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If parrots can talk, why can’t monkeys?

It’s commonly thought that most primates aren’t able to speak because of their vocal anatomy, but we now know it’s because of neural connections

Koko Gorilla
Koko the gorilla with her trainer, Penny Patterson, May 1977.Barney Peterson (Getty)

Speech is the articulation of complex sounds for communication. It’s an ability that humans share with other animals such as parrots, dolphins, elephants and even sea lions, but not with their primate cousins. Several attempts at teaching apes to speak have ended disastrously. Viki, a chimpanzee raised from infancy by humans, only managed to learn a few words, like mom and dad. The famous Koko, a gorilla that learned more than 1,000 thousand hand signs to communicate, never uttered a single word. Curiously, some of the best explanations for this inability have come from deer, felines and koalas, but it hasn’t exactly been a straightforward journey.

Charles Darwin theorized that monkeys had the vocal anatomy needed for speech, but lacked the necessary neural mechanisms. This was the most popular hypothesis until 1969, when Philip Lieberman published an article in Science that disrupted everything. Lieberman studied the vocal anatomy of a monkey corpse and concluded that other primates could not produce as many vowels as humans because of the position of their larynxes. The larynx of a newborn human is positioned at the top of the neck, but it descends permanently by the time a child is two years old. This change doesn’t happen in other primates.

Liberman’s discovery cemented the idea that a descended larynx is a prerequisite for speech. Paleoanthropologists began to search for hominid fossils that would enable them to determine larynx positions and thereby date the origins of language. Lieberman then reconstructed the soft tissues of a Neanderthal’s vocal tract and deduced that its larynx was elevated, more like a chimpanzee larynx. He never suggested that Neanderthals completely lacked any capacity for language, but he did raise serious questions about the ability of other hominids to speak.

Towards the end of the 20th century, evolutionary biologist W. Tecumseh Fitch realized a crucial fact – all the existing evidence was based on the anatomy of dead primates. Surprised that this had not been done before, Fitch used x-ray imaging to study the vocal tracts of live animals while they were voicing sounds. He was amazed to observe that their larynxes at rest remained high and then descended during vocalization to a position very similar to the human larynx.

Fitch’s study demonstrated that the mammalian vocal tract is dynamic, thereby challenging Lieberman’s conclusions about the speech abilities of other primates. In The Evolution of Language, Fitch writes, “Even the early hominids could produce many clear and understandable phonemes simply by doing what all mammals do: reconfiguring the vocal anatomy while vocalizing.”

However, humans still seemed to be the only animals with a permanently descended larynx. Soon after publishing his study, Fitch received an email from David Reby, a French biologist specializing in deer. Reby asked for Fitch’s thoughts on some strange movements he observed in the throats of deer during vocalization. Fitch realized that they were similar to those found in other mammals, but the resting position of a deer’s larynx was in the middle of the throat, much like a human larynx. In 2001, Fitch and Reby published “The descended larynx is not uniquely human.”

Fitch then embarked on a search for other animal larynxes positioned like ours, and found them in all the big cats of the genus Panthera (lions, tigers, jaguars and leopards). He even went to Australia to dissect marsupials, and discovered that koalas are also in the club. One can imagine how surprised Fitch was when his bibliographic review discovered an article from 1921 that described the koala’s phonatory system (voice box), but failed to mention its resemblance to the human larynx.

The discovery that humans are not the only animals with a permanently descended larynx called into question the importance of its position for speech. While lion roars are certainly impressive, they are simple vocalizations and nowhere near as complex as articulated sounds. All these new discoveries led scientists to find a new function for the descended larynx, and so the vocal size exaggeration hypothesis emerged.

Large animals tend to produce lower-pitched calls than smaller ones because they have larger larynxes with longer vocal folds. This is the reason for the correlation between the size of an animal and the pitch of its sounds. Animals that are nocturnal or that live in dense forests use deeper voices to appear larger and scare off potential threats.

When a larynx descends, it elongates the vocal tract and results in deeper voices. This is obvious in our own species. Pubescent males experience a series of body changes that make them appear larger. Their shoulders broaden, their beards grow, and their larynxes descend a second time. Fitch writes, “The larynx originally descended in hominids in order to exaggerate size, which served as a preadaptation for speech abilities that came later.” This has important implications because it challenges the earlier notion that there were hominids with descended larynxes that could not speak.

All these studies indicate that apes have all the anatomical characteristics necessary for speech. The reason they don’t is purely neural. Humans have much better control of the larynx, not because of its position, but because of the neural connections that connect it to the brain. Parrots don’t even have a larynx, but they have wonderful control of their speech organ, which enables them to articulate intelligible words and phrases.

All this is a clear example of the importance of comparative research, of how we can study other species to better understand them and ourselves as well. And once again, Darwin was right.

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