Endotherms maintain constant body temperature through physiological and behavioral adjustments. Behavioral thermoregulation is an important factor influencing energy balance. We exposed the leaf-eared mouse, Phyllotis darwini, to temperatures corresponding to its natural thermal range and studied two forms of behavioral thermoregulation: diminishing surface to volume ratio by huddling and heat dissipation by increasing physical contact with the substrate (flattening). We predicted that at low ambient temperatures (T(a)) huddling would be used as a heat conservation mechanism and at high T(a) flattening would be used for heat loss. We simultaneously measured oxygen consumption (VO2) and flattening, in response to three independent factors: huddling, T(a), and body mass. Each experiment was a 6-h VO2 trial where five virgin females were measured at constant T(a). We performed this protocol for two body mass groups, small (ca. 40 g) and large (ca. 70 g), in a metabolic chamber. Treatments were groups with and without the ability to huddle at five different T(a), ranging from 5 degrees to 35 degrees C. A significant interaction between all three factors was found. Huddling and flattening were used as strategies for conserving or dissipating heat, respectively, and the shift between both strategies occurred at the lower limit of thermoneutrality. At T(a) below thermoneutrality, huddling was a more effective way of reducing metabolic requirements and was more efficient (H(E)) in small individuals than large individuals. So, by huddling, small individuals save more energy. At high T(a), flattening appeared to be an equally useful mechanism for heat loss, for both large and small animals.