Lead researcher Kerry Jordan, a psychologist at Utah State University, Logan, US, said that this sort of evidence "shows that [animals] have these precursors to math very early on in the evolutionary line and early on in development." Jordan and colleague Elizabeth Brannon, of Duke University in Durham, North Carolina, US conducted the study over two eight-year-old female Rhesus macaques.
These monkeys were taught 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.
To test the skill, the researchers flashed dots of different sizes onto a screen and at the same time animals were made to hear 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.
They found that both the monkeys performed better than 50:50 ratio. One could add the sights and sounds with 72 pct accuracy, the other did it with 66 pct accuracy.
However, both the animals 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, five and six.
Jordan said that the monkey's ability to add numbers seen and heard together makes sense in the wild.
"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," New Scientist quoted Jordan, as saying.
"It would do this by combining information on how many animals it could see with how many it could hear," she added.
The study appears in journal Cognition.