Modern koalas are known for their cuteness, nearly exclusive eucalyptus-leaf diet, and the unexpectedly weird noises they make.

Now, new research into their ancient ancestors shows that the koalas' odd appeal arose through the evolutionary interplay between an increasing reliance on an odd food supply and the need to maintain distinct ear structures for hearing each others' bellows.

By studying the skulls of koala predecessors that lived five to 24 million years ago in the Miocene, an Australian team argues that evolution reshaped the animals faces to enable them to eat the tough leaves while maintaining their specialized communication anatomy.

"The unique cranial configuration of the modern koala is therefore the result of accommodating their masticatory adaptations without compromising their auditory system," write the researchers, led by Mike Archer, a paleontologist at the University of New South Wales, in a paper in the Journal of Vertebrate Paleontology.

The koalas communicate across the Australian forests by making low-frequency calls that are shockingly uncute (listen to the embedded video). The upside to the sounds is that they can travel longer distances – they act like the long waves of AM radio instead of the shorter waves of FM. The researchers hypothesize that the ancient koalas evolved their communication system at a time when the Australian continent was drying out and the koala habitat becoming less dense. By lowering the frequency of their calls, they were able to maintain communication in the sparser forests.

To hear the lower frequencies, they evolved an ever larger apparatus in the middle ear. Modern koala bellows can travel more than 2,500 feet.

Meanwhile, as the Miocene wore on, those same forests were increasingly dominated by the eucalyptus, which became the koalas' main food source. To make use of that resource, though, they had to add chewing power to deal with the tough leaves. And it's that combination of evolutionary quirks that yielded the strange skull of Phascolarctos cinereus, the modern koala.

Citation: "Cranial Anatomy of Oligo-Miocene Koalas (Diprotodontia phascolarctidae): Stages in the Evolution of an Extreme Leaf-Eating Specialization" by Julien Louys, Ken Aplin, Robin M.D. Beck, and Michael Archer in the Journal of Vertebrate Paleontology 29(4):981–992, December 2009

Image: Dorothy Dunphy.

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