Living in a seasonal environment poses challenges for small mammals, such as bats, reliant on insects as their primary food source. Bats may adeptly navigate these energetic challenges by reducing their metabolism and body temperature, entering a state of torpor. Particularly during the winter, bats remain torpid for extended periods, but are dependent on sufficient energy reserves to survive until spring. With the onset of autumn and declining temperature, bats face the challenge of building their fat deposits during a time of decreasing food availability. Bats may therefore transition to cooler roosts to initiate torpor, thereby reducing energy expenditure. However, little is still known about torpor use or roost selection by bats in autumn. This study explores the factors influencing roost selection and torpor use and --duration in two bat species during this critical transition period between the breeding and overwintering season. We show that date in autumn is a stronger driver of torpor use than prevailing ambient temperature, and that bats employ specific strategies in which they first increase daytime torpor use before also increasing the use of nighttime torpor during the pre-hibernation fattening period, most likely to facilitate rapid fat accumulation. Notably, bats commenced nighttime torpor use after spending entire days in torpor. These findings underscore the dynamic nature of torpor and the energy-saving strategies employed during the crucial pre-hibernation period, marking the transition from summer to winter.
