Koala bears have a mating call about 20 times lower than the pitch it should be given its small size, but the discovery of an extremely unique organ seems to hold the answer.

According to a press release, the organ, which researchers identified as the larynx, was located outside the marsupial's voice box and is unlike any other land-dwelling mammal. The researchers' study was published Monday in the journal Current Biology.

"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," Benjamin Charlton, of the University of Sussex, said in the release. "We also demonstrated that koalas use these additional vocal folds to produce their extremely low-pitched mating calls."

Charlton said the sound is produced by a continuous series of inhalation and exhalation, resulting in a bellowing mating call more similar to a belch or a donkey's braying. The study's lead author also noted the sound could be mistaken for snoring and is "actually quite loud."

In relation to the koala bear's small size, the mating call's low-pitch is highly irregular. The tone the koala is able to achieve is the kind of sound that should come out of something the size of an elephant. Size affects vocal pitch because the laryngeal dimensions of the vocal folds tend to conform to the lowest frequency possible.

Charlton said the laryngeal vocal folds of a koala are not unlike other animals', but their location is highly irregular. He said the folds are two long fleshy lips that lie just outside the larynx where the oral and nasal cavities meet.

According to the researchers' findings, this is the first living example of a mammal using another organ to help produce sound. As a result, Charlton and his team will examine other mammals to see if such an organ exists.

"To our knowledge," Charlton said, "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."