Why do koalas hug trees? Turns out it's less about looking cute than staying cool.
A new thermal imaging study shows that koalas rest against tree trunks because the bark's cool-to-the-touch temperature helps keep them from overheating in the heat of their Australia habitat.
"We found trunks of some tree species can be over five degrees Celsius cooler than the air during hot weather," Natalie Briscoe, a University of Melbourne researcher who led the study, said in a written statement.
For the study, Briscoe and her colleagues observed the behavior of 30 koalas during warm-weather days in Melbourne, Australia. Using a portable weather station/thermal imaging camera mounted on a long pole, the scientists were able to measure what temperature the koalas were feeling way up in the trees.
What did the researchers find?
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Thermal image of a male koala lying on the lower limb of an E. ovata tree during hot weather; by resting against the coolest part of the tree, koalas can reduce how much heat they need to lose by evaporative cooling.
“When we took the heat imagery it dramatically confirmed our idea that 'tree hugging' was an important cooling behavior in extreme heat," study co-author Dr. Michael Kearney, a researcher at the University of Melbourne's Department of Zoology, said in the statement. “Cool tree trunks are likely to be an important microhabitat during hot weather for other tree-dwelling species, including primates, leopards, birds and invertebrates."
A separate University of Sydney study conducted in 2013 showed that koalas seek out tall trees in cool low-lying areas, such as gullies, for shelter from the hot sun.
"Koalas have a fussy diet of eucalyptus, so they need a range of these food trees, but they need shadier trees too such as the Belah and Kurrajong varieties," Dr. Matthew Crowther, a wildlife management lecturer at the University of Sydney, told The Guardian in October 2013. "One of the biggest problems we face is the loss of these trees. As heat waves increase, we'll need more of these trees to maintain a healthy population as koala distribution changes."
Why don't koalas survive the heat by panting or licking their fur like other mammals? Briscoe said that doing so could leave the animals dehydrated.
"Access to these trees can save about half the water a koala would need to keep cool on a hot day," she said in the statement. "This significantly reduces the amount of heat stress for koalas." Point taken.
The new study was published online June 4, 2014 in the journal Biology Letters.