Tree-hugging keeps koalas cool

A report published in Biology Letters today reveals how koalas keep cool in a heat wave - by hugging a tree.

A team of scientists studied the behaviour of koalas during the hot Australian summer to figure out how they keep cool. The animals boost their ability to cool down during heat waves by cuddling up to tree trunks which are colder than air temperature.

Lots of animals sweat to lose excess body heat into the environment when the sweat evaporates from their skin. Koalas don't sweat but pant and lick their fur instead to loose heat in the same way, a process called evaporative cooling. However, when water is scarce shedding liquids can be dangerous and when humidity is high cooling down this way is inefficient. Instead, in hot or humid weather, animals must modify their behaviour and seek out cooler microclimates or refuges.

The researchers observed koalas during the winter and the summer, recording their posture, activity, height and location in trees. They also kept track of the microclimates koalas were exposed to using a portable weather station.

The scientists found that when the weather was hot the koalas took refuge lower in the trees than when the weather was milder. Using a thermal camera to snap the trees the team found that closer to the ground they were at their coolest. The Acacia mearnsii tree, a species used by the koalas more often in hot weather, was up to 8.9°C cooler than air temperature at the base and 6.7°C at the mid trunk.

In high temperatures the team more often observed koalas with their limbs out stretched and positioned as if hugging the lower branches or trunks of the trees. Using a mathematical model they predicted that cuddling up to the cool trunks was boosting the koala’s loss of excess heat in the sweltering temperatures. The calculation showed hugging trees could slash koalas’ requirements to cool off through evaporative cooling by more than half, saving precious water and increasing survival.

‘With extreme heat events predicted to become more severe and frequent under climate change, understanding how animals use local microclimates is crucial,’ say the team who hope their results might guide conservation efforts to protect these vital refugees for koalas and other tree-dwelling animals.