Simple calculations suggest koalas should have high-pitched voices. That's because the pitch generated by an object is linked to its size, and usually animals' vocal chords tend to be large or small according to the mass of their bodies.

But koalas have relatively low voices, especially males, which produce bellowing sounds during the mating season that alternately sound like a donkey braying and a frog vomiting. The average pitch of this bellow is 20 times lower than an animal that weighs 8 kilograms (18 lbs.) and more typical of an animal the size of an elephant, according to a study published today (Dec. 2) in the journal Current Biology.

So what's the koalas' secret? The animals actually have an extra "organ" outside the larynx, which contains the vocal chords that mammals and other animals use. In the koala's case, the vocal chords consist of long fleshy folds of tissue in the soft pallet between the upper throat, or pharynx, and the nasal cavities. When the koalas breathe in, they can push air through these "velar vocal folds," as the authors call them, to make low-pitched sounds, according to the study. This is quite unusual, they wrote.

"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, a study co-author and researcher at the University of Sussex, said in a statement. "We also demonstrated that koalas use these additional vocal folds to produce their extremely low-pitched mating calls." [Video: Weird Organ Gives Koalas Deeper Voices]

In their study, Charlton and colleagues examined the larynxes and throats of 10 male koalas and found these vocal folds in the animals. Female koalas have also been known to bellow and should also be studied, the researchers wrote.

According to the scientists, this is the only known example of an organ outside the larynx producing vocalizations, beside special structures called "phonic lips" that whales use to make echolocation clicks. Those clicks are sound waves that whales bounce off other objects to orient themselves and "see" their surroundings, according to the study.

Get the Monitor Stories you care about delivered to your inbox. By signing up, you agree to our Privacy Policy

Email Douglas Main or follow him on Twitter or Google+. Follow us @livescience, Facebook or Google+. Article originally on LiveScience.

Copyright 2013 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.