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3 December 2013, 06:00
Male koala bears really know how to big themselves up to impress the ladies.
They possess an extra voice organ that allows them to sound as large as an elephant, scientists have discovered.
Experts had been puzzled by the fact that the male koala's mating call is 20 times deeper in pitch than it should be for an animal of its size.
Now they have uncovered the koala's macho secret - a second set of vocal cords not seen in any other terrestrial mammal.
"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 study leader Dr 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 consists of a loud bellow, similar to a donkey braying, but much lower pitched. In fact, it is the kind of sound more likely to be made by an elephant, said the scientists.
The size of animals is related to the pitch of their voices because of the restrictions it places on the vocal cords in the larynx, or voice box. Large vocal cords, or folds, vibrate at a lower frequency than small ones. Hence, house cats meow while lions roar.
Koalas have found a way round these limitations by evolving a separate set of vocal folds in a different location. They look like two long, fleshy lips situated just above the larynx at the junction between the mouth and nasal cavities.
Whales are the only other mammal known to possess anything similar.
"To our knowledge, the only other example of a specialised sound-producing organ in mammals that is independent of the larynx are the phonic lips that toothed whales use to generate echolocation clicks,'' said Dr Charlton.
The scientists made the discovery after carrying out anatomical studies of 10 male koalas. Besides the normal vocal cords, they found "much larger and previously undocumented'' folds in an oval opening within the soft palate.
"The distinctive shape and the position of these 'velar vocal folds' indicate that they could be used to produce sound as air is sucked in through the nostrils during inhalation,'' the researchers wrote in the journal Current Biology.
They were able to hear the sound made by the organ by using a pump to suck air through the vocal folds. A video camera attached to a flexible tube made it possible to film them vibrating.
"Further studies are now warranted to investigate whether this remarkable adaptation is shared with other mammals, or whether it is unique to koalas,'' the scientists added.