Have a listen to the koala above. These noises – which sound more like a revving motorbike than the call of a furry marsupial – are produced as the koala breathes in and out. As it inhales, it makes a bellowing sound a bit like snoring. Then, as it exhales, the sound is more similar to a loud belch.
These sounds have an extremely low pitch – more typical of an animal the size of an elephant. Usually, the pitch of an animal’s call is related to the size of its larynx – the organ in an animal’s neck that’s often called the voice box. Stretched across the larynx are the vocal cords, which vibrate to produce sounds. The longer these vocal cords, the lower the sound that can be produced. Think of it like notes on a guitar, which increase in pitch as you press on the fretboard and shorten the strings.
So how can koalas produce such low sounds? Researchers led by Dr Benjamin Charlton of the University of Sussex found that the marsupials have a second set of vocal cords that are over three times longer than those in the larynx. These are located outside the voice box, where the oral and nasal cavities connect.
“We demonstrated that koalas use these additional vocal folds to produce their extremely low-pitched mating calls,” says Benjamin Charlton at the University of Sussex. “To our knowledge, the only other example of a specialized sound-producing organ in mammals that is independent of the larynx are the phonic lips that toothed whales use to generate echolocation clicks.”
It’s not known exactly why koalas have evolved their ultra-low calls, but it could be to transmit information more clearly, or to help males advertise their quality to the opposite sex. After all, nothing says “I love you” quite like a big, brassy belch.