Activity during natural hibernation in three species of Vespertilionid bats

1. Patterns of winter-activity in three species of bats (Myotis mystacinus, M. daubentoni, M. dasycneme) were studied in three hibernation quarters in The Netherlands. The methods of investigation involved: individual marking, automatic recording of intracave and extracave flights and assessment of the frequency of movements between hibernation sites by short-interval searches. 2. Although differing in details, the species studied show close agreement in most aspects of hibernation behaviour. Major differences probably exist with other species in other climates. 3. Arrivals in the cave takes place gradually from September to December. Most bats leave the cave between the middle of March and the middle of April. There are consistent specific and sexual differences in the timing of hibernation. Females tend to arrive and to leave earlier than the males. Departure from the cave in spring is correlated with the prevailing temperature, while the arrival in autumn is not. 4. Population turnover during the winter is small. No more than an estimated 10% of the bats venture outside the cave upon waking. Recorded extracave migrations took place mainly in autumn and over distances not exceeding 1 km. 5. Microhabitat selection is such that the bats throughout the winter tend to concentrate in the coldest parts of the cave. Low outside temperatures cause cooling of the cave near the entrance during the winter season, and the bats respond by shifting their positions from the rear to the front of the cave, whereas on occasional warm days they tend to move to the rear. In the front part of the cave where stronger air circulation and temperature fluctuations prevail, bats tend to hide in fissures, while in the thermoconstant rear they prefer exposed locations. 6. Myotis mystacinus and M. daubentoni are solitary hibernating bats. In M. dasycneme 10% of the locations were shared simultaneously by two or more individuals. Observed matings were concentrated in autumn. 7. Some bats return to exactly the same locations in consecutive winters. A positive correlation between sequences of locations occupied by (presumably) the same individuals in different years leads to the hypothesis that the selection of locations, which were occupied in former years, is connected to an endogenous annual program clock. 8. The frequency of intracave movements shows large seasonal variation, being relatively high in autumn and spring, and low in mid-winter. The occurrence of the same patterns in the rear part of the cave under almost constant conditions suggests the endogenous nature of a seasonal variation in arousal frequency. 9. The frequency of movements from one resting site to another was positively correlated with ambient temperature. High outside temperatures (directly or indirectly, by reversed thermocirculation) had a positive effect on the frequency of movements from locations in the front part of the cave. 10. The daily distribution of recorded flights shows that the activity of the hibernating bats is almost exclusively nocturnal in October. With a decrease in activity towards mid-winter, flights become more uniformly distributed over the 24 hrs and their mean shifts significantly to a later part of the night. The increase in activity in spring is accompanied by the return of nocturnalism. 11. During the hibernation extracave flights take place mainly between sunset and sunrise. Occasional outward flights in daytime are usually immediately followed by returns into the hibernaculum. 12. Weight loss of the bats is rapid in the first half of the hibernation season and occurs at a slower rate in the second half. It is concluded that feeding does not occur during the major part of the winter. 13. Energetical implications of winter activity in bats are discussed, and compared with other hibernators. The arousals from hibernation constitute a major proportion of bat energy expenditure during the winter. This expenditure of energy with the possibility of starvation in spring is among the greatest hazards to which temperate zone bats are exposed. The hypothesis is advanced that the energetic balance between gain due to successful feeding activity on mild autumn and spring days and loss due to arousal and movements gives positive survival value to an endogenous seasonal variation of arousal frequency