So. That's the sound a koala makes. Huh.
It's not really very cute, is it?
But it does get the job done. Specifically, this is the call of the horny male koala—and this sound is such an effective mating technique that simply hearing it can cause female koalas to go into heat. (Insert your own Koala Justin Bieber joke here.) Scienceline explains the importance of koala "bellowing":
The timbre of a koala's bellow seems to have something to do with its size, age and androgen (a sex hormone) concentration. Older males bellow more, bigger males bellow longer, and those with more androgen have deeper bellows. One study found that when the male koala is 2-4 years older than the female, the mating is more likely to be successful, leading some to suggest that females use male calls to gage their relative sexiness.
Via Bora Zivkovik