Physiological traits affecting the distribution and wintering strategy of the bat Tadarida teniotis

The ability to enter torpor at low ambient temperature, which enables insec tivorous bats to survive seasonal food shortage, is often seen as a prerequ isite for colonizing cold environments. Free-tailed bats (Molossidae) show a distribution with a maximum latitudinal extension that appears to be inte rmediate between truly tropical and temperate-zone bat families. We therefo re tested the hypothesis that Tadarida teniotis, the molossid species reach ing the highest latitude worldwide (46 degrees N), lacks the extreme physio logical adaptations to cold that enable other sympatric bats to enter furth er into the temperate zone. We studied the metabolism of individuals subjec ted to various ambient temperatures in the laboratory by respirometry, and we monitored the body temperature of free-ranging individuals in winter and early spring in the Swiss Alps using temperature-sensitive radio-tags. For comparison, metabolic data were obtained from Nyctalus noctula, a typicall y hibernating vespertilionid bat of similar body size and convergent foragi ng tactics. The metabolic data support the hypothesis that T. teniotis cann ot experience such low ambient temperatures as sympatric temperate-zone ves pertilionid bats without incurring much higher energetic costs for thermoge nesis. The minimum rate of metabolism in torpor was obtained at 7.5 degrees -10 degrees C in T. teniotis, as compared to 2.5 degrees-5 degrees C in N. noctula. Field data showed that T. teniotis behaves as a classic thermo-con forming hibernator in the Alps, with torpor bouts lasting up to 8 d. This c ontradicts the widely accepted opinion that Molossidae are nonhibernating b ars. However, average body temperature (10 degrees-13 degrees C) and mean a rousal frequency (3.4 d in one bat in January) appear to be markedly higher than in other temperate-zone bat species. At the northern border of its ra nge T. teniotis selects relatively warm roosts (crevices in tall, south-exp osed limestone cliffs) in winter where temperatures oscillate around 10 deg rees C. By this means, T. teniotis apparently avoids the risk of prolonged exposure to energetically critical ambient temperatures in torpor (<6.5 deg rees-7.5 degrees C) during cold spells. Possibly shared by other Molossidae , the physiological pattern observed in T. teniotis may clearly be linked t o the intermediate latitudinal extension of this bat family.