Koalas don't hug trees because they love them. The trees keep them cool, scientists say.

On a hot day in Australia, a koala's thick fur makes it hard to stay cool. The over-heated marsupials don't seek shelter. And water can be hard to come by. In the wild, many koalas die when the heat becomes extreme, according to a report published Wednesday in the journal Biology Letters.

The authors of the study guessed that koalas must do something to cool off in hot weather. To figure out what that was, they put radio collars on 37 koalas in southeastern Australia. They tracked koalas' movements during the winter of 2009 and the summer of 2010-11. In the land Down Under, summer lasts from December to March.

Watching Their Every Move

The researchers kept detailed records of what the koalas did. They noted their body position and whether they liked high or low branches. The scientists also used a portable weather station. The device recorded the temperature near where the koalas made themselves comfortable.

For comparison, the scientists also observed 130 koalas that weren't wearing collars.

What they discovered was that the koala behaved differently on hot days than on cooler days. In the summer, the animals positioned themselves in trees in a way that exposed more of their surface area. This often meant hugging a tree trunk or a large branch closer to the ground, with arms and legs outstretched. In the winter, koalas climbed higher in the tree and farther out onto the branches.

Why would different parts of the tree be appealing in different seasons? To answer this, the researchers took the temperatures of four species of trees in the heat of summer. Three of the trees were eucalyptus and one was an acacia.

With heat-imaging cameras, they discovered that the tree trunks were cooler than the branches or canopies. The trunks were also cooler than the surrounding air. This seemed to explain why koalas stayed closer to the base of the tree in the summer and up higher in winter.

Of all the trees tested, the coolest ones were the Acacia mearnsii trees, the scientists found. In summer, the koalas spent 29 percent of their time in these trees, but only 5 percent of their time in the winter. Koalas can't eat the leaves of these trees. But now scientists believe they know why they're so appealing to koalas, especially in summer.

The Math Behind The Tree Hugging

The scientists needed to check their theory. So they came up with an equation to determine how much heat a koala could lose by hugging a cool tree. The equation took into account the thickness of a koala's fur and how it conducts heat. They also measured how much of the koala's fur is in contact with the tree. They calculated that a 25-pound male koala could shed 68 percent of its excess heat on a 95-degree day by hugging a cool tree in a shady spot. That could mean the difference between life and death when water is scarce.

It's a clever way to chill out. And koalas probably aren't the only animals that realize it, the researchers wrote.

"Cool tree trunks are likely to provide important microhabitat for a broad range of tree-dwelling species ... during hot weather," they wrote. That would include primates, leopards, birds and insects.

This will help humans predict how animals will adapt to a warming climate. It will also help them choose the kinds of environments most critical to preserve, the researchers added.