Koalas spend a lot of their time asleep, clinging to tree branches. But far from being lazy, it seems tree-hugging is an efficient way to keep cool. The trick should ensure that koalas lose half as much water through sweat as they otherwise would.
Koalas (Phascolarctos cinereus ) regularly endure temperatures above 40 °C. So Natalie Briscoe from the University of Melbourne in Australia expected them to move into shadier areas when the temperature rose. But, in fact, they hang out in shady areas most of the time anyway.
"The koalas' behaviour didn't make a lot of sense to us," says her colleague Michael Kearney. "As it got hotter, they started moving down to the tree trunks and hugged really tightly to the trunks."
So Briscoe used infrared cameras to measure the temperatures of the trunks and found they were much cooler than the air on hot days. At that point, "it was absolutely obvious what they were doing," says Kearney.
Aircon trunks
Briscoe tracked 37 koalas and watched their behaviour. As well as shifting to lower parts of the trunks in hot weather, they also changed their posture, tending to hug branches when it was very warm, but sit upright in mild weather. In extremely hot conditions, they even spent time hugging the trunks of wattle trees, which they can't eat, but which have cooler trunks.
The trees are probably cooled by the roots sucking up water from underground where the sun doesn't heat the soil so much, says Kearney.
A computer model suggested that, on a typical hot day, a koala that switched to hugging the lower tree trunks would lose half as much water through sweat as one that didn't. That's good for an animal that has thick fur and almost never drinks water.
Kearney says other mammals often adapt their behaviour to keep cool, just as humans do. But this is the first time tree-hugging has been shown to be used this way. "It could be what leopards in Africa are doing too," says Kearney.
Koalas' eucalyptus habitat is shrinking and their climate is warming. "Understanding what it is about the trees that makes a suitable habitat is absolutely essential for conservation," says Bill Ellis of the University of Queensland in Brisbane.
Koalas also make terrifying growling sounds using their unique vocal organs
Journal reference: Biology Letters, DOI: 10.1098/rsbl.2014.0235
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
