Field biologists almost invariably report relative humidity as a measure of moisture in the air and assume that relative humidity somehow predicts evaporative water loss from an animal. In this paper, I use the vapor pressure gradient to show that, under conditions of constant relative humidity, evaporative water loss from the body surface of a hibernating bat can vary by more than 100%, depending on ambient temperature. Potential evaporative water loss at constant relative humidity is an increasing curvilinear function of ambient temperature for a torpid bat that has a surface temperature equal to surrounding air temperature, but a decreasing curvilinear function of air temperature for an aroused bat in the hibernaculum. Under some circumstances, evaporative loss actually can be greater in a hibernaculum with higher relative humidity than in one with lower relative humidity. When examining potential differences in evaporative water loss between sites, habitats, or treatments, biologists should consider the absolute (not relative) level of ambient moisture, as well as the surface temperature of the animal, which greatly affects the tendency of water molecules to evaporate.
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Vol. 16 • No. 1