Koalas have a surprisingly low mating call; it is 20 times lower than it should be in proportion to their size and researchers believe the tone can be attributed to a never-before-seen organ.

The sound-producing organ is located outside of the voice box (larynx), a Cell Press news release reported.

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

The sound of a koala looking for a lover can be compared to a donkey's braying. The call is a "continuous series of sounds on inhalation and exhalation," the news release reported; but the noises sound closer to a person snoring than an agitated donkey.

The noise that comes out of the small marsupial is low enough to be compared to the call of an elephant. The size of the animal, and its laryngeal vocal folds, usually determine how low its voice can go.

Koalas have defied this rule of nature by changing the location of their vocal folds. Charlton described the organ as "two long, fleshy lips in the soft palette, just above the larynx at the junction between the oral and nasal cavities."

They look basically identicle to the vocal folds of other mammals, but their location is significantly different.

"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," Charlton said.

The findings mark the first evidence that an organ other than the larynx can produce sound in a land mammal.

The team plans to take a closer look at the sound-producing organs of other mammals to see if there are other undiscovered systems.