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29 September 2011 Last updated at 05:40 By Victoria Gill Science reporter, BBC Nature
The males bellow during the mating season, probably to attract females and intimidate males The loud, grumbling bellow that emerges from a male koala sounds very unlike that of a cute, laid-back creature. Now scientists have discovered the anatomy behind the strange sound that males make during mating season.
Male koalas have very long vocal tracts - structures in their throats that produce the sounds.
Their vocal tract anatomy is so unusually specialised, in fact, that they are able to make sounds that make them sound far larger than they are.
The study, reported in the Journal of Experimental Biology, used medical imaging to reveal that a male koala's voice box, or larynx, sits very low in its throat. This "descended larynx" was thought to be a uniquely human feature - something that allows us to make the sounds we need for speech.
It was only in 2001 that scientists found that red deer also had a descended larynx. Its discovery in koalas now supports the theory that it evolved in even more branches of the evolutionary tree, probably to allow males to distinguish themselves vocally from females.
The researchers studied the koalas at a sanctuary called Lone Pine in Queensland, Australia.
As well as recording their bellows, the team also carried out a medical scan on one male koala, which revealed the marsupial's strange vocal anatomy.
"A permanently descended larynx hasn't been documented in marsupials before," said Ben Charlton from the University of Vienna, Austria, who led the study. "It was believed that only humans had [this, and] that it was an essential adaptation for the creation of vowel sounds."
Dr Charlton explained to BBC Nature that the thing that made koalas "sound big" was not the just the very low pitch of their bellows, but the "quality" of the sound coming from their long vocal tract.
The effect works like a musical instrument; when an animal's "voice box" vibrates to make a sound, this sound echoes inside the tube that is its vocal tract. The dimensions of the tube change the sound.
So a violin and a cello can make a sound that is exactly the same pitch, but the cello sounds very different - richer and larger.
The sound of a koala's call, the researchers found, would "predict" a vocal tract length of 50cm. This is almost the entire body length of a koala. So when they bellow, the animals sound bigger than a bison.
The medical scans also revealed a muscle deep in the koala's chest that the researchers think might pull the voice box even further down into its chest cavity as they bellow, enabling them to exaggerate their size even more.
Dr Charlton explained that koalas had evolved to "sound big", "probably because it's important for intimidating other males."
There is also some evidence that the bellows attract females.
Red deer also have a descended larynx, a feature which had been thought to be unique to humans David Reby is a psychologist from the University of Sussex in the UK who specialises in mammal communication. He led the team that, 10 years ago, discovered male red deer had a descended larynx.
Dr Reby explained how this study in koalas added to the wider evolutionary story of vocal communication. Finding out why male koalas evolved to make such deep, grumbling bellows could help us understand why male and female humans have very different voices, he said.
"Men have a 20% longer vocal tract than women," Dr Reby said. "To find out the evolutionary origin of this, we need to better understand how these [differences] evolved in other species.
"That's why Ben's work is really exciting, because he's not only studying other mammals but also marsupials."
The scientists would ultimately like to take live scans of the animals as they bellow - internal snapshots of what happens as they produce sound.
Dr Reby commented: "It would be difficult, but if we could get some good footage, we'd be able to figure out what was really going on."
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