The complexity of human language may set us apart from the animals, but a new study of geladas suggests how it may have developed from the behavior of our primate ancestors.

Geladas, sometimes called gelada baboons, are a highly social species of monkey from the high mountains of Ethiopia that make unique lip-smacking vocalizations to each other called "wobbles." Wobbles are produced mainly by adult males seeking the attention of females, and are produced by inhaling and exhaling while lip-smacking, punctuated by grunts.

Other primates display non-vocal lip-smacks during social encounters, but geladas are the first nonhuman primates observed to vocalize while lip-smacking. Most other monkeys and apes make vocalizations without moving their lips, jaw, or tongue, and those sounds tend to be monosyllabic and without much variation in pitch and volume.

Geladas' wobbles, on the other hand, have an undulating rhythm that sounds surprisingly like human speech. The new findings, published today in the journal Current Biology, suggest that lip-smacking vocalizations could have been an evolutionary step towards human speech.

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

Bergman began his fieldwork studying geladas in 2006, and was unnerved by the eerily humanlike sounds the monkeys made to each other. "I would find myself frequently looking over my shoulder to see who was talking to me, but it was just the geladas."

Geladas,
Geladas, native to the remote mountains of Ethiopia, are the only nonhuman primate known to communicate with a speech-like, varying rhythm. [Credit: Current Biology, Bergman]

He puzzled over the nature of the gelada wobbles until last year, when he read a study in Current Biology about macaque monkeys' lip-smacking behavior which suggested that vocalization while lip-smacking may have been an evolutionary precursor to human speech. Macaques make sounds called "girneys" along with complex facial movements like lip-smacking, but the sounds and movements do not appear to be synchronized. Still, they occur closely enough to suggest the possibility of them working together in other species.

That study piqued Bergman's interest in comparing the geladas' vocalizations to the rhythms of human speech. He recorded and analyzed the rhythm of the wobbles, and discovered that at 6-9 hertz (Hz), they do indeed have a similar frequency to human speech. Like that of human speech, the rhythm of geladas' lip-smacking wobbles corresponds to the periodic movements of the mouth and allows complexity.

Bergman suggests that lip-smacking may serve the same basic purpose as human language: in addition to allowing the exchange of information, it enhances social interactions.

It is unclear what kind of meaning geladas' wobbles might carry, but it's clear from observations of the monkeys that they facilitate social behavior. Geladas live in family units that often combine to form larger foraging bands of hundreds of animals, and spend much of their time sitting, munching on grass, and socializing.

While Bergman's findings indicate a possible evolutionary pathway for language development, he acknowledges that much more research must be done to flesh out how human speech developed from other forms of animal communication. Many species of animals make sounds structured like human speech, and a gene called FOXP2 has been identified as important to both human language and animal communication.

As Bergman concluded his paper: "There is much to be explored about the evolution of human speech, perhaps most importantly how the production of complex sounds came to represent complex meanings."