THURSDAY, Sept. 29 (HealthDay News) -- The bellowing of male koalas during mating season is a boast about their size, researchers report.

They found that the largest koalas produce the deepest baritone bellows, a message to females and rival males that they're the biggest dudes in the neighborhood.

The researchers also discovered that koalas are one of the few animals with a descended larynx, which makes the vocal tract longer. A longer vocal tract emits deeper sounds. This means that the longer vocal tracts of the largest koalas produce the lowest baritone bellows.

The study appears in the Journal of Experimental Biology.

The bellowing of koala males may have driven the evolution of their descended larynxes, according to study author Benjamin Charlton of the University of Vienna, Austria.

"Individuals that could elongate their vocal tracts by lowering the larynx may have gained advantages during sexual competition by sounding larger, and this would drive the evolution of laryngeal descent," he explained in a journal news release.

More information

The Friends of the Koalas has more about koalas.