In just about every koala photo, the furry Australian marsupial is clinging to a tree. Now, scientists have figured out why: To the koala, a tree trunk is an air conditioner. A new study by a team of Australian and U.S. researchers found that koalas seek out and hug trees to keep themselves from overheating.

It's Getting Hot in Here

Koalas don’t den, so they have to find other ways to cool off. Panting helps, through a mechanism called evaporative cooling, but the animals also lose precious water that way. To figure out just how dependent koalas are on trees for cooling, Natalie Briscoe of the University of Melbourne and other researchers tracked the behavior of 37 koalas during winter and summer, observing their interaction with trees. They also noted tree type, height and location, and measured the microclimate of the sites using portable weather stations mounted on an extendable pole. During hot weather, the koalas “appeared to be hugging the trunks or large lower branches” of trees, which can be five degrees cooler than the air, the researchers reported in the paper, published today in the journal Biology Letters. They spent more time there, too: The koalas used the lower areas of trees 65 percent more often on sweltering days than during milder weather, according to the study, part of a broader investigation of how koalas are influenced by climate.

Crucial Cooling

“Our modeling shows that hugging a cool tree trunk during a typical hot day in southeastern Australia can halve the amount of heat koalas need to lose via evaporative cooling,” Briscoe wrote in an email. “This behaviour is likely to help them cope with hotter or longer extreme heat events.” That extra cooling can mean the difference between life and death: During heat waves, hugging trees increases survival rates. But the trees with the coolest trunks – a type of tree called Acacia mernsii -- may not always be available within koala habitat, and searching out those trees could put koalas at greater risk of getting hit by a car or being attacked by a dog, Briscoe adds. Other tree-dwelling species use the tree-cooling strategy too, including leopards, various birds, and even bugs. As heat waves become more frequent – and more intense – in response to climate change, Briscoe and her co-authors concluded, cool tree trunks will become even more important for forest dwellers’ survival.

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