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

Abstract

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.