Male koalas have a specialized chest gland that produces a musky scent for marking territory and attracting mates.

The sternal gland of the male koala (Phascolarctos cinereus) is a specialized organ located in the center of the chest. It begins developing around age three as the animal reaches sexual maturity. This gland produces a thick, oily secretion containing over 40 different chemical compounds, including fatty acids and esters, which create a unique chemical signature for each male.Research published in journals such as PLOS ONE shows that these scent marks communicate critical information about a male's health, age, and social status. During the breeding season—typically August to February in Australia—males become more aggressive and increase scent-marking frequency. They grasp tree trunks and rub their chests vigorously to deposit the musk, which other koalas can detect from considerable distances.This olfactory communication is vital for koalas because they are generally solitary animals living in low-density populations. Scent marks maintain social hierarchy without constant physical confrontation, conserving precious energy. Females use these chemical cues to assess the fitness of potential mates, often preferring the scent of larger, more dominant males. This behavior ensures that robust genetics are passed to the next generation within the eucalyptus forest ecosystem.