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Side-by-side thermal images of the same koala lying on the lower limb of an E. ovata tree during hot weather. By resting against the coolest part of the tree koalas can reduce how much heat they need to lose by evaporative cooling. Image Credit: Steve Griffiths

Koalas hug trees to keep cool

  • BY Signe Cane |
  • June 04, 2014

Koalas hugging cool trees can reduce their body temperatures by almost 70 per cent

A KOALA SPRAWLED on the trunk of a eucalyptus isn't just resting its paws; it's hugging the tree to stay cool in hot weather, according to researchers from the University of Melbourne.

"If we all had infrared vision, we'd have known all along what koalas are doing," says lead author of the study Dr Michael Kearney, senior lecturer in zoology at the University of Melbourne.

It was Michael's student, zoology postgraduate Natalie Briscoe who first stumbled across this phenomenon while measuring the microclimates of koala habitats in south-eastern Australia.

The team had a thermal-imaging camera on hand, and were surprised to discover that the tree trunks - the ones with koalas splayed across them - were on average more than 5°C cooler than the surrounding temperature.

Koalas hug trees for quick cooling

In hot weather animals and birds often avoid overheating via 'evaporative cooling'. Humans produce sweat, which evaporates and cools the skin in the process.

Animals that don't have sweat glands often pant like dogs or lick their fur to achieve the same effect. However, for tree-dwelling animals saliva is a luxury they can't spare.

"Koalas are stuck up on a tree. They pant and lick their fur, but that's emergency stuff," explains Michael. "They don't have much in the way of drinking water. So, if you're a koala, you want to minimise the amount of water you lose."

The researchers tracked 37 radio-tagged koalas and measured the temperatures surrounding them up in the canopy. After gathering data on tree trunk temperatures and calculating how much heat koalas need to shed, the team was able to determine that tree hugging could cool koalas down by as much as 68 per cent, thus reducing the need to evaporate precious water.

"What they've done is really neat," says Dr Bill Ellis, a wildlife researcher at the University of Queensland who didn't participate in the study. "This is something I've been thinking about for years."

Koalas seek non-food trees to cool down

The researchers measured four species of eucalyptus and one type of acacia. Notably, the acacia provided even more cool relief during heatwaves, which may explain why the marsupials hang out in a tree they don't actually eat from. But the scientists are not entirely sure why trees are so cool in the first place.

"My guess is that trees have thermal inertia, meaning they heat up and cool down more slowly than the ambient temperature," says Michael. "It might also have to do with the cool groundwater they pull up through their roots."

The findings may also hint at the reason why koalas are less prevalent in the north of Australia. "Previously we thought koala spread was either related to predators or humidity," says Bill.

"It changes the way we think about habitat requirements for koalas," Michael agrees. "But it might only work in places that are less hot."

With the warming climate koalas might be hugging more trees in the future, and more research will be needed in other areas of Australia.

"Where we study koalas, it gets up to 40 degrees," Bill points out. Hugging a tree in these conditions might not be efficient enough.

The study was published today in Biology Letters.

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