Koalas Use Additional Vocal Folds to Produce Low Pitched Mating Calls

Researchers have found that Koala's have an additional organ that helps produce the deeper bellow during the mating season.

Koalas have a well earned reputation of having a lethargic lifestyle but they get a little active during the mating season and produce rumbling bellows in order to communicate. It was believed that the bellows produced by the males were probably to attract females and intimidate other males.

But researchers at the University of Sussex, discovered that the koalas, through their low pitched bellowing actually communicate their size.  The researchers noticed that the Australian marsupials possess a specialized sound producing organ that no other terrestrial animal has.  What is strikingly interesting is the location of  the organ, which is found outside the voice box, the larynx.

"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, said in a press statement. "We also demonstrated that koalas use these additional vocal folds to produce their extremely low-pitched mating calls."

Charlton explains that these bellow calls are produced in a continuous series of sounds on inhalation and exhalation. These bellows are quite loud.

With a descended larynx koalas have a longer vocal tract. The sound produced is linked to the size. Larger koalas have longer vocal tracts, which due to lower resonance produce deeper sounds. Therefore, the largest koala with longer vocal tracts tells the females and also hints to males of how big it is. Larger males have a lower tone than the smaller animals.

A previous study  by Charlton found that the muscle which attached the larynx to the sternum further pushed the larynx  down. He also suspected that the boastful bellows of the koalas led to the evolution of the descended larynx.

In the current study, Charlton terms the newly found flaps as 'velar vocal folds' like two long fleshy lips in the soft palette that are placed on top of the larynx at the passage between the oral and nasal cavities. Though they may resemble the laryngeal vocal folds of other mammals, what remains unusual is their location.

"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 collected data makes it the first evidence of a land dwelling mammal carrying an organ other than the larynx that is solely responsible for the production of sound. The team now plans to investigate whether other mammals have this vocal adaptation or if it is completely unique to the koalas.

Click Here to see the video of Koalas velar fold vibration at 10-45 Hz.

The study was reported in the Cell Press Journal Current Biology.