Potential energy expenditure by litter-roosting bats associated with temperature under leaf litter during winter
In temperate portions of North America, some bats that remain active during winter undergo short periods of hibernation below leaf litter on the forest floor during episodes of below-freezing weather. These winter roosts may provide above-freezing conditions, but the thermal conditions under leaf litter are unclear. Further, little is known of the relationship between temperatures under litter and potential energy expenditure by bats. Therefore, I characterized thermal conditions below leaf litter, compared temperatures encountered under different litter depths, and evaluated the quality of these sites as hibernacula based on potential energy use by eastern red bats (Lasiurus borealis) during winter in forests of the Ouachita Mountains, Arkansas, USA. Over an averaged 24-h period, there was no significant difference in temperature among different depths of leaf litter, but temperatures under litter remained significantly warmer than air temperatures, especially during nighttime and under snow cover. Temperatures below leaf litter were significantly warmer on south-facing slopes than north-facing slopes, but predicted metabolic rates did not differ among aspects. Predicted metabolic rates of eastern red bats were lowest under the deepest leaf litter measured (8cm) and highest under ambient air conditions. Depending on depth of leaf litter cover, predicted energy savings based on O² consumption from roosting under litter were 1.9 to 3.1 times greater than remaining in ambient air during periods of freezing weather and around 5.6 times greater when roosting under leaf litter with snow cover. A model for predicted total energy consumption (estimated as the total oxygen consumption during a 24-h period) by eastern red bats indicated that when roosting below leaf litter, energy consumption would be reduced with greater ground temperatures, greater leaf litter moisture, and when located on south-facing slopes. Predicted metabolic rates and total energy consumption may provide more insight on the quality of roost sites for wintering bats than temperature of roost sites alone.