Hot climate, hot koalas: the role of weather, behaviour and disease on thermoregulation.

Thermoregulation is critical for endotherms living in hot, dry conditions, and maintaining optimal core body temperature ( T b ) in a changing climate is an increasingly challenging task for mammals. Koalas ( Phascolarctos cinereus ) have evolved physiological and behavioural strategies to maintain homeostasis and regulate their T b but are thought to be vulnerable to prolonged heat. We investigated how weather, behaviour and disease influence T b for wild, free-living koalas during summer in north-west New South Wales. We matched T b with daily behavioural observations in an ageing population where chlamydial disease is prevalent. Each individual koala had similar T b rhythms (average T b  = 36.4 ± 0.05°C), but male koalas had higher T b amplitude and more pronounced daily rhythm than females. Disease disrupted the 24-hr circadian pattern of T b . Koala T b increased with ambient temperature ( T a ). On the hottest day of the study (maximum T a  = 40.8°C), we recorded the highest ( T b  = 40.8°C) but also the lowest ( T b  = 32.4°C) T b ever documented for wild koalas, suggesting that they are more heterothermic than previously recognized. This requires individuals to predict days of extreme T a from overnight and early morning conditions, adjusting T b regulation accordingly, and it has never been reported before for koalas. The large diel amplitude and low minimum T b observed suggest that koalas at our study site are energetically and nutritionally compromised, likely due to their age. Behaviour (i.e. tree hugging and drinking water) was not effective in moderating T b . These results indicate that T a and koala T b are strongly interconnected and reinforce the importance of climate projections for predicting the future persistence of koalas throughout their current distribution. Global climate models forecast that dry, hot weather will continue to escalate and drought events will increase in frequency, duration and severity. This is likely to push koalas and other arboreal folivores towards their thermal limit.