Sussex academics have discovered why koala bears have such low voices.

Dr Benjamin Charlton and Dr David Reby reporting in journal Current Biology, revealed koalas’ low-pitched voice was explained by the animal’s voice box – larynx – being unique.

The pitch of male koalas’ mating calls is about 20 times lower than it should be, given the Australian marsupial’s relatively small size, they revealed.

“We have discovered that koalas possess an extra pair of vocal folds that are located outside the larynx, where the oral and nasal cavities connect,” says Dr Charlton.

“We also demonstrated that koalas use these additional vocal folds to produce their extremely low-pitched mating calls.”

The koala’s bellow calls are produced as a continuous series of sounds on inhalation and exhalation, similar to a donkey’s braying, Dr Charlton said. On inhalation, koala bellows sound a bit like snoring. As the animals exhale, the sound is more reminiscent of belching.

Dr Reby added: “To our knowledge, the only other example of a specialised sound-producing organ in mammals that is independent of the larynx are the phonic lips that toothed whales use to generate echolocation clicks.”