Koalas hug trees to keep cool, say scientists

Trees act like thermal heat sinks to help the marsupials cope with the extreme heatwaves caused by climate change, say scientists

This thermal image shows the massive difference in temperature between the koala's fur and the trunk of the tree. (Steve Griffiths)

Koalas spotted clinging to tree trunks or sprawled across branches aren’t just hanging out, say scientists, they’re keeping cool.

A new study published in the journal Biology Letters used thermal cameras to show that during heatwaves in Australia, the koalas used the surprisingly cold surfaces of trees like an icepack.

“They’re just stuck out on the tree all the time so when hot weather comes they’re completely exposed to it,” Dr Michael Kearney of the University of Melbourne told the Guardian Australia.

“The fur on their tummy is quite a lot thinner than the fur on their backs, so they’re pushing that fur and that part of their body as much against the tree as possible.

"Any way that they can lose heat that doesn’t involve losing water is going to be a huge advantage to them. Dumping heat into the tree is one of those methods.”

Animals and birds avoid overheating in hot weather through this ‘evaporative cooling’ process, but while koalas can pant and lick their fur like dogs to produce a similar effect, these methods are ‘emergency only’ for a creature that can’t easily refill on water.

Koalas don't have sweat glands and so have to find other ways to stay cool through evaporation.

The researchers noticed that in the winter koalas stay higher up in the trees, nearer to the leaves, but in hotter weather they move further down - behaviour that seems unusual but makes perfect sense if they are using the tree trunks as heat sinks.

It’s thought that the trees stay cool because of their strong thermal inertia (meaning they heat up and cool down more slowly than their surroundings) and by pulling up cool groundwater into their trunks.

The team measured the temperature of four species of eucalyptus and one type of acacia and found that on days as hot as 39C the trees could be as much as 7 or 8 degrees cooler. When they used thermal cameras to check their theories they saw it was “obvious” what the animals were doing.

“You could see the koala sitting on the coolest part of the tree trunk with its bottom wedged right into the coolest spot,” Dr Kearney, a co-author on the paper, told the BBC.

The studies were conducted as part of a larger research project into the effects of climate change on land-dwelling animals in Australia.

The researchers predict that this tree-hugging behaviour will become increasingly common as the continent experiences longer and hotter heatwaves – but that once temperatures go past 40 degrees even this might not be enough to help koalas cool down.