There is little doubt that non-human primates like Koko the gorilla are very intelligent. Koko, for example, uses sign language to communicate with people, telling them that she loves her pet cats, Miss Black and Miss Grey. Koko, however, is noticeably the strong and silent type, at least when it comes to speaking our language. She doesn't say a word.

They may not always show it, but new research, published in the journal Science Advances, suggests that non-human primates, even monkeys down on the food chain, have the vocal anatomy to produce clearly intelligible human speech. The discovery negates a long-standing theory that monkeys, gorillas, chimps and the like do not talk as we do because they are incapable of creating the sounds required for the skill.

"I hope that this new data dispels forever the widespread myth that monkeys and apes cannot speak because of anatomical limitations of their vocal tract," lead author Tecumseh Fitch of the University of Vienna's Department of Cognitive Biology told Seeker.

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Fitch, senior author Asif Ghazanfar, Bart de Boer and Neil Mathur investigated the range of movements that primate vocal anatomy could produce. Using X-ray videos, they captured and then traced the movements of a macaque's tongue, lips, larynx and more as the monkey vocalized, ate and made facial expressions. The researchers then used these X-rays to build a computer model of a monkey vocal tract, allowing them to answer the question: What would monkey speech sound like, if a human brain were in control?

You can hear the results, first with the monkey model saying, "Will you marry me?" and then, "Joyeux Noel (Merry Christmas in French)."

(Recordings courtesy of Asif Ghazanfar, Princeton Neuroscience Institute; Image 1 Credit: Paul Asman and Jill Lenoble, Flickr; Image 2, showing Tecumseh Fitch in his lab: University of Vienna)

The experiment worked out great, and yet we're still left with the fact that monkeys and apes do not talk as we do. The explanation turns out to be more complex, and controversial, than you might think.

First, Fitch and his team believe that most mammals possess flexible, speech-ready vocal tracts. He said, "It seems clear that this type of flexibility evolved early on, for reasons other than vocalization, probably initially for food processing—manipulating and swallowing food."

He suspects that humans evolved at least two important changes to our brains that give us a communication edge.

Fitch explained, "We have direct connections between our motor cortical neurons and the neurons that actually control the vocal tract musculature, particularly those in charge of the larynx; and we have much more substantial connections, within our cortex, between the auditory cortex—responsible for hearing sounds—and the motor cortex, responsible for making sounds."

Fitch says there are many theories attempting to explain how humans evolved both the brain and the vocal tract for speech. One of his favorites was formulated by famed British naturalist Charles Darwin, who theorized that our ancestors initially evolved to become "singing apes," or kind of a cross between gibbons and songbirds and being able to learn new songs. This musical ability, Darwin suspected, emerged first, and then later was put to use in speech.

Photo: X-ray of a macaque vocal tract. Credit: Asif Ghazanfar, Princeton Neuroscience Institute

Fitch thinks it is unlikely we could teach non-human primates to speak, save for the remote chance that genetic engineering in future might make this possible.

Laurie Santos, a psychology professor at Yale University, told Seeker the paper "opens whole new doors for finding the key to the uniqueness of humans' unparalleled language ability."

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On the other hand, Constance Scharff, a professor in the Department of Animal Behavior at Free University Berlin, indicates we may undervalue the communicative skills of animals, many of which—like parrots—are clearly very vocal.

Scharff told Seeker that she is glad the new study "puts another nail in the coffin of the idea that the absence of speech in macaques cannot be explained by an unsuitable vocal tract." Scharff also agrees that monkeys "do not seem to have the same regions and neural connections in their brains that humans use."

But, she quickly added, "there are other ways imaginable to achieve speech." She pointed out that parrots, seals and elephants either use quite different brain regions to vocalize, or the underlying systems remain largely unknown.

"As many experiments have recently shown," she added, "animals might not do things under natural conditions, but are capable of doing them when trained and prompted, such as sea lions and parrots moving to a beat."

"I am aware that so far the evidence in macaques points against 'speech-ready' neural hardware, but I think we do not know enough about all the ways brains can produce sounds in a speech-like way to say, 'Macaques don't because their brains can't.'"

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