Tree-hugging koalas demonstrate a novel thermoregulatory mechanism for arboreal mammals

Abstract

How climate impacts organisms depends not only on their physiology, but also whether they can buffer themselves against climate variability via their behaviour. One of the way species can withstand hot temperatures is by seeking out cool microclimates, but only if their habitat provides such refugia. Here, we describe a novel thermoregulatory strategy in an arboreal mammal, the koala Phascolarctos cinereus. During hot weather, koalas enhanced conductive heat loss by seeking out and resting against tree trunks that were substantially cooler than ambient air temperature. Using a biophysical model of heat exchange, we show that this behaviour greatly reduces the amount of heat that must be lost via evaporative cooling, potentially increasing koala survival during extreme heat events. While it has long been known that internal temperatures of trees differ from ambient air temperatures, the relevance of this for arboreal and semi-arboreal mammals has not previously been explored. Our results highlight the important role of tree trunks as aboveground 'heat sinks', providing cool local microenvironments not only for koalas, but also for all tree-dwelling species.

Keywords: behavioural thermoregulation; biophysical models; climate change; koala; microclimate selection.

Figure 1.

Koala posture under mild (grey; n = 121) and hot (black; n = 102) conditions in southeastern Australia. Postures were divided into five categories, ranging from lowest surface area (left) to highest surface area (right), with examples provided below the axis.

Figure 2.

(a) Thermal image of a koala hugging the cool lower limb of a tree, illustrating a posture typically observed during hot weather, (b) Thermal profiles (mean surface temperature – air temperature ± s.e.m.) of four dominant tree species (Eucalyptus obliqua, E. viminalis, Acacia mearnsii and E. ovata), (c) Predicted heat loss requirements (W) of koalas with (grey) and without (black) tree-hugging behaviour during a typical hot day in southeastern Australia. Dashed line indicates air temperature (right axis).