Why can't monkeys talk like us? Their vocal tract might not be the problem.

A limited vocal tract might not be what's holding non-human primates' tongues, according to new research.

A talking monkey seems like a thing of science-fiction, cartoons, and goofy advertisements. But new research suggests monkey speech may be closer to reality than commonly thought.

Macaques actually have vocal anatomy capable of human-like speech, according to a study published Friday in the journal Science Advances. 

"This suggests that what makes people unique among primates is our ability to control the vocal apparatus, not the apparatus itself," Thore Jon Bergman, an evolutionary biopsychologist at the University of Michigan who was not part of the research, writes in an email to The Christian Science Monitor.

Researchers studying non-human primates had previously hypothesized that the shape, size, or structure of the vocal anatomy in the animals' heads might be holding them back from making the sorts of sounds that make up human speech. To test this idea, scientists decided to look at the vocal anatomy itself.

In the 1960s, linguist and cognitive scientist Philip Lieberman conducted a study in which he and his colleagues examined the vocal anatomy of a dead rhesus macaque and built a computer model of the motions it might have been able to make. That model found that the rhesus monkey could only produce a limited range of vowels compared with humans.

But some researchers recently weren't satisfied with the study of a cadaver, so they decided to study a live macaque. 

The team captured X-ray video of a monkey vocalizing, making facial expressions, and eating. Then, they used this data to build computer models and parameters of what motions the macaque's vocal tract may be capable of – similar to Dr. Lieberman's 1969 study.

And, as one of the study authors Asif Ghazanfar, a neuroscientist at Princeton University in New Jersey, says, those models suggest "the range of different sounds that a living macaque can produce actually overlaps quite a bit with the sounds that a human can produce."

This study provides "strong evidence to believe that macaque monkeys have a vocal tract capable of supporting a spoken language," Marcus Perlman, a cognitive scientist at the Max Planck Institute for Psycholinguistics in the Netherlands who was not involved in the research, writes in an email to the Monitor. 

This could help researchers figure out how speech evolved in our own ancestors, Dr. Perlman says. "Presumably this potential extends to great apes and thus would characterize a baseline for all of our hominid ancestors. It appears clear now that the vocal anatomy of our ancestors was not an impediment to the evolution of speech."

So perhaps it was the neural anatomy or some other factor that was key to evolving human speech, researchers suggest.

"This is a solid piece of modeling and experimental work," Adriano Reis e Lameira, an anthropologist and cognitive scientist at England's Durham University who was not involved in the research, writes in an email to the Monitor. But, he says, the results are not new.

"They have gathered the type of data that confirms that monkey vocal tracks are speech-ready," Dr. Lameira says. "But, the text reads as if Lieberman stopped doing research in 1969, which could not be further from reality. Lieberman - who the authors seem to be trying to disprove - has published extensively explaining how our understanding of monkey vocal tract physiology has developed since 69. In fact, Lieberman has several (!) papers clarifying that monkey vocal tracts can produce a wide range of vowels."

Lieberman himself agrees. "They have done an excellent job of showing that monkeys have a wider range than I did in 1969," he tells the Monitor in a phone interview. And although this suggests monkeys have the vocal anatomy to produce somewhat human speech-like sounds, he says, there are some key nuances missing from the monkeys' abilities.

The new research suggests that the monkeys can produce five vowel sounds like humans, but they are missing a crucial one, says Lieberman, who is now a professor emeritus at Brown University in Providence, R.I.. The /i/ vowel, like in "beet," is thought to be particularly special for human speech. And the macaques' vocal anatomy doesn't appear to be capable of making that sound.

"We could talk without these sounds," Lieberman says, "But it wouldn't be an effective means of communication."

It's not just about vowels when considering whether or not non-human primates or human ancestors could speak, Max Planck's Perlman adds. "There is a good reason to think that our ancestors would also have possessed a vocal anatomy capable of articulating various consonants. For example, the famously ‘inarticulate’ chimpanzee Viki was able to produce the words mama, papa, and cup, which exhibits three different consonants (/m/, /p/, and /k/). And with control over vocal fold vibration, /p/ could be made into /b/ and /k/ into /g/."

"Considering all of the current evidence, it appears clear that our great ape ancestors would have had a vocal anatomy capable of articulating enough vowels and consonants to provide a fairly substantial substrate for a spoken language," he says.

Neurobiologists like Princeton's Dr. Ghazanfar and other researchers say this suggests those non-speaking primates lacked the neural pathways for speech. But Lameira says it's not as simple as ruling out the vocal anatomy-based hypothesis.

"There is a growing body of evidence from ALL great ape species that there are little neural limitations - our closest relatives can vocally learn new vowel-like and consonant-like calls, both in the wild and captivity!"