At this rate a monkey might prove the Riemann hypothesis. Rhesus macaques have been shown to possess yet another numerical talent once thought unique to humans – they can simultaneously count audible beeps and dots on a computer screen.
Their ability to comprehend numbers not as just discrete images or sounds, but as abstract representations that can be combined suggests that such maths skills aren't unique to humans, says Kerry Jordan, a psychologist at Utah State University, Logan, US, who led the new study.
This sort of evidence "shows that [animals] have these precursors to math very early on in the evolutionary line and early on in development," she says.
Jordan and colleague Elizabeth Brannon, of Duke University in Durham, North Carolina, US, trained two eight-year-old female macaques to equate beeps to dots on a computer screen. So if a monkey heard seven beeps, it knew to tap a square on the screen displaying seven dots.
Next, the researchers tested the monkeys’ training in adding dots and beeps together.
The animals were presented dots of different sizes flash onto a screen. At the same time they heard a series of short tones.
To determine if the monkeys could combine the two, Jordan and Brannon showed the animals a screen with two numerical choices, represented as dots – one the correct sum, one incorrect.
Both monkeys did better than 50:50 – one added the sights and sounds correctly 72% of the time, the other 66% of the time.
Both monkeys tended to make mistakes when the right and wrong answers were numerically similar. For instance, if the choices were one and eight, the animals rarely got it wrong. But they found it harder to choose between, say, five and six.
People make the same kind of errors when making snap numerical judgements, such counting the number of people in a crowd, says Jordan, which is further evidence that our abstract maths skills aren't unique.
The monkey's ability to add numbers seen and heard together makes sense in the wild, says Jordan.
"If you have an animal trying to make a decision to defend its territory, it's going to want know how many other animals it has to deal with," she says. It would do this by combining information on how many animals it could see with how many it could hear.
Irene Pepperberg, a psychologist at Harvard University in Cambridge, Massachusetts, who trained a parrot named Alex to add small sums, says the paper confirms observations in the wild.
Flycatchers, for instance, seem to communicate their mood to other birds using a numerical combination of song and wing motions. The more wing flicks and songs, the more likely it is to attack another bird, she says.