Periodic arousals in hibernating mammals: is evaporative water loss involved?

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1. Using existing data on the rate of cutaneous and pulmonary evaporative water loss (EWL) for hibernating Little Brown Bats (Myotis lucifugus) and on the duration of torpor bouts, body temperature (Tb) and oxygen uptake (VO2) of Golden‐mantled Ground Squirrels (Spermophilus saturatus), the rate of EWL was modelled for ground squirrels hibernating at ambient temperatures (Ta) of –2, 2, 4 and 8 °C. 2. Total EWL showed a curvilinear response to Ta, being lowest at 2 °C and increasing with both increasing and decreasing Ta. EWL at –2 °C was about equal to that at 4 °C. The duration of torpor bouts showed the same curvilinear response to Ta and torpor bout duration at –2 °C was similar to that at 4 °C (8·5 vs 8·3 days, respectively). 3. At Ta≥ 2 °C, where Tb of torpid S. saturatus is not metabolically defended, torpor bout duration is significantly related to Tb, VO2 and EWL, with the three variables having similar r2 values. 4. Using the regression equations generated at Ta≥ 2 °C to predict torpor bout durations at –2 °C, where Tb is metabolically defended, shows that the three variables do not have equivalent predictive abilities. Tb and VO2 predicted torpor bout durations of 15·2 and –40·4 days, respectively, compared with observed durations of 8·5 days at –2 °C. EWL predicted torpor bout durations of 8·4 days or only 0·1 days less than that observed at –2 °C. 5. The relation between torpor bout duration and total EWL was insensitive to major variations in cutaneous EWL. Over Ta ranging from –2 °C to 8 °C, a stepwise multiple regression including Tb, VO2 and EWL as independent variables identified EWL as the only variable significantly correlated with torpor bout duration. 6. Our analyses suggest that torpor bout duration may be influenced by EWL, indicating that animals may need to obtain free water when they arouse. An analysis of the structural and temperature characteristics of ground squirrel hibernacula suggests that they may function as a biological condensing tower. Throughout winter, water should evaporate from the warm lower levels and condense in the colder upper regions and so free water may be available to animals when they arouse.