Evaporative Water Loss by Hibernating Little Brown Bats, Myotis lucifugus

We measured total evaporative water loss (EWL)for little brown bats (Myotis lucifugus) hibernating at 2° and 4°C. Total EWL was not influenced by body mass or temperature but was directly related to the difference in water vapor pressure (ΔWVP) between the tissue surfaces and the atmosphere. Evaporative water loss (mg bat⁻¹ d⁻¹) is 3763 ΔWVP (kPa). Metabolic water production, estimated at 1.68 mg bat⁻¹ d⁻¹, will not compensate for EWL except at relative humidities greater than 99.3% at normal hibernating temperatures. Net water losses range from 3.62 to 28 88 mg bat⁻¹ d⁻¹ at temperatures and humidities ranging from 2° to 4°C and 90% to 98%. Torpor durations predicted on the basis of net EWL correspond closely to those observed in laboratory and field studies, which suggests that water loss may be implicated in arousals from hibernation. We also measured respiratory frequency and tidal volume using a head-body plethysmograph. Bats showed an arrhythmic breathing pattern with bouts of apnea (31.6± 1.4 min) punctuated by ventilation bouts lasting 1.6± 0. 1 min and containing 71.2 ± 0 1 breaths. Tidal volume was 47± 2 μL, and minute volumes were 100.6 μL min⁻¹. Pulmonary water loss was only 0.3%6 of the total EWL because of the low ventilation rates