The loud grunting of the usually sleepy male koala is not limited to alpha males attracting mates.
Listen to the sounds of a male koala:
Scientists previously thought the loud grunting was limited to a single alpha male, but new research from the University of Queensland shows that many male koalas in a given area belt out their snarls, hoping to attract females, sometimes from long distances.
"Females can travel up to 500 m out of their home range...We observed one female who got down on the ground and made a beeline toward the sound," says lead researcher Dr Bill Ellis.
It seems that can females detect physical clues within these bellows because the most successful males tend to be the bigger koalas (Phascolarctos cinereus), sometimes two kilograms heavier than their rivals.
"It's pretty clear that the bellow a big male makes has a different frequency, so it's possible the female can tell the size of the male or even detect the body condition of the male from it," says Bill.
The researchers fitted satellite tracking collars on 12 koalas (six of each gender) and recorded their bellowing, which they then compared to the movements made by the koalas.
During the summer mating season, males amplified their bellowing and the females travelled in response, more so than males. The researchers concluded the primary motive of the bellow is to attract females, rather than to ward off rivals.
Koala expert Dr Stephen Phillips says that the innovative study confirmed what koala specialists had been aware of for some time. "We had collectively noticed there seemed to be preferential mate selection, driven by the female," he says. "We see females travelling some distance to mate with a particular male."
Stephen says there are usually one or two big males within a home range that are responsible for the majority of the offspring; some alpha males mate with up to four females, although occasionally others will be successful.
Male koalas also sometimes seek out bellowing males, or females which call out in distress, to aggressively confront the competing male.
In animals, the length of the vocal tract is a good indicator of body size; however, male koalas are able to exaggerate their vocalisations to produce loud sounds which are incongruous with their size. Bill says it's possible the koalas employ a circular breathing pattern when they bellow.
"When we're talking it is pretty clear we're breathing out, but I'm not 100 per cent convinced which sounds are exhalation and which are inhalation when the koala bellows," he says.
Bill and his colleagues have been observing the koalas on St Bee's Island off the coast of Queensland for over ten years, and have been recording their bellows for three.
The research was published in Behavioural Ecology in February 2011.