Given tests that measure the ability to estimate amounts, monkeys held their own against a group of Duke University undergraduates. The study required that estimations be made quickly, with no time for counting.
Humans are good at precise calculations; monkeys are not. But Jessica Cantlon, a brain researcher at Duke University, said monkeys are good at something called "fuzzy math."
Experts say one reason monkeys are good at estimating may be that they often need to assess quantities in a hurry.
"It's much more like estimating than the verbal mathematics you learn in school," Cantlon said.
To find out how monkeys' fuzzy math stacks up against humans', Cantlon tested two female rhesus monkeys named Boxer and Feinstein (after the senators). The monkeys watched a video screen.
"They would see one set of dots and then there would be a little delay," Cantlon said. "They would see a second set of dots, and then they'd be given two choices. And their task was to press the choice that represented the sum of those two sets of dots."
When Boxer and Feinstein were right, they got Kool-Aid. And they were right about 75 percent of the time.
Cantlon then gave Duke students the same exact task, rewarding them with cash instead of Kool-Aid. They were right about 90 percent of the time — not a great deal better than the monkeys.
Had the students been given more time, they would have done much better, Cantlon said. Able to count the dots out loud or in their heads, they would add up the number to close in on the exact amount.
"When you take away language from a human during a math task like this," Cantlon said, "they end up looking just like a monkey. You see these remnants of these more primitive mathematical abilities that are still kicking around in humans."
For monkeys, those abilities don't include counting. Cantlon said they're probably good at making quick estimates because they often need to assess quantities in a hurry — like whether they're outnumbered by an enemy.
Cantlon said young children probably do something very similar before they learn formal arithmetic. The results of the study appear in the journal Public Library of Science Biology.