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Bellowing koalas sound as big as a bull

Big as a bull During mating season, bellowing koalas may be boasting their body size, sounding as large as a bison, according to a new study by Austrian and Australian scientists.

Startlingly sounds emanate from male koalas as they bellow during their mating period in November and December.

This new research, published in The Journal of Experimental Biology has found their cries seem to contain information on their size, and might be used to intimidate rivals or attract mates.

"We've shown, for the first time, there's variation in the bellows that correlate with body size in the males," says co-author Dr Bill Ellis, part of the Koala Ecology Group at the University of Queensland.

"To our ear, we can't actually hear much difference between a large and not so large koala, but it seems koalas can tell," he says.

To see if the bellows changed with body size, scientists recorded koalas at the Lone Pine Koala Sanctuary in Queensland, and then measured their heads. Head size correlates closely to body length in koalas, and is easier to observe.

Analysis of the recordings showed the difference was not in pitch, but in vocal tract resonance, or formant.

"Formants are a resonant frequency of the vocal tract," says study lead author Dr Ben Charlton, a research biologist at the University of Vienna.

"The vocal tract has air in it, and it vibrates at certain frequencies preferentially," he says. "Formant perception is critical for vowel perception [in humans]"

In koalas, it is the spacing of formants that changes with body size, with larger koalas having lower formant spacing, explains Charlton.

According to the study, formant spacing is an honest cue for body size, as they are inextricably linked to vocal tract length, and that length is ultimately constrained by the chest and sternum.

Exaggerated sound

Though koalas are being honest about their size, the trait is highly exaggerated compared to other species, says Ellis.

"If you just looked at the vocal tract resonances, you would expect it came from a much larger animal," he says.

Koala bellows sound as though their vocal tract is 50 centimetres long, nearly the length of their entire body.

Their larynx, or voice box, is descended deep in the throat, and they might pull it even lower using a large muscle embedded in the chest cavity, according to the paper.

Ellis is already looking ahead to the next study.

"We're really keen to take it to the next step now," he says. "If we can manipulate these sounds to make a larger male sound like a smaller male, and then observe the movement of the females, then we can really start to understand their language."