Suominen, K. M. | bioRxiv Science

bioRxiv2024

Supersize me: torpor assists pre-hibernation fattening in a boreal bat

Hibernators face an energetic dilemma in the autumn at northern latitudes; while temperatures and food availability decrease, hibernating species need to build fat deposits to survive the winter. During this critical fattening phase, insectivorous boreal bats use torpor to build and conserve their reserves. However, we still know little about temporal variability in torpor use employed by bats in autumn and how decreasing temperatures and food availability in combination with increasing individual body mass impact this. Here we present two general hypotheses for explaining temporal torpor patterns observed in a boreal bat (Eptesicus nilssonii), in which torpor use I) facilitates rapid mass gain or II) conserves stored body mass. Although temporally separated in our dataset, temperature, insect abundance and body mass throughout autumn in the study system indicate that E. nilssonii reaches a majority of its overwintering mass before the onset of increasing daily and nightly torpor use. In combination with generally low food availability by this point in time, these observations suggest that torpor expression might be intended to conserve gained reserves. Our study is intended as a first proof-of-concept for disentangling temporal drivers of torpor in bats during the pre-hibernation fattening phase.

ecology↗

bioRxiv2024

Thermoregulation and Diurnal Roost Selection of Boreal Bats During Pre-Hibernation Period

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 night-time torpor during the pre- hibernation fattening period, most likely to facilitate rapid fat accumulation. Notably, bats commenced night-time 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.

ecology↗