Rhesus monkeys are able to match sounds and facial expressions. Their skill may be an evolutionary forerunner to our ability to interpret the signals, researchers say.

Like many other animals, monkeys communicate through facial and vocal expressions. But scientists hadn't tested whether animals other than humans can put the two together. Rhesus monkeys (Macaca mulatta) make a variety of calls both in the wild and in captivity. Their two most common are a friendly "coo" and a noisy threat call.

When making a long coo call, the animals pout their lips and open their mouths slightly. If they're in danger, the monkeys make a short, threatening call by opening their mouths wider, the researchers say.

Friendly 'coo' callCourtesy: A. Ghazanfar

Asif Ghazanfar and Nikos Logothetis of the Max Planck Institute for Biological Cybernetics in Germany studied 11 adult monkeys.

They adopted a "preferential looking" method that psychologists say human infants use to help figure out who is speaking and where the voice is coming from before they learn to speak.

The researchers played silent side-by-side videos of the expressions that were synchronized to the call sounds. They predicted that if the monkeys matched the sounds and faces, they would spend more time looking at the match video.

'Threat' callCourtesy: A. Ghazanfar

They found on average, the monkeys looked straight at the matching face 67 per cent of the time. They also gazed at the match face making a coo call for longer than the matched threat call.

"The presence of multimodal perception in an animal's communication signals may represent an evolutionary precursor of humans' ability to make the multimodal associations necessary for speech perception," the pair wrote in Thursday's issue of the journal Nature.