In the Australian outback, summer temperatures are scorching and can exceed forty degrees. A koala's thick fur (Phascolarctos cinereus) can be quite a problem in conditions like these.

These endearing animals, however, unlike other marsupials, do not retreat to burrows to lower their body temperature — they hug trees. The image of a koala nestled in a eucalyptus tree is a classic one, but scientists are now offering a new interpretation of it.

"If we had infrared vision, we would have already known why they do it," writes Natalie Briscoe, assistant to Michael Kearney, a zoology lecturer at the University of Melbourne who leads the project. By using a thermal imaging camera to measure the microclimates of koala habitats, Briscoe found that the tree branches on which koalas rested were more than 5 degrees cooler than the surrounding air.

The researchers monitored 37 radio-collared individuals to track the koalas' habits and measure surrounding temperatures. The data collected allowed scientists to observe variations in the animals' behaviour according to the climate. On cooler days, below 25 degrees, they sit in the higher branches of trees, while on hotter days they favour the lower branches, against which they stretch out so as to expose the largest possible surface area to contact with the tree.

When it is very hot, koalas also descend from their favoured trees — eucalyptuses, from which they obtain both food and fluids by eating up to one kilogram of leaves a day — to move onto acacias, whose trunks are even cooler. The study, published in the journal Biology Letters, establishes that hugging trees allows koalas to cool down by up to 68 per cent, thereby reducing the need to evaporate precious bodily fluids.

This clever cooling strategy is likely used by other animals too, argues Professor Michael Kearney. "Tree trunks are fantastic habitats and host a wide range of tree-dwelling species, especially during the hottest hours, including primates, leopards, birds and invertebrates." This discovery will help biologists predict the adaptive strategies animals use to cope with the constant rise in temperatures, providing important clues as to which habitats are most important to preserve.