June 13, 2014 7:09 pm

Animal behaviour: Tree-hugging – a bear necessity for cooler koalas

Thermal imaging shows that the trunks of some acacia and eucalyptus species are cooler than their surroundings
©S Griffiths

Australian researchers have discovered why koalas hug trees. Thermal imaging shows that the trunks of some acacia and eucalyptus species are as much as 5C cooler than their surroundings in hot weather.

Scientists at the University of Melbourne observed 30 koalas living on French Island, off the coast of Victoria. They found that, although panting and fur-licking helped cool the animals, this behaviour could lead to dehydration. Lying with legs wrapped around the lower limb of a tree seemed a better strategy. “Access to these trees can save about half the water a koala would need to keep cool on a hot day,” says lead researcher Natalie Briscoe.

The team used a thermal camera and a portable weather station to assess the conditions available to the koalas in different places within the trees where they live. During heatwaves they were more likely to hug the trunks and lower branches than in cooler weather, when they lived higher in the canopy. They also frequented acacias, the species with the coolest trunks, more in extreme heat than in normal conditions, when they preferred eucalyptus trees.

Koalas are vulnerable to thermal stress – as many as a quarter of some populations have died in heatwaves. But the researchers’ findings are significant to a wider range of arboreal animals, as climate change brings more extreme weather.

“Cool tree trunks are likely to be an important microhabitat during hot weather for other tree-dwelling species including primates, leopards, birds and invertebrates,” says Michael Kearney, co-author of the study published in the journal Biology Letters. “The availability of cooler trees should be considered when assessing habitat suitability under current and future climate scenarios.”

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