A male koala’s come-hither routine in mating season involves a call that sounds almost like a bullfrog crossed with a creaky, heavy old door opening:

However, when scientists listened closely to the belch-like croons of the lovesick koala, something didn’t quite add up. The frequency of the sounds an animal makes depends on its vocal folds, meaning that vocalizations are usually in proportion to an animal's size. But a koalas’ voices are about 20 times deeper than expected for an animal of their stature -- in fact, you’d expect these kind of low tones from something more the size of an elephant.

How does the koala emit such a bassy belch from such a tiny frame? It turns out that he has a special, previously unknown organ that helps bring his tenor down to a bass. An international team of researchers described the find on Monday in a paper published 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," lead author Benjamin Charlton, a 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."

Charlton and colleagues have dubbed the newly found flaps the “velar vocal folds.” They’re a pair of long, fleshy lips located right at the junction of the koala’s oral and nasal cavities, just above the larynx (voice box). The scientists confirmed that male koalas use these folds to produce the low-frequency bellowing sounds with video and sound recordings.

"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," the authors wrote.

Next, Charlton and colleagues will be investigating the anatomy of female koalas, to see if they too possess velar vocal folds. Female koalas are known to bellow sometimes, but not as often as their brothers. The researchers are also curious as to whether other mammals might have special adaptations for making much lower-frequency sounds than expected.

Previously, some of the scientists had attributed the koala’s low-pitched voice to the fact that the animal has a “descended larynx” -- a voice box that sits low in the throat. Humans and red deer also have this feature. But now it appears there’s another adaptation that helps to keep the koala’s bellow properly baritone.

SOURCE: Charlton et al. “Koalas use a novel vocal organ to produce unusually low-pitched mating calls.” Current Biology published online 2 December 2013.