Koala love (AP)
A girl koala peeping out from behind eucalyptus branches. Photo by Reuters
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A group of koalas gather atop a fence. Photo by AP

Koalas spend most of their time asleep but during mating season, the male wants potential partners to know he's very much awake. And the bizarrely loud burps and groans the "bear" emits are not only pitched sexily low: they're extraordinarily loud - and originate from a unique organ never seen before, say scientists.

When looking for a mate, the boy koala emits a deep, reverberating bellow at a pitch scientists say is about 20 times lower than it should be, given the Australian marsupial's relatively small size.

Now, researchers reporting in the Cell Press journal Current Biology on December 2 say they have discovered their secret: koalas have a specialized sound-producing organ never before been seen in any land-dwelling mammal.

Key to the working of this peculiar organ is its location outside the larynx.

"Koalas possess an extra pair of vocal folds that are located outside the larynx, where the oral and nasal cavities connect," says Benjamin Charlton of the University of Sussex, on the team that demonstrated koalas using use these extra vocal folds to produce their extremely low-pitched mating calls.

Snore in, burrrp out

The koala's bellows are a continuous series of sounds based on inhalation and exhalation, similar to a donkey's braying, Charlton says.

On inhalation, the bellows sound a bit like snoring. As the animals exhale, the sound is more akin to belching. And, Charlton says, "They are actually quite loud."

They are also incredibly low-pitched, more typical of an animal of much larger size.
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.

Charlton says koalas have bypassed that constraint by putting those vocal folds in a new location. The folds may not look all that different from the laryngeal vocal folds of other mammals, but their location, says Charlton, 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," Charlton says.

How one finds a unique fold in the throat

The study was conducted on male koala specimens euthanized at Brisbane's Moggill Koala Hospital after being hit by cars or otherwise injured to a point beyond rehabilition, in a six month period between November 2012 and April 2013. Charlston stresses that the so-called bears – they aren't - were not destroyed for the research.

To study how the vocal folds work, Charlston says he sucked air at different speeds through the organs of three freshly euthanized male koalas, filming the various rates of vibration at frequencies between 10 and 45 hertz.

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. But he hopes that the reporting the odd discovery will "excite the curiosity of non-scientific audiences, and in doing so, serve to raise the profile of this increasingly endangered species," he says.

Another extraordinary vocalization discovered among Australia's marsupials is that of an animal at the other end of the cuteness rainbow - the Tasmanian devil.  This carnivore too boasts extraordinary range of voice including burps and eerie howls, when provoked, but it isn't necessarily looking for love.