The Misuse of Relative Humidity in Ecological Studies of Hibernating Bats

Abstract

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 t...