance, the questions were 1 +7, 2 + 6, 3 + 5, 4 + 4, 5 + 3, 6 + 2 and 7 + 1).

As the scientists report tonight in the online journal PLoS Biology, “monkeys perform approximate mental addition in a manner comparable to college students tested on the same addition task.” To wit: the monkeys got about 76 percent of the questions right, compared to 94 percent for the students.

You may be sneering that this isn’t addition at all; it’s just memorizing the look of two arrays and mentally combining them. So to see whether the monkeys were not really adding but instead merging the spatial extent of the two sets of dots, the scientists varied that spatial extent. About one-quarter of the time, the area covered by the wrong answer more closely matched the total area of the dots in the two addends than the right answer did—that is, the dots were crowded together or spread apart in the right answer but had spacing comparable to that in the two addends in the wrong answer. The monkeys weren’t fooled, still answering correctly most of the time, “indicating that they based their choices on the numerical sum of the objects, not their surface area,” say the Duke scientists.

Okay, so it's not calculus. But as scientists probe for the evolutionary roots of human abilities, both cognitive and moral, expect more such discoveries that challenge many claims of human uniqueness.

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