It is a question that many have asked (well, in film at least): What would happen if a chimpanzee could say “No”? Previous research suggested their inability to talk was due to limitations in their vocal tract anatomy. Well, new research suggests that it could be more to do with their brains than their mouths, as experiments with macaque monkeys have found that they have the vocal capability, but not the cognitive know-how to speak. 

The international team of scientists looked into the vocal capabilities of macaque monkeys by using X-ray videos to record the movements and changes in their lips, tongue, and larynx when making noises. This is a marked change from previous experiments into the vocal abilities of primates, which have tended to use dead specimens to examine their vocal anatomy.

The data gathered from the videos was then fed into a computer model that could simulate the macaques' vocal ability and range based on their physical attributes. They found that the primates possessed the anatomy to produce sounds used in human speech, such as vowels, and would even be able to produce full sentences. The resulting noises are “clearly intelligible” even if they do sound fairly creepy to our human ears.

Listen to what a macaque would sound like asking “will you marry me?” below. Warning: you may not be able to sleep afterward.

This, they claim, means that what is lacking is not the vocal apparatus, but the cognitive ability instead.

Because the researchers carried out their work on the vocal anatomy of macaques, they think it could have implications for many other species of monkeys. Macaques are classed in a group known as “old world monkeys”, which includes the likes of baboons and langurs, but which eventually gave rise to the apes, including chimpanzees and, obviously, us. This suggests that the vocal anatomy may be shared by our closest living relatives.

^{Macaque MRIs used to convert the monkey's skull diameter measurements to create a vocal tract model. Credit: Fitch et al. Sci. Adv. 2016;2:e1600723}

“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 co-author Asif Ghazanfar of Princeton University in a statement. “Even if this finding only applies to macaque monkeys, it would still debunk the idea that it's the anatomy that limits speech in nonhumans. Now, the interesting question is, what is it in the human brain that makes it special?”

Their findings mean that some focus may now shift to examine the neural networks behind human speech, and what it is that is so different between us and our closest evolutionary cousins that allow us to produce such a massive range of noise and sounds, even though we have such similar anatomy.

“It's going to force us to think more carefully about how speech evolved,” said Ghazanfar. “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.”