Stage of pregnancy dictates heterothermy in temperate forest-dwelling bats

•Data combination of two years using temperature correction.•Analysis of torpor use in three bats species during pregnancy on a daily basis.•Close to parturition torpor depth was not species-specific.

Bats face high energetic requirements, as powered flight is costly and they have a disadvantageous surface-to-volume-ratio. To deal with those requirements energy saving mechanisms, such as heterothermy (torpor), have evolved. Torpor during pregnancy, however, reduces rates of foetal development and consequently prolongs pregnancy. Therefore, heterothermy has a great effect on reproduction, as an unhindered parturition can only be assured by high body temperatures. Regardless of these adverse affects of torpor the energetic requirements of bats during reproduction urge for energy savings and bats are known to enter torpor during pregnancy. The species in the current study differ in their torpor patterns and thus their heterothermic strategy. However, we hypothesized, that species-specific heterothermic behaviour should be revoked at the end of pregnancy. We analyzed skin temperatures of Myotis bechsteinii, Myotis nattereri and Plecotus auritus during pregnancy and found no differences in torpor depth between species during the last phase of pregnancy. Furthermore, we could show that individuals entered torpor frequently during pregnancy and only minimized torpor during the last stage of pregnancy. This suggests that close to the end of pregnancy, heterothermy is restricted but not species-specific and the required energy is allocated otherwise.