Male koalas seeking sex get all shouty and spread out, partly to reduce competition from the other males.
Bellowing seems to prevent fights between the males, as well as letting females know exactly who they are in the breeding season, says University of Queensland researcher William Ellis.
It is not 100 per cent clear how all that bellowing, usually between 10pm and 4am throughout September to December, attracts the females, Dr Ellis says.
But females can identify each male's signature call and seem to be attracted to different males each year - so it could be luring them to new partners to prevent inbreeding.
Koalas are cute and very well-known, but there is not much information about their social and mating behaviours.
That is why Dr Ellis and colleagues from Australian, Japanese and US universities were using GPS tracking to map the social system of about 25 wild koalas, including one male named Chuck, at St Bees Island near Rockhampton.
Their paper, in the journal PLOS One, says they thought the koalas would fight during mating season.
But they found their behaviour helped males avoid physical confrontation.
"They can tell who is the biggest, and stay away from him," Dr Ellis told AAP.
That is lucky because fights last longer in breeding season and brawls in tree tops can end in fatal falls.
Their shouts also allow male koalas to space themselves apart, with little direct mating competition, Dr Ellis said.
Curiously, the females spend more time together in the breeding season, something the researchers want to explore more.
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