Now scientists have discovered the secret behind the Australian animal’s unusually deep voice: a special sound-producing organ not found in any other land mammal.
Koala bellows have a pitch about 20 times lower than they should be given the animals’ size—it’s actually more typical of an elephant-size animal. Male koala bellows, for instance, are so fearsome that sound designers used recordings of them to create the T. rex roars in the movie Jurassic Park.
For male koalas, this adaptation is essential to their love lives: A deep voice is attractive to female koalas choosing a mate.
A male koala, Iraga, in his enclosure at the zoo in Dresden, Germany, on November 22. [Photograph by Sebastian Kahnert, AP]
Many animals, including koalas and people, produce sounds by passing air from the lungs over folds of skin located in the larynx, or voice box. When these folds, the vocal cords, vibrate, they make a sound. The size of the vocal cords determines the pitch of the sounds they create, so smaller animals will typically produce higher-pitched calls than larger ones.
The Lowdown on Low Voices
Male koala bellows are produced as a continuous series of sounds on inhalation and exhalation, like a donkey’s braying. Study co-author Benjamin Charlton, of the University of Sussex in the U.K., explained in a statement that during inhalation, koala bellows sound like snoring, and during exhalation, they sound more like belching.
The scientists harvested body parts from wild koalas that had been humanely euthanized for other reasons. When scientists looked at the voice boxes of male koalas, they found their vocal cords weren’t large enough to create the animals’ extremely low-pitched mating bellows. But further examination revealed a second, much larger pair of vocal folds located outside of the larynx, where the oral and nasal cavities connect.
Charlton and his colleagues used a combination of physical, video, and acoustic analyses to demonstrate that the newly discovered vocal folds outside of the larynx are capable of producing extremely low-pitched sound as the koala inhales air through its nostrils.
It’s the first evidence in a land-dwelling mammal of an organ other than the larynx that is devoted to producing sound.
The only other example of a specialized sound-producing organ in mammals that is independent of the larynx is the phonic lips that toothed whales use to generate echolocation—or the natural sonar that helps them find prey, said Charlton, whose study is published in the journal Current Biology.
But Charlton and colleagues plan to keep looking to find other animals with such low voices.