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

Abstract

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.