Koalas have a reputation of being lazy animals (see figure 1). Of course, sleeping about 19 hours a day, and spending 3 out of the 5 remaining hours eating, only adds to their ‘street rep’. But even these lazy marsupials have to mate. And that’s when it gets interesting (at least to some biologists).

Figure 1: Sleeping koala. Awww...

(Source: Wikimedia Commons, author: Dingy)

During mating season, the males bellow rather impressively (click here for an example, courtesy of the Australia Koala Foundation). Intrigued by their calls, an international team of researchers went looking for the messages contained within the koala’s call. According to the team, the bellowing might reveal information about the size of the individual. Bigger males produce deeper resonances than smaller ones. But even the small males sound bigger than a bison.

How might this be possible? The team hypothesized that koalas might possess a descended larynx, making their vocal tracts longer, enabling lower resonances to be produced.

Aided by the use of MRI scans and post-mortem studies, the researchers found the koala’s larynx indeed descended. Down to the third and fourth cervical vertebra to be precise. The team also discovered that the muscle attaching the larynx to the sternum was anchored deeply in the thorax and might pull the larynx even further down.

After recording bellows and measuring skull size (as a proxy for body size), the team found indeed that bigger males always had lower resonances than their smaller colleagues. Based on the acoustics, the researchers calculated that koalas can make themselves sound as if they possess a vocal tract of half a meter, almost as long as the animal itself. It is suspected that the resonances of the oral and nasal tracts are used simultaneously to sound bigger.

The team concludes:

Taken together, these findings show that the formant spacing of male koala bellows has the potential to provide receivers with reliable information on the caller's body size, and reveal that vocal adaptations allowing callers to exaggerate (or maximise) the acoustic impression of their size have evolved independently in marsupials and placental mammals.

Reference

Charlton, B.D.; Ellis, W.A.H.; McKinnon, A.J.; Cowin, G.J.; Brumm, J.; Nilsson, K. and Tecumseh Fitch, W.(2011). Cues to body size in the formant spacing of male koala (Phascolarctus cinereus) bellows: honesty in an exaggerated trait. Journal of Experimental Biology. 214, pp. 3414 – 3422. doi:10.1242/jeb.061358.