Have you ever wondered why humans can speak but chimpanzees can’t? After all, they are our closest animal relatives, but even ones raised by humans don’t say a word. But your pet parrot will spend all day talking nonsense, and birds aren’t even the same class of animal as humans! Read on to learn more about how humans are unique in the way we speak.
Humans are able to create speech sounds through our unique vocal tract, which consists of our voice box, vocal cords, oral cavity, tongue, teeth, lips, and nasal passage. That’s a lot of parts! Each part of the vocal tract is manipulated to produce the wide variety of speech sounds that humans can make. For example, think of the words “man” and “nan”. The only difference between these two words is the consonant sound at the beginning, either /m/ or /n/. And the only difference between these two consonants is the position of the lips (closed for /m/ and open for /n/) and tongue (lowered for /m/ and pressed against the roof of the mouth for /n/). It is remarkable that we are able to produce two distinct sounds with such minor changes to the vocal tract!
But don’t chimpanzees and humans have the same vocal tract elements? They do, and you might think that would make chimpanzees capable of speech, but the vocal tracts of apes have enough differences that they simply can’t produce speech sounds the way humans do. In the 1950s, a couple of scientists raised a chimpanzee named Viki like a human child and started training her to speak at five weeks old. By the age of three, Viki could only speak the words “mama”, “papa”, and “cup”. This video shows that these words were so degraded that they were difficult to understand, and there are three main reasons why. First, apes have vocal membranes attached to their vocal cords, making speech sounds more difficult to control. Second, apes have laryngeal sacs in their throats that can be filled with air to produce the loud whoops associated with non-human primates, which limit the range of vowel sounds that apes are capable of producing. Finally, the root of an ape’s tongue is closer to the oral cavity than a human’s tongue, meaning an ape’s tongue has a more limited range of motion.
So how is it that a pet parrot, who has a beak instead of lips, can speak, but a chimpanzee can’t? Parrots create sounds by making minute changes to the way air passes through a body part known as the syrinx, which humans do not have. The syrinx allows parrots, songbirds, and corvids to mimic a wide variety of sounds that they encounter in their environments. But while parrots can mimic human speech, there is some debate as to whether they actually understand what they are saying.
The next time you say something, pause to think about all the ways your vocal tract moves, and know that human speech is more unique than you might have ever guessed!