Sublethal effects of extreme heat on a critically endangered marsupial

Context Extreme heat events are intensifying under climate change, yet their sublethal effects on threatened mammals remain poorly understood. Aims We investigated behavioural responses of the critically endangered western ringtail possum (Pseudocheirus occidentalis; ngwayir) to extreme heat on the Southern Swan Coastal Plain of Western Australia. Methods Using tri-axial accelerometer collars, we quantified diel activity patterns of 10 ngwayirs across two peppermint (Agonis flexuosa) woodland populations during summer of 2024–25, encompassing several heatwaves. Generalised additive models were used to examine how activity varied with time of day, sex, and daily maximum temperature. Key results Activity was strongly structured by temperature and diel period. On very hot days (>40°C), nocturnal activity declined by up to 43% in males and 31% in females compared with cooler days (27°C), with the largest reductions during early-evening peaks. Activity increased later at night as temperatures declined, indicating temporal shifts in foraging effort. These results demonstrate pronounced behavioural sensitivity to thermal conditions below lethal thresholds. Conclusions Ngwayirs reduce nocturnal activity during extreme heat, consistent with short-term behavioural thermoregulation. Implications Sublethal heat exposure significantly alters daily activity patterns in this critically endangered marsupial. Such behavioural suppression likely carries energetic costs that could reduce fitness and reproductive output. Integrating behavioural responses into climate-vulnerability assessments is essential for predicting species persistence under increasing heatwave frequency.