Deep voices aren't just for crooners like Elvis Presley, Frank Sinatra, and Nat King Cole — male koalas attract mates with low-pitched bellows as well. The secret behind their ability to sing so low, however, lies in the unique location of a pair of vocal folds not found in any other land mammal, a new study reveals.

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

A male koala's mating call is around 20 times lower than normally would be expected from a mammal its size. Special folds on the layrnx and how much they constrain often limit vocal frequencies, and due to the logistics involved, smaller animals tend to have higher-pitched calls than larger ones. A koala's call, however, is far more akin to an elephant's than a similar-sized mammal.

The key to a koala's baritone crooning is in the fact that its folds are located independently of the larynx, between the oral and nasal cavities.

"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 says.

The team of scientists involved in the koala study hope to look at other land-dwellers to see if the koala's folds are truly unique or simply just the tip of the iceberg.

You can read the full published study detailing the findings in the journal Current Biology.