Australia's koalas cope with extreme heat by resting against cooler tree trunks, new research has revealed.
Thermal imaging uncovered the koalas' cool plan, confirming that they choose to hug trees that can be more than 5°C cooler than the air during hot weather.
Researchers observed the behaviour of 30 koalas during hot weather at French Island, Victoria. Co-author Andrew Krockenberger from James Cook University in Cairns, in far north-east Australia, says heat wave events can hit koala populations hard.
"We know that about a quarter of the koalas in one population in New South Wales died during a heat wave in 2009," Professor Krockenberger said.
"Understanding the types of factors that can make some populations more resilient is important." Koalas also pant and lick their fur to cool down, but that can lead to dehydration.
"Access to these trees can save about half the water a koala would need to keep cool on a hot day," lead researcher Dr Natalie Briscoe, from the University of Melbourne, said.
"Access to cool tree trunks would significantly reduce the amount of heat stress for koalas." Co-author Dr Michael Kearney said the findings were important as climate change is bringing about more extreme weather.
Researchers used a portable weather station on a long pole to measure what the koalas were experiencing in the places they chose to sit, compared to other places available to them.
"When we took the heat imagery it dramatically confirmed our idea that 'tree hugging' was an important cooling behaviour in extreme heat," Dr Michael Kearney said.
"Cool tree trunks are likely to be an important microhabitat during hot weather for other tree dwelling species including primates, leopards, birds and invertebrates.
"The availability of cooler trees should be considered when assessing habitat suitability under current and future climate scenarios."
Professor Krockenberger's research includes some of Australia's warmest koalas - the population on Magnetic Island, in the country's tropical northeast.
"These findings underscore the importance of trees to koalas especially, in the context of climate extremes," he said.
"In this study the coolest trees were acacias. They're not a koala food tree, but clearly they can be important when it comes to coping with the heat."
The study is published in the current edition of Biology Letters.