Thermal imaging shows koalas cool down in extreme temperatures by hugging trees.
Koalas cope with extreme heat by hugging cool trees according to results of a thermal imaging study of the marsupials living on French Island, 80 kilometres south-east of Melbourne.
The researchers found that, unlike panting and licking their fur to keep cool, when temperatures exceeded the high 30s tree hugging both cooled and reduced the degree of dehydration for the animals, which rarely drink but get water from their food.
Led by Natalie Briscoe from Melbourne University's botany department, the study showed there could be as much as five degrees difference between the tree trunk temperature and the air temperature.
The study suggests cool tree trunks will become increasingly important habitat as extreme heat events become more frequent and severe in the future.
The koalas capitalised on the cool trees by flopping on the larger branches or hugging the trunk – both positions which maximised the surface area of the body when it came in contact with the tree.
"Access to these trees can save about half the water a koala would need to keep cool on a hot day," she said.
Melbourne University ecologist Michael Kearney said one plausible explanation was that trees suck up cool water from deep underground.
"So where the ground temperature is cool ... the effect of that water coming up through the tree could create quite a big offset," he said.
Dr Kearney said koalas, which have thinner fur on their bellies, also appeared to prefer cooling off in acacias over eucalypts in hot conditions.
According to the CSIRO's Action Plan for Australian Mammals, koalas are now considered threatened in every habitat type in the country.
"Koalas are animals which are stuck out there in the weather," Dr Kearney said. "Unlike their close relative the wombat which can retreat to a burrow, they don't have retreats so they are vulnerable."
Published in Biology Letters on Wednesday, the study suggests cool tree trunks will become increasingly important habitat as extreme heat events become more frequent and severe in the future. It also highlights the importance of old, established trees.
The researchers, from Melbourne University, James Cook University, La Trobe University and the University of Wisconsin, added the findings had implications for other tree-dwelling species, including primates, leopards, birds and invertebrates.