Global climate change exposes small endotherms to high ambient temperatures (Ta), increasing the risk of dehydration and hyperthermia. These threats are disproportionately severe on cold adapted small endotherms with limited heat tolerance. Using temperature telemetry and flow through respirometry, we investigated the thermoregulatory strategies of lesser long-eared bats (Nyctophilus geoffroyi) inhabiting a temperate ecosystem. We predicted this population would have low heat tolerance limits and would initiate active thermoregulation at low Tas. We also predicted that, similar to other populations, females would have higher heat tolerance limits and a more conservative water economy than males given their reproductive ecology places a high demand on water reserves. In line with our predictions, the subcutaneous temperature (Tsub), whole animal resting metabolic rate (waRMR) and whole animal evaporative water loss (waEWL) data suggest that they experience heat stress at low Tas. Females could tolerate higher experimental Tas (females = 44 degrees C, males = 42 degrees C) and initiated evaporative cooling at a significantly higher Ta (37.7 degrees C) than males (33.1 degrees C), which is considerably lower than other populations of the same species adapted to arid environments. Our results highlight how different environment conditions and contrasting reproductive ecologies between sexes influence the thermoregulatory capacities and strategies of small endotherms. Although this population does not currently face consistent threats from lethal hyperthermia or dehydration, increasing Tas could expose them to sub-lethal fitness costs such as progressive mass loss due to higher energy expenditure for thermoregulation.