Thermal implications of roost site selection in hibernating Plecotus townsendii

Call Number:

Keywords:

Abstract:

Winter survival for hibernating bat species depends on an ability to locate appropriate thermal environments and modify energy use patterns. Populations of Townsend's big-eared bats, Plecotus townsendii [Corynorhinus townsendii], successfully hibernate within magmatic caves in the cool sagebrush-steppe desert of the eastern Snake River Plain. The present study characterized thermal habitat within a natural hibernaculum, quantified thermal resource selection by roosting Plecotus townsendii, and evaluated the influence of the thermal environment on body temperatures and heat balance in torpid Plecotus townsendii. For the duration of the study (August 1992 to May 1993), a datalogger and array of thermocouples, deployed at fixed locations throughout the hibernaculum, provided continuous information on seasonal changes in cave temperature patterns as well as point-in-time characterizations of the thermal environment. With the automated temperature data, frequency distributions of available temperatures were created and compared to the distribution of bats within the thermal space. During periodic cave visits, supplemental manual temperature measurements were taken. Bats were sampled periodically throughout the winter for body temperature (Tb), body weight, and sex determinations. To allow for inter-cave comparisons, additional environmental temperature and roost location data were collected from four nearby Plecotus townsendii hibernacula. These data were of much lower resolution than those collected within the main study cave but proved invaluable for delineating preferred temperature ranges for the mid-winter period and for supporting asserted generalizations. Within the primary study cave, thermal resource selection was not evident early in the hibernation season. During mid-winter, strong thermal selection was displayed. Sixty-seven percent of the hibernating population was occupying roosts with environmental temperatures between 0.0° and 1.0°C; [areas of this temperature range] comprised only 16.6% of the surface area of available thermal habitat. Mean roost temperature (Trst) varied seasonally. During mid-hibernation, no measured Trst was above 3.0 °C in any hibernaculum despite the availability of warmer roosts. Caves not providing mid-winter temperatures below 3.0°C did not support hibernating populations of Plecotus townsendii. Differences based on sex or body weight were not significant. Patterns of Tb varied seasonally. Early season bats tended to resist cooling over the range of selected roost temperatures (0.9° to 3.8°C). The Tbs of mid season bats closely tracked the environment down to the sample mean Trst ( = 0.39 °C; SD ± 1.17) but elevated below this point. Although early season bats occupied warmer roost sites than mid season bats, body temperature to operative temperature differentials were not significantly different (p = 0.1). This finding suggested similar metabolic costs. Mid season bats fell into three groupings: bats exhibiting little change in body temperature to operative temperature differentials over a range of Tbs, bats exhibiting increases in body temperature to operative temperature differentials with linear increases in Tbs, and bats with low body temperature to operative temperature differentials and low Tbs. Plecotus townsendii in southeastern Idaho choose roost site temperatures close to freezing. Selected temperatures cluster near a critical thermal minimum down to which uniform endogenous heating is maintained. Below this temperature endogenous heat input is increased to prevent potentially lethal effects. Metabolic control over the range of selected temperatures is evident. Understanding habitat requirements, limits, and their basis are invaluable for the management and protection of sensitive species such as Townsend's big-eared bat.

Notes:

ELECTRONIC FILE - Zoology: Mammals