Australia's koala bears are perhaps the world's best known and loved "tree huggers," but it's not about food or about a love of eucalyptus, researchers say; it's about staying cool.
The furry creatures are using the trunks of the trees as natural "heat sinks" to draw heat from their bodies in the Australian heat, a study by researchers at the University of Melbourne has found.
Cooler temperatures found in the trunks and thicker, lower branches of trees provide relief for the koalas, which unlike many other tree-dwelling mammals do not shelter in dens or hollows.
"They're just stuck out on the tree all the time so when hot weather comes they're completely exposed to it," study leader Michael Kearney says.
Studying koalas near Melbourne, the researchers saw them moving to lower, cooler branches and laying flat along tree trunks during extremely hot weather.
The region enjoys relatively cool temperatures for the greatest part of the year, the researchers said, but in the summer temperatures can reach 104 degrees Fahrenheit, and that's when the change in koala behavior was noticed.
And it's not just a matter of having a lie-down or a nap in the heat, as the use of thermal imagery to study the animals revealed, Kearney says.
"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," he says. "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."
The tree trunks measured as much as 9 degrees cooler than the surrounding ambient air, the researchers found, possibly because they pull in lots of groundwater that's cooler than the air temperature, he said.
Koala bears normally cool themselves by panting, allowing heat to escape their bodies in the evaporating moisture in their mouths.
In hugging cooler tree trunks during extreme heat, they can save up to half the amount of water that would have been used in panting to cool themselves, the study found.
Koalas are likely to resort to this "tree-hugging" behavior more and more as climate change brings on heat waves that are more extreme and longer in duration, he says.
The study has been published in the journal Biology Letters.
