By Victoria Gill
Science and nature reporter, BBC News
Usually quiet males can become quite vocal
The classic image of a koala is a quiet little marsupial, sitting contentedly in a tree chomping eucalyptus leaves.
But these unassuming bear-like animals can make a racket - producing a deep, growling bellow.
Now researchers in Australia have discovered that male koalas' bellows draw in females.
The team also believes that female koalas are able to work out how attractive a potential mate is by the sound of his voice.
Dr William Ellis from the University of Queensland, who led the study, has been observing a group of koalas on St Bees Island for 10 years.
This national park's isolation, 30km off the coast of Queensland, makes it an ideal spot to observe the creatures in the wild.
Dr Ellis and his colleagues, who report their findings in the journal
Behavioural Ecology
, caught and fitted satellite tracking collars to 12 koalas - six males and six females.
"We have these solar-powered mobile phones that record sound for a few minutes every half hour and upload it to a server here in Brisbane - so I can hear how much bellowing is going on," he explained.
The researcher says the males "advertise themselves" by bellowing.
Whatever they're doing vocally, it seems really quite complex
"We see these big spikes in female movement when there is more bellowing going on, so it really seems that the females are going and searching out the males."
The team thinks that the females' mate-seeking excursions benefit the animals by reducing the risk of inbreeding.
Dr Ellis explained: "A current theory is that when females come into oestrus, they're more likely to respond to the bellows and more likely to leave their home range."
Big voice
The findings could also help explain why male koalas are much larger than females.
Larger males appear to be able to bellow for longer, so "the females might be able to tell which males are bigger by the bellows", said Dr Ellis.
The team are currently taking their investigation of koala language one step further, working with an acoustics expert from the University of Vienna in Austria, Dr Ben Charlton.
He is studying the recordings of each koala's voice to find out what information the females may be extracting from a male's bellow.
"Whatever they're doing vocally, it seems really quite complex," said Dr Ellis.
"For years, people thought that only the alpha male bred - that [his bellows] attracted lots of females.
"But there isn't just one male that dominates breeding, so it's not as simple as we thought."
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Whatever they're doing vocally, it seems really quite complex
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