Koalas are known for having an unpleasant temperament. For the most part, however, they tend to avoid aggressive behaviors that require expending a lot of energy. Females that are pregnant or have small offspring tend to be the most aggressive and territorial.
Male koalas sometimes attack other males in order to drive them from their spot in a tree. The aggressor attacks by sinking his claws into the victim's shoulders and biting repeatedly. When the stronger koala wins, he bellows and marks the tree as his own. Koalas sometimes fight when they are climbing over or passing one another in a tree.
Adult male koalas use very loud bellows to communicate with other males and to attract females. They do this at any time of year, but it is most frequent during mating season. They use the sound to exaggerate their size to intimidate other males and because females are attracted to sounds from the bigger males.
Female koalas also bellow, scream and snarl in order to indicate that they are in distress or when defending their territory. Young koalas emit a squeaking noise when they are in distress. This noise transforms into a squawk as the koala gets older.