Trees provide koalas with more than food and shelter – they also serve as air conditioners, scientists say.
On a hot day in Australia, a koala's thick fur is not necessarily an asset. The marsupials don't retreat to dens or dugouts, and the water that would help them cool off can be hard to come by. In the wild, the animals "can suffer high mortality during extreme heat events," according to a report published Wednesday in the journal Biology Letters.
The authors of the study figured koalas must do something 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 posture and their preferences for high or low branches. The scientists also used a portable weather station to assess the microclimate conditions just a couple of feet from where the koalas made themselves comfortable.
For additional data, the scientists observed another 130 koalas in the area that weren't wearing collars.
What they discovered was that the koalas' posture on hot days was quite different than on cooler days. In the summer, the animals arrayed 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 further 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% of their time in the summer but only 5% 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, especially in the summer months.
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 dissipate by hugging a cool tree. After taking into account the thickness of a koala's compressed fur, the thermal conductivity of that fur and the surface area of the fur touching a tree (among other factors), they calculated that a 25-pound male koala could shed 68% of its excess heat on a 95-degree day by hugging a cool tree in a shady spot. That, in turn, 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, including primates, leopards, birds and invertebrates during hot weather," they wrote.