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Keeping cool. Thermal imaging reveals the temperature difference between a tree, a hot koala, and the surrounding air.

Keeping cool. Thermal imaging reveals the temperature difference between a tree, a hot koala, and the surrounding air.

Steve Griffiths

Trees Become Refrigerators for Koalas

When they wrap their limbs around the wide trunks of acacia trees, koalas aren’t just trying to keep from falling—the trees help the marsupials cool off, researchers have discovered. The findings could impact koala conservation efforts as well as forecasts on how climate change will affect the animals.

“I think this is really exciting,” says koala ecologist William Ellis of the University of Queensland, St. Lucia, in Australia, who was not involved in the new work. “The way koalas use their environment appears to be quite complex, and this just adds another layer to it.”

Koalas are known for their limited diet—they survive solely on the leaves of particular species of eucalyptus trees. Even their water is mostly obtained from the leaves, making dehydration a danger, especially during Australia’s heat waves. But despite their reliance on eucalyptus for food and water, the animals also hang out on the trunks of other trees.

“We were puzzled why they’d spend time and energy swapping trees when they were so clearly hot and exhausted,” says zoologist Michael Kearney of the University of Melbourne, Parkville, in Australia. He and his colleagues suspected that the koalas might be trying to find spots with the most shade or breeze. To test their hypothesis, they used a miniature weather station, attached to a pole for easy transportation and control, to gather data on the weather around—and within the branches of—different trees. First, they collected data around trees in southeastern Australia that 37 radio-tagged koalas spent time around during different days. Then, they got a random sampling of readings on hundreds of nearby trees that the koalas didn’t choose to relax on. Finally, to get more information on how heat moved in and out of the trees, the researchers snapped thermal images of the four species of trees most often used by the koalas—three eucalyptus species and the Australian acacia.

Whereas koalas sat in acacia trees only 5% of the time on cooler days, they were in the acacia trees almost a third of the time when the temperature exceeded 35°C. The preference couldn’t be explained by local temperature, humidity, or wind speed, which were all similar around the four tree species, and the acacia trees provided no extra shade. But the thermal imaging revealed something else. During hot weather, all four tree species had trunks cooler than the air temperature. Yet whereas eucalyptus trunks were less than 2°C cooler than the surrounding air, acacia tree trunks averaged almost 7°C colder. So the koalas spent more time on the acacia trees in hotter weather to help cool themselves down and avoid water loss, the team reports online today in Biology Letters.

“This shows the many tricks that animals have up their sleeve to survive in harsh environments,” Kearney says. “I think it also emphasizes that even warm-blooded animals can be dependent on their environment for temperature regulation.”

The new observation suggests that when scientists are trying to restore, protect, or recreate ideal koala habitats, they need to focus on more than just food-bearing trees—cool trunks may be key to helping the animals survive droughts and heat waves. “This certainly has implications for choosing suitable reserves for koalas,” Kearney says. “Now all of a sudden there’s something really important that we weren’t considering in the past.”

Ellis says more research will be needed, though, to determine whether the findings hold true in all koala habitats—Kearney’s team studied koalas only in the southern end of the animals’ range, where the climate is generally less harsh. “I’d be interested to see if the advantage of these trees holds true in other areas with increased humidity or longer periods of heat,” Ellis says. “It may be that the animals can’t dump heat into a tree after a certain time.”