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According to researchers, the male koala bears' mating calls have a very low pitch for such a small sized animal. Based on the koala bears' small size, researchers explained that the pitch heard in mating calls should logically be higher. The researchers stated that the koala bears' pitch is more in line with sounds that would come out of a enormous animal, such as an elephant. However, the Australian marsupial's pitch is roughly 20 times lower than it should be and now, researchers believe that they have identified the cause for this low-pitched call.

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

After examining the biology of the koala bears, the researchers found that the extra pair of vocal folds help enhance the sound and make the mating call louder. The team explained that when koala bears inhale and exhales, a series of sounds are created. When the bears inhale, the call sounds very similar to snoring. When the koalas exhale, the vibrations create a sound that is similar to belching.

The team described the vocal folds as two long, fleshly lips that sit right above the larynx. They are located at the junction between the oral and nasal cavities, and although the folds are similar to folds found in other animals, the main difference is the location. The researchers believe that the location of the folds is responsible for the low-pitch mating call.

"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 stated.

The study, "Koalas use a novel vocal organ to produce unusually low-pitched mating calls," was published in Current Biology.