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Vocal lips the secret to koala's boom box

Big voice Koalas use vocal folds located outside the larynx to make mating calls that are more suited to an animal the size of an elephant, new research has found.

The loud snorting sounds are produced by a previously unidentified body part known as velar vocal folds, which makes male koalas sound bigger than they actually are.

"Terrestrial mammals use their larynx to produce vocalisations," says study lead author Dr Benjamin Charlton from the University of Sussex in the United Kingdom.

"Because the dimensions of the laryngeal vocal folds constrain the lowest frequency that can be generated, smaller species typically give calls with higher frequencies than larger ones."

Despite their relatively small size, male koalas produce mating calls with an exceptionally low fundamental frequency (pitch) that would normally be expected from a far larger animal, he says.

"The aim of our study was to find out how koalas produce these unusually low-pitched mating calls."

Dull roar

The folds were first identified during detailed dissections of a koala in Germany. Charlton then collaborated with Allan McKinnon at Moggill Koala Hospital in Queensland to dissect further specimens.

He wanted to show that the folds are found throughout the species, and weren't just an abnormality in one specimen.

"The koala's velar vocal folds are two long fleshy lips that are located in the soft palette (or velum), just above the larynx where the oral and nasal portions of the pharynx connect," he says.

While they look very much like the laryngeal vocal folds that most mammals use to produce their calls, Charlton says they are highly unusual because they are not located inside the larynx.

"Furthermore, because the velar vocal folds are over three times longer and around 700 times heavier than [normal] laryngeal vocal folds they can oscillate at lower frequencies and produce lower-pitched sounds, [similar to] a guitar string."

The low notes of the bellow are produced when the koala breathes in and the researchers were able to reproduce mating bellows in koala specimens by mimicking inhalation and sucking air through the folds. A strategically placed video camera confirmed the folds' role in sound production.

Independent adaptation

Charlton says that velar vocal folds may have evolved independently in koalas specifically to produce this species' characteristically low-pitched mating calls.

They are the only example of a specialised sound producing organ in a land-dwelling mammal that is independent of the larynx. The only other known example of an independent, specialised sound producing 'instrument' in mammals are the phonic lips used by toothed whales to generate echolocation clicks.

"We might expect to find velar vocal folds in other closely related marsupials that produce disproportionately low-pitched calls for their size," Charlton says. "However, because velar folds are not documented in any species of placental or marsupial mammal, it also seems likely that this remarkable adaptation evolved independently in the koala."

Big call

It remains uncertain why koalas need to make such low-pitched calls.

"In previous studies we have shown that the vocal tract resonances (known as formant frequencies) of koala bellows provide reliable information about the identity and size of male callers," Charlton explains.

"Because low pitch improves the perception of formant frequencies in humans, we think that the koala's remarkably low pitch facilitates the transfer of this important information.

"Another possibility is that low pitch acts as a direct cue to male quality, but this remains a topic for future studies."

The research is published today in the journal Current Biology.