Male koalas produce deep bellows during the breeding season to attract females and challenge rivals.
A koala bellow is surprisingly low and powerful for an animal of its size. The sound is produced with help from an unusual extra vocal structure located outside the ordinary laryngeal vocal folds. This anatomy lets males create frequencies lower than expected from their body size.
Bellows can travel through eucalyptus woodland and help males advertise their presence. Females may use calls to assess nearby males, while males can respond to one another and avoid unnecessary physical encounters. Koalas also make other sounds, including bellows, screams, grunts, and softer contact vocalizations.
The common image of koalas as silent animals is therefore misleading. Their mostly nocturnal lifestyle and time spent high in trees can make their communication difficult for people to notice, but breeding males can be extremely vocal.