Sounds made by a little-known monkey living in Ethiopia’s mountain grasslands may hint at the origins of human speech. Unlike most other primates, which communicate in strings of short, relatively flat-toned syllables, geladas possess uncannily human-like vocal tempos and undulations.

“When we first started working with geladas in 2006, we noticed sounds like people were talking around you,” said evolutionary biologist Thore Bergman of the University of Michigan. “Most primates only make a few sounds, but geladas produce a complex stream with a rhythm similar to language.”

Key to the gelada vocalizations, described today by Bergman in Current Biology, is the ability to smack their lips. Underlying that seemingly simple action is a rich synchrony of lips, tongue and the hyoid bone beneath them.

'The ability to produce complex sounds might have come first.'

Though the monkeys moved their lips without without actually vocalizing, the researchers speculated that lip-smacking could have been a precursor to human speech, setting a tempo for what would become the sonic foundations of language.

Bergman builds on that notion. He shows that geladas sometimes use lip-smacking to shape their calls, giving them a human language-like quality. Geladas were already known to possess an extremely rich vocal repertoire; lip-smacking adds to that richness.

An open question, said Bergman, is whether the lip-smacking vocalizations have some special significance. “We don’t know much about the function,” he said. “It will be interesting to see if the fact they produce these complex sounds allows them to communicate things other monkeys might not be able to.”

The possibility that early ancestors of humans may have shared this ability raises a linguistic chicken-egg-problem, Bergman added.

“The ability to produce complex sounds might have come first. Then, when we could do that, we could attach meanings and communicate in more sophisticated ways,” he said. “Or it could be that, as we needed to communicate more, we developed an ability to produce a greater variety of sounds.”

Whatever the order, vocal complexity is likely intertwined with social complexity. Baboons are closely related to geladas, but use fewer vocalizations and don’t smack their lips. Perhaps not coincidentally, baboons live in relatively small, short-lived groups.

Gelada groups stay together for many years, with females having especially long-lived relationships. Often groups come together in bands of several hundred individuals. “It’s a very complex social system. They have some of the largest groups of any primate,” Bergman said. “These very large group structures may be linked to vocal complexity. There’s some evidence across primate that bigger groups make more sounds.”

Citation: “Speech-like vocalized lip-smacking in geladas.” By Thore Bergman. Current Biology, Vol. 23 No. 7, 8 April 2013.

Video: Thore Bergman/Current Biology