Hot days increase the risk of heat-stress-related deaths in endangered koala populations

Heat exposure has been recognized as an increasingly important driver of wildlife vulnerability worldwide, making it essential to understand how rising temperatures affect species with limited physiological flexibility. We used a case-crossover analytical framework from environmental epidemiology to evaluate the association between ambient temperature and koala admission into care and mortality across New South Wales, Australia. We examined 11 862 koala rescue admissions recorded between 2000 and 2022. Risk of admission and mortality for adult koalas increased once the 7-day average maximum daily temperature exceeded 27°C, with risk continuing to rise at higher temperatures. Exposure above 30°C increased the odds of admission or death 1.5-3.5 times compared with individuals exposed to 25°C. Mapping heat-related risk indicated that inland north-west populations face the greatest exposure to high temperatures. We list intervention guidelines to support prevention, preparedness and response in high-risk regions. Our results highlight how even moderate temperature increases can elevate mortality risk in a thermally sensitive mammal, underscoring the need to identify and mitigate heat-related threats for wildlife as climates continue to warm. We adapted a technique used in human health to wildlife veterinary admission data and provided a novel framework for quantifying the impacts of extreme heat on vulnerable free-ranging populations.