An international team of biologists led by Dr Benjamin D. Charlton from the University of Sussex has discovered that koalas (Phascolarctos cinereus) possess a unique sound-producing organ that has never before been seen in any other land-dwelling mammal.
A koala in the Great Otway National Park, Victoria, Australia. Image credit: David Iliff / CC-BY-SA 3.0.
During the breeding season, male koalas produce low-pitched mating calls that are characterized by a continuous series of inhalation and exhalation sections, and an extremely low fundamental frequency. The pitch of these calls is about twenty times lower than it should be, given the koala’s relatively small size.
“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 Dr Charlton, who is the lead author of the study published in the Current Biology.
“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. On inhalation, koala bellows sound a bit like snoring. As the animals exhale, the sound is more reminiscent of belching. And they are actually quite loud.”
The calls 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. The folds are two long, fleshy lips in the soft palette, 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.
“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,” Dr Charlton said.
The study represents the first evidence in a terrestrial mammal of an organ other than the larynx that is dedicated to sound production.
Dr Charlton said the team will now look more closely at other mammals to find out whether this vocal adaptation is truly unique to koalas.
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Bibliographic information: Charlton BD et al. Koalas use a novel vocal organ to produce unusually low-pitched mating calls. Current Biology, vol. 23, no. 23, pp. R1035-R1036; doi: 10.1016/j.cub.2013.10.069
