During the hot Australian summers, mammals run the very serious risk of overheating. One important avenue for heat loss for mammals and birds is the cooling through evaporation that occurs during sweating, panting or fur licking. The problem is that evaporative cooling is costly; large amounts of water are lost and when water is a limited resource, animals must use other heat-loss mechanisms in order to avoid overheating and dehydration. Many Australian mammals face this conundrum during the very dry and hot summers. Koalas, in particular, obtain most of their water from the eucalyptus leaves they eat and may not even have access to water during dry periods. This means that in order to conserve water, they must use other physiological or behavioural mechanisms to cool down. Natalie Briscoe and her colleagues from the University of Melbourne, La Trobe University, James Cook University and the University of Wisconsin, were intrigued by this possibility and decided to study koalas in their natural environments to determine whether these cuddly marsupials would change their behaviour during the hot months in order to keep their cool.
Briscoe and her team radio-tracked koalas during the winter and summer months and recorded behaviours such as posture, activity, and height and location in the tree, as well as microclimate and habitat data. They also recorded the temperature of different parts of trees using a thermal imaging camera. Finally, they used a biophysical model of heat exchange to predict what the koalas' heat-loss requirements are under different environmental conditions and the potential for conductive heat loss when koalas are in direct contact with cooler tree trunks and branches.
Their biophysical model predicted that during hot days, hugging trees would allow koalas to lose enough heat such that the need for evaporative cooling could be substantially reduced or even entirely abolished. Indeed, Briscoe's observations indicate that koalas choose thicker tree trunks and branches (larger surface area for conductive heat loss) during hot weather and they were more often observed on the main tree trunks rather than on branches at these times. Koalas also used non-food trees more often during hot weather than during mild conditions. Interestingly, these species of trees were also cooler than food trees, sometimes up to 8.9°C lower than the environmental temperature, providing a large temperature gradient to aid in heat loss.
When we think of koalas, we usually form the mental image of a cuddly bear-like marsupial holding on to a eucalyptus tree. Rarely do we think, however, that ‘tree hugging’ is much more than a way of passing time for koalas or of being close to their lunch, and that, in fact, it might be a crucial behaviour for their survival during hot weather. With increasingly hotter and dryer seasons predicted as climate changes, and during the already recurring heat waves of the Australian summers, tree trunks could be important heat refuges for many arboreal species, including koalas.
