Macaque monkeys have the anatomy for human speech, so why can’t they speak?

December 14, 2016

Researchers used X-ray videos (right) to capture and 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 behaviors. (credit: Illustration by Tecumseh Fitch, University of Austria, and image courtesy of Asif Ghazanfar, Princeton Neuroscience Institute)

While they have a speech-ready vocal tract, primates can’t speak because they lack a speech-ready brain, contrary to widespread opinion that they are limited by anatomy, researchers at Princeton University and associates have reported Dec. 9 in the open-access journal Science Advances.

The researchers reached this conclusion by first recording X-ray videos showing the movements of the different parts of a macaque’s vocal anatomy — such as the tongue, lips and larynx. They then converted that data into a computer model that could predict and simulate a macaque’s vocal range.

Audio file of researchers’ macaque vocal model uttering the same phrase “Will you marry me?,” synthesized with the same noisy source (credit: W. Tecumseh Fitch et al./Science Advances)

Audio file of an adult human female saying “Will you marry me?,” resynthesized with a noisy source (credit: W. Tecumseh Fitch et al./Science Advances)

The model was used to create computer-generated audio clips (above — may not work with older browsers) that simulate what a macaque (top) might sound like if it could speak, compared to a human female (bottom). The clearly audible phrase: “Will you marry me?”

The researchers found that a macaque would be able to produce comprehensible vowel sounds — and even full sentences — with its vocal tract if it had the neural ability to speak, according to co-corresponding author Asif Ghazanfar, a Princeton University professor of psychology and the Princeton Neuroscience Institute.

“The key conclusion from our study is that the basic primate vocal production apparatus is easily capable of producing five clearly distinguishable vowels … the worldwide norm for human languages, and many of the world’s languages make do with only three vowels,” the researchers note in their paper. “The common stop consonants (/p/, /b/, /k/, and /g/) along with a variety of other consonantal sounds (for example, /h/, /m/, and /w/) would be easily attainable by a macaque monkey.”

The work was supported by the National Institutes of Health and European Research Council advanced and starting grants.

Abstract of Monkey vocal tracts are speech-ready

For four decades, the inability of nonhuman primates to produce human speech sounds has been claimed to stem from limitations in their vocal tract anatomy, a conclusion based on plaster casts made from the vocal tract of a monkey cadaver. We used x-ray videos to quantify vocal tract dynamics in living macaques during vocalization, facial displays, and feeding. We demonstrate that the macaque vocal tract could easily produce an adequate range of speech sounds to support spoken language, showing that previous techniques based on postmortem samples drastically underestimated primate vocal capabilities. Our findings imply that the evolution of human speech capabilities required neural changes rather than modifications of vocal anatomy. Macaques have a speech-ready vocal tract but lack a speech-ready brain to control it.