Babbling Sounds of Monkeys Share Rhythms with Human Speech
A gelada baboon in Simien Mountains National Park, Ethiopia. Photo by A. Davey via Flickr.
Scientists studying the evolution of speech have long puzzled over why there are no good models in primates. While primates share many traits with humans — they’ve been known to play, grieve, fight, even laugh — speech isn’t one of them.
With one possible exception. A group of wild monkeys from the Ethiopian highlands called geladas, which are closely related to baboons, make gutteral babbling noises that sound eerily human-like. And they do it while smacking their lips together. The combination of lip smacking and vocal sounds is called a “wobble.” A study in this week’s issue of the journal Current Biology analyzed the rhythm of the wobble and found that it closely matched that of human speech.
Geladas are found only in Ethiopia, in a national wilderness where steep, jagged cliffs surround high-altitude grasslands. (They sleep on the cliffs and feed in the grasslands.) Thore Bergman, the study’s lead author, spends a month or two every year in the Simien Mountains National Park there studying the animals.
Lip smacking is a common animal behavior. Bergman describes it as “a rapid opening and closing of the lips.” There are different styles, he says. Some animals move their tongues in and out too.
But male geladas are the only documented case of primates that make noise while they lip smack. That’s the “wobble.” Bergman was interested in whether the wobble contained a speech-like rhythm.
The vocal sounds made by lip smacking in wild Gelada monkeys have similarities to human speech, which you can watch in this video from the University of Michigan.
Talking is actually very rhythmic. All human speech contain patterns — loud sounds and quiet sounds that alternate as much as three to eight times a second, or at 3 to 8 hertz (Hz).
“That rhythm is universal across all languages,” Bergman said. “If you listen to language, it’s not a smooth steady hum — there are loud parts, quiet parts and pauses.”
He recorded the gelada sounds, digitized them and ran them through a software program designed to analyze speech. The model calculates the intervals between each loud noise. And those intervals, he found, fall solidly within the range of human speech. As noted, the range for human speech is 3 to 8 Hz — that means three to eight loud noises per second. The geladas clocked in at 7 to 9 Hz — a slightly more rapid, but similar rhythm.
“What’s incredible about Thore’s study is that he showed, in a specialized species of baboons, that under certain circumstances they combine rhythmic facial expression and vocal chord movement,” said Asif Ghazanfar, an assistant professor at Princeton University who also studies the biology of primate communication.
What it doesn’t show necessarily, he cautioned, is a direct evolutionary line from primate speech to human speech.
“What it’s showing is this possibility for rhythmic expression and vocal output,” Ghazanfar said. “This possibility exists and geladas have exploited it. But it doesn’t show a direct relationship between what we can do and what geladas can do.”
Still unknown is precisely what purpose the gelada vocalizations have, Bergman said. Are they using the sounds to communicate? Do they carry extra meaning?
And the really interesting question, said Ghazanfar, is why is it only seen in geladas? “What is the change in circuitry that allows them to coordinate facial expressions and vocal expressions? What is it about geladas that allows them to do that?”
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