The sound made by male koalas to attract females during breeding season is a bellow. A male’s bellow is a deep, resonant call that advertises his presence and can make him sound larger than he really is.
Koalas produce these calls with an unusual vocal anatomy. Their vocal folds include structures that allow them to generate exceptionally low-frequency sounds for their body size, giving the bellow its surprising depth. The call can travel through eucalyptus woodland and help males communicate with potential mates and rivals.
Females pay attention to male bellows, and larger-sounding males may attract more interest. Male koalas also bellow when changing trees, so the call is not limited to the instant of mating. Females can bellow too, but their calls are generally softer and may occur alongside snarls, wails, and screams.
A bellow is easy to confuse with “roar,” especially because it sounds powerful and carries across the forest. In zoological terminology, however, “bellow” is the established description for the deep breeding call associated with male koalas. It is not a whistle, chirp, or ordinary birdlike call.