Energy‐Exchange Analysis of the Belding Ground Squirrel and Its Habitat

An analysis is made of energy exchange between Belding ground squirrels (Citellus beldingi beldingi) and the microhabitats which they occupy during their annual summer period of activity. A natural population was observed, and the microclimates of six different types of microhabitats which the squirrels typically occupied are quantitatively described. Methods are derived for calculating the average radiation absorbed by a geometric model which is used to represent the squirrel at different orientations to the sun. The average absorbed radiation is plotted as a function of air temperature in climate diagrams that describe the thermal conditions occurring hourly in the microhabitats on clear, partly cloudy, and stormy days. The physiological and physical properties of the animals which affect energy exchange were measured in an open—flow metabolic apparatus. Body temperature, surface temperature, metabolic rate rate of evaporative water loss, and conductance are expressed as functions of effective ambient temperature. The capabilities of the squirrels for maintaining energy balance under extreme combinations of air temperature, absorbed radiation, and wind speed were calculated from physiological data by using an energy—budget equation, and are expressed on climate diagrams similar to those describing the thermal environments of the microhabitats. By comparing a climate diagram for a microhabitat with that for a squirrel, one can determine whether or not the squirrel is capable of maintaining thermal equilibrium in that microhabitat during any hour of the day. The net gain or loss of energy can be calculated from the climate diagrams for an animal in an environment in which it cannot maintain thermal equilibrium. The rate of change in body temperature resulting from energy imbalance is discussed in terms of its significance for animals occupying thermally unsuitable habitats. A method is proposed for comparing the nearly blackbody radiation environment of a metabolic chamber to complex natural thermal environments. With this technique it is possible to use laboratory data taken for a species at different blackbody ambient temperatures to estimate the physiological responses of that species to any natural environment where the wind speed, air temperature, and absorbed radiation are known.