Like a lot of humans, monkeys might not be able to do calculus. But a new study shows that they can learn and rapidly apply abstract mathematical principles.

Previous work has shown that monkeys and birds can count, but flexible applications of higher mathematic rules, the study authors asserted, "require the highest degree of internal structuring"—one thought largely to be the domain of only humans.

So researchers based at the Institute of Neurobiology at the University of Tubingen in Germany set out to see whether rhesus monkeys could learn and flexibly apply the greater-than and less-than rule. They tested the monkeys with groups of both ordered and random dots, many of which were novel combinations to ensure that the subjects couldn't have simply memorized them. The monkeys were cued into applying either the greater-than or less-than rule by the amount of time that elapsed between being shown the first and second group of dots.

"The monkeys immediately generalized the greater than and less than rules to numerosities that had not been presented previously," the two researchers, Sylvia Bongard and Andreas Nieder, wrote. "This indicates that they understood this basic mathematical principle irrespective of the absolute numerical value of the sample displays." In other words: "They had learned an abstract mathematical principle."

But the researchers were after more than simple ape arithmetic. "If and how mathematical rules can be represented by single neurons," they wrote, "has remained elusive." So during the experiment, they recorded the activity of randomly selected neurons in the lateral prefrontal cortexes of the rhesus monkeys. They chose that region of the brain because functional imaging (fMRI) studies have shown that rule-based arithmetic activates that part of the brain in humans, too. 

The data revealed that in the observed part of the brain, the majority of the neurons selected were involved in applying the mathematical rules (with equal portions activating for the greater-than and less-than applications) rather than obtaining and retaining the sensory details. The researchers concluded that this begs for a processing model that has "specific 'rule-coding' units that control the flow of information between segregated input, memory and output."

What does this mean for us, the only calculator-punching primates? The researchers noted that it likely points to a much older evolutionary root to abstract math: "These neuronal circuits…could readily be adopted in the course of primate evolution for syntactic processing of numbers in formalized mathematical systems."

The research was published online January 18 in Proceedings of the National Academy of Sciences.

Image of rhesus monkey courtesy of iStockphoto/GlobalP