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 is not an asset. The over-heated marsupials don't retreat to dens or dugouts to keep cool. 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 figured koalas must have some way to cool off in hot weather. To figure out what it was, they put radio collars on 37 koalas in southeastern Australia and tracked their movements during the winter of 2009 and the summer of 2010-11. In the land Down Under, summer lasts from December to March.

The researchers kept detailed records of these koalas' activities, noting their body position and their preferences for high or low branches. The scientists also used a portable weather station to record the temperature just a couple of feet from where the koalas made themselves comfortable.

For comparison, the scientists observed 130 koalas that were not wearing collars.

A Cooler Arrangement

They discovered that koalas behaved differently on hot days than on cooler days. In the summer heat, the animals arranged 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 ventured higher up in the tree and farther out onto the branches.

Why would different parts of the tree be appealing in different seasons? To answer this question, the researchers took the temperatures of four species of trees - three eucalyptus and one acacia - at the peak of summer.

Using a thermal-imaging camera and software, they discovered that the tree trunks had lower surface temperatures 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 ventured farther up the tree in winter.

Of all the trees tested, the coolest ones were the Acacia mearnsii trees, the scientists found. As it happens, these were the trees where koalas spent 29 percent of their time in the summer 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 the Acacia mearnsii is so appealing to koalas, especially in the summer months.

Other Animals Chill Out As Well

To make sure that their trees-as-air-conditioners theory was plausible, they came up with an equation to determine just how much heat a koala could shed by hugging a cool tree. They had to take into account the thickness of a koala's compressed fur and how much its fur conducts heat. Another factor is how much of the koala's fur touches 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 was scarce.

It's an ingenious strategy for chilling 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. The species would range from primates and leopards to birds and insects.

Knowing this will help humans predict how animals will adapt to a warming climate. People would also have some of the information they need to decide what kinds of habitat are most critical to preserve, the researchers added.]]>