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Tree-huggers: koalas cuddle up to keep cool

Animals that live in hot climates have various behavioral adaptations to help keep them cool. Kangaroos lick their wrists; rats lick their testicles. New-world vultures urinate on themselves. Humans and horses sweat.

These behaviors all take advantage of the ability of evaporation to lower temperature by transferring heat energy from an animal’s body to the air. However, for animals that live in places that are both hot and dry, using valuable water stores to cool off may put them at risk of dehydration. Now, as described in a new study published in Biology Letters, it seems that koalas have figured out a way to stay both cool and dry: by hugging trees.

Thermal image of koala hugging a cool tree trunk during hot weather. Photo by Steve Griffiths.

Koalas (Phascolarctos cinereus) are arboreal marsupials that are native to eastern Australia. They depend almost entirely upon eucalyptus trees for shelter and food. Because eucalyptus leaves have very limited nutritional content, koalas are sedentary creatures that sleep for up to 20 hours of the day, often by splaying spread-eagle along large branches.

Researchers from universities in the U.S. and Australia decided that perhaps this sleeping style had more to it than met the eye, so they used thermal cameras to photograph koalas napping at French Island in the state of Victoria. They found that hugging trees enabled koalas to lose 68 percent of their excess body heat.

“When we took the heat imagery it dramatically confirmed our idea that ‘tree hugging’ was an important cooling behavior in extreme heat,” said Michael Kearney of the University of Melbourne.

The authors believe the koalas were able to keep cool because the trees remain significantly cooler than the ambient air temperature.

“We found trunks of some tree species can be more than five degrees Celsius cooler than the air during hot weather,” said lead researcher Natalie Briscoe, from the University of Melbourne.