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Koalas’ low-pitched voice explained by unique organ
The pitch of male koalas’ mating calls is about 20 times lower than it should be, given the Australian marsupial’s relatively small size, University of Sussex research reveals.
Dr Benjamin Charlton and Dr David Reby, reporting in the Cell Press journal Current Biology on December 2 have discovered the koalas’ secret: they have a specialised sound-producing organ that has never before been seen in any other land-dwelling mammal. The key feature of this newly described organ is its location outside the voice box, or 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,” says Dr Charlton, a Leverhulme Trust Early Career Fellow in the School of Psychology at the University of Sussex. He adds: “We also demonstrated that koalas use these additional vocal folds to produce their extremely low-pitched mating calls.”
The koala’s bellow calls are produced as a continuous series of sounds on inhalation and exhalation, similar to a donkey’s braying, Dr Charlton explains. On inhalation, koala bellows sound a bit like snoring. As the animals exhale, the sound is more reminiscent of belching. And, as Dr Charlton says, “they are actually quite loud.”
They are also incredibly low-pitched, more typical of an animal the size of an elephant. Size is related to pitch in that the dimensions of the laryngeal vocal folds normally constrain the lowest frequency that an animal can generate. As a result, smaller species will typically give calls with higher frequencies than larger ones.
Koalas have bypassed that constraint by putting those vocal folds in a new location. Dr Charlton describes the folds as two long, fleshy lips in the soft palate, just above the larynx at the junction between the oral and nasal cavities. They may not look all that different from the laryngeal vocal folds of other mammals, but their location is highly unusual.
Dr Reby says: “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.”
The combination of morphological, video, and acoustic data in the new study represents the first evidence in a terrestrial mammal of an organ other than the larynx that is dedicated to sound production. Dr Charlton says that he and his colleagues will now look more closely at other mammals to find out whether this vocal adaptation is truly unique to koalas.
Notes for Editors
'Koalas use a novel vocal organ to produce unusually low-pitched mating calls', in Current Biology, Benjamin D. Charlton, Roland Frey, Allan J. McKinnon, Guido Fritsch, W. Tecumseh Fitch, David Reby (December 2013 Volume 23, Issue 23).
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