Koalas make a deep, rumbling bellow that some people call a roar. This is more commonly used by males but has been observed in females as well.
Male koalas use their bellows to attract mates and warn other males to stay away. This bellow is unusually deep for such a small animal and is often compared to a lion's roar or an elephant's bellow. Koalas have a special organ outside of their larynx to produce their rumbling roars.
Koalas can also make higher pitched squeals, which are usually signs of discomfort or distress. Baby koalas, called joeys, use high-pitched yips to communicate with their mothers. Although koalas usually ignore the squeals of other koalas, mothers respond to their joeys' cries.