Gelada monkeys in conversation. (Image courtesy: Clay Wilton/ University of Michigan)

The lip-smacking sounds of rare African monkeys - geladas - have similarities to human speech, finds a new study. 

Many non-human primates are known to show lip-smacking behaviour, which involves rapid opening and closing of mouth parts in a speech-like fashion. But their calls are just one or two syllables and they lack rapid fluctuations in their pitch and volume.

Researchers from the University of Michigan in Ann Arbor have found that the geladas, a baboon-like primate that are found in the remote mountains of Ethiopia, are the only apes to communicate with a speech-like, undulating rhythm. The vocal lip-smacks that the geladas use in friendly interactions produce a sound which is called a "wobble." The "wobbles" have a rhythm that structurally resembles human speech.

Researchers suggest that the lip-smacking behaviour could have been a significant step in the evolution of human speech. "Our finding provides support for the lip-smacking origins of speech because it shows that this evolutionary pathway is at least plausible," Thore Bergman, of the University of Michigan in Ann Arbor, said in a statement. "It demonstrates that nonhuman primates can vocalize while lip-smacking to produce speech-like sounds."

For their study, Bergman and his colleagues recorded the unusual sounds of the geladas in the Simien Mountains National Park in Ethiopia. They analyzed the recordings of geladas' vocal sounds and tracked their duration and frequency. Their analysis showed that there is a close match between the intervals in gelada sounds and speech, which were not seen in any primate vocalization.

"When geladas 'speak,' that there aren't discernible English words," Bergman said. "However, if you are with geladas, you sometimes get the impression that people are talking around you-something I hadn't noticed in years of working with other monkeys," he said.

Bergman and his team are further planning to study if the vocal lip-smack sounds have a special function for the geladas.

The details of the study, "Speech-like vocalized lip-smacking in geladas," are published in the journal Current Biology.

Below is a video showing gelada lip smacks and wobbles. The You Tube video was posted by University of Michigan