Koalas may look cute and placid but come the mating season, the males produce a bellow that… well… is not the sound you expect them to make. As they inhale, they sound like a loud, creaky door. As they exhale, they sound like someone belching vigorously. Put these together, and you get a continuous racket that sounds like an angry Wookiee.

The bellows are surprising to passers-by, but they perplex scientists too. Koalas just shouldn’t be able to make a sound that low.

Mammals make calls using an organ in our throats called the larynx, or voicebox. When air passes through the larynx, it vibrates a pair of membranes called the vocal folds (or vocal cords). These create sound waves