Macaque monkeys possess the vocal anatomy to produce “clearly intelligible” human speech but lack the brain circuitry to do so, a new study has found.
The findings, which could apply to African and Asian primates known as Old World monkeys suggest that human speech stems mainly from the unique evolution and construction of our brains, and is not linked to vocalisation-related anatomical differences between humans and primates.
“Now nobody can say that it’s something about the vocal anatomy that keeps monkeys from being able to speak – it has to be something in the brain,” said Asif Ghazanfar, a professor at the Princeton University in the US.
“Even if this finding only applies to macaque monkeys, it would still debunk the idea that it’s the anatomy that limits speech in non-humans,” Ghazanfar said.
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Researchers studied the range of movements that primate vocal anatomy could produce.
Previous examinations of primate vocal anatomy conducted on cadavers had concluded that monkeys and apes have a very limited range of sounds they could produce relative to humans.
Researchers used X-ray videos to capture and then trace the movements of the different parts of a macaque’s vocal anatomy such as the tongue, lips and larynx during a number of orofacial behaviours.
The data was converted into a computer model that could predict and simulate a macaque’s vocal range based on the physical attributes recorded by X-ray.
Human speech stems from a source sound produced by the larynx that is changed by the positions of the vocal anatomy such as the lips and tongue, Ghazanfar said.
The researchers plugged the source sound of a macaque’s grunt call into their computer model of the primate’s vocal anatomy.
They found that a macaque could produce comprehensible vowel sounds – and even full sentences – with its vocal tract if it had the neural ability to speak.
While a macaque would be understandable to the human ear, it would not sound precisely like a human, researchers said.
Since the study shows that macaques express nearly the same range of physical movements as humans during vocalisation, primates could be used as models for understanding early human speech development and human speech evolution, Ghazanfar said.
“Their value as a model system for studying the parts of the brain that directly control the biomechanics of orofacial movements during speech and other vocal behaviours will increase,” Ghazanfar said.
“Moreover, it’s going to force us to think more carefully about how speech evolved, how our brain is uniquely human and how we can use these model animals in the future to understand what goes wrong when we are unable to speak,” he said.