Koalas aren’t the most social of animals: they move mostly at night, live mostly-independent lives, and don’t group together or socialize, except when they’re planning to mate. So how do they find each other when they need to? Male koalas use deep resonating calls, or bellows, to send messages across the dense eucalyptus forest about where they are – but new research by Dr Benjamin Charlton (University of Vienna) and collaborators in Brisbane, Australia, shows these calls also relay important information about the caller’s size. Their work was published this week in the international peer-reviewed journal The Journal of Experimental Biology (doi:10.1242/jeb.061358).
When it comes to koala mating and territory defense, size matters.
Bigger male koalas have higher reproductive outputs and females may preferentially select larger males to sire their offspring – which are then also likely to be larger and more competitive. Furthermore, larger males are better able to defend good territories.
Dr. Charlton and colleagues found that male koalas have a particular vocal anatomy that maximizes the resonance of their bellows, thus giving individuals a deep, low-frequency call that makes them sound big, tough, and attractive. This is the first time that a permanently-descended larynx – previously found in humans and red deer - has been discovered in a marsupial. A descended larynx increases the spectrum of vocalisations that can be produced, including those in a deeper range.
According to Dr. William Ellis, our own UQ koala expert and co-author on the paper, the finding “is especially exciting because it confirms our impression that intense sexual selection in the koala operates despite their seemingly solitary nature. They are an excellent model for investigating parameters of mate choice and evolutionary ecology, as well as being an iconic, if vanishing, species.” The group is following up the study with work to determine the precise function of these bellows in koala ecology.
