Koalas (Phascolarctos cinereus) cope with extreme heat by resting against cooler tree trunks, according to a new study led by Dr Michael Kearney from the University of Melbourne’s Department of Zoology.

In Australia, 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. Understanding the types of factors that can make some populations more resilient is important,” said co-author Prof Andrew Krockenberger from James Cook University.

Dr Kearney, Prof Krockenberger and their colleagues observed the behavior of 30 koalas during hot weather at French Island, Victoria.

They 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 the places available to them.

“Koalas were observed hugging cool tree trunks,” said Dr Natalie Briscoe of James Cook University, the first author on the study.

“We found trunks of some tree species can be over 5 degrees Celsius cooler than the air during hot weather. Access to these trees can save about half the water a koala would need to keep cool on a hot day. This significantly reduces the amount of heat stress for koalas.”

Prof Krockenberger added: “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.”

“Access to cool tree trunks would significantly reduce the amount of heat stress for koalas,” Dr Kearney explained.

“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.”

The team also found that while panting and licking their fur helps koalas cool down, this can also lead to dehydration.

The results of the study are published in a paper in the journal Biology Letters.

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Natalie J. Briscoe et al. 2014. Tree-hugging koalas demonstrate a novel thermoregulatory mechanism for arboreal mammals. Biol. Lett., vol. 10, no. 6; doi: 10.1098/rsbl.2014.0235