Despite the tree-dwelling mammal having a cute and furry appearance, a 15 pound koala is as loud as a cow weighing more than a tonne, a study found.
Researchers discovered the marsupial emitted a louder sound as a way of attracting sexual partners during mating season.
The team of Australian and Austrian scientists, writing in The Journal of Experimental Biology, also found their cries were a way of boasting about their body size and intimidate rival lovers.
Using complex medical imaging they discovered the sounds were louder because their larynx had “descended” and sat deeper in their throat and chest than other species.
This was similar to human development because as a person grows up, their larynx also becomes lower, and deeper, as they learn more complex language and speech.
"A lot of times people in the bush might hear a Koala calling… you have this cute fluffy animal but on the other hand (hear) this booming voice which jolts them a bit,” said Dr Bill Ellis, a co-author of the study.
"There are some interesting parallels in the structure of the actual vocal tract. This descendant of larynx koalas have is similar to that in some of the big cats but also in humans.”
“In adult humans we have really complex speech we should expect that similarly in koalas - it might be complex as well.”
Dr Ellis, the director of the Koala Ecology Group at the University of Queensland, added: "To our ear, we can't actually hear much difference between a large and not so large koala, but it seems koalas can tell.”
During their research, the scientists recorded 140 of the animals at the Lone Pine Koala Sanctuary in Queensland and measured their heads and body lengths.
They also carried out a medical scan on one male koala, which disclosed the marsupial's strange vocal anatomy.
An analysis of the recordings found the difference was not in the pitch but in “vocal tract resonance”. The bellows gave the impression their vocal tract was nearly 20 inches long or almost entire length of their body.
Dr Ben Charlton, from the University of Vienna, Austria, who led the study, said: “A permanently descended larynx hasn't been documented in marsupials before.
"It was believed that only humans had (this)… and that it was an essential adaptation for the creation of vowel sounds."
Dr Ellis said their noises were a way of “making themselves sound a lot bigger than they are".
They also found that as a female searched for a mating partner, they identified who issued the loudest bellows and calculated how big they were.
Previous studies using GPS tracking, specialists listening devices and DNA technology found larger males had better breeding success than their smaller rivals.
“It's quite apparent that there are some attributes that are closely correlated between the size and that one male can tell how big another male is,” Dr Ellis said.
"There are a number of strategies that koalas use for breeding and ... it isn't like an arms race to be the biggest koala.”
Earlier this year the Australian government warned koalas were facing serious risk of extinction, thanks to predators, climate change and rampant human development.