Bats are most diverse in the tropics, but there are no quantitative data on torpor use for energy conservation by any tropical bat in the wild. We examined the thermal biology, activity patterns and torpor use of two tree-roosting long-eared bats (Nyctophilus geoffroyi, 7.8 g) in tropical northern Australia in winter using temperature telemetry. Bats commenced activity about 20 min after sunset, ended activity about 2.5 h before sunrise and entered torpor everyday in the early morning even when minimum ambient temperatures (T (a)) were as high as 23°C. On average, bats remained torpid for almost 5 h, mean minimum skin temperature (T (skin)) measured was 22.8 ± 0.1°C and daily T (skin) minima were correlated with T(a). Our study shows that even in the tropics, torpor is frequently employed by bats, suggesting that worldwide most bat species are heterothermic and use torpor for energy conservation. We propose that the ability of employing torpor and the resulting highly plastic energy requirements may partially explain why these small insectivorous bats can inhabit almost the entire Australian continent despite vastly different climatic and likely trophic conditions. Reduced energy requirements also may permit survival in degraded or modified habitats, reduce the need for foraging and reduce exposure to predators. Thus, the ability to employ torpor may be one important reason for why most Australian bats and other heterothermic mammals have not gone extinct whereas many obligatory homeothermic mammals that cannot employ torpor and have high energy and foraging requirements have suffered high rates of extinctions.