Tree-hugging beats the heat
Briscoe, NJ 2015, Temperature, vol. 2, no. 1, pp. 33-35.
Koalas change their behaviour to use temperature variation in microclimates to cool themselves down by hugging cool tree surfaces during hot weather to increase heat loss. They were observed to minimise evaporative water loss through spending more time in shaded areas and hugging trunks of trees of the species Acacia , which has a cool trunk temperature.
Radio-tracking devices were placed on koalas and their behaviours recorded over different temperature days. The koalas were found to select microclimates with lower solar radiation loads on hot days (air temperature of greater than 30 °C). On hot days, koalas were also disproportionately found on Acacia trees, a species they don’t feed on, sitting on the main trunk and moving closer to the ground with increasing air temperature. Acacia were found to have trunk temperatures of 5 °C cooler than the air temperature, with the base and main trunk being the coolest part of the tree. By pressing themselves against the tree trunks, the koalas could increase conductive heat loss and decrease their exposed surface area for radiative heat gain, and thereby decrease the need for evaporative heat loss by half. Koalas have previously been observed to use evaporative cooling through panting and salivating on their wrists and face at temperatures above 30 °C, but this increases risk of dehydration, demonstrating that conductive heat loss is a less costly heat loss mechanism. Previous research has also found that desert birds and flying foxes cool themselves on cool tree trunks, suggesting a wide range of species may use this technique. The tree trunks in this study had cool ground water being drawn into the tree through transpiration and minimal direct solar radiation, lowering their temperatures. There was a range in thermal profiles between tree species, suggesting properties including root depth and bark are important in trunk temperature determination.
Understanding koala behaviour around temperature variation facilitates better conservation programs by creating improved habitats for koalas, such as by adding trees with significant shade and cool trunk surfaces rather than just food trees. This knowledge will become increasingly important as global warming changes koala habitats.
Summarised by Laura Wait
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