Nest insulation: Energy savings to brown lemmings using a winter nest.

Energy metabolism of brown lemmings in summer pelage was measured over long periods at several air temperatures, with and without a real nest or artificial nest material. Resting metabolism of lemmings at T a=-16°C was 43% higher than that of lemmings in nests. As T a increased, the difference between resting metabolism of animals with and without nests decreased and was similar at T a=20°C. The energy saved at rest is equivalent to a reduction of approximately 40% in the thermal conductance. Independent estimates of energy savings due to nest insulation by analysis of cooling curves of a lemming model with and without a nest suggest a 46% reduction in thermal conductance due to the nest. At T a=0°C, baby lemmings huddled in a nest had equilibrium temperature excesses (T b-T a) four to five times higher than isolated nestlings outside the nest. These data indicate that there is a substantial energy savings at ecologically relevant air temperatures, and that energy savings increase as T a decreases. If the insulative value of the nest is similar whether the animal is in summer or winter pelage, these data suggest that heat production of a resting lemming would be 0.88 W (about 1.6 times BMR), while in nests at subnivean air temperatures typical of Barrow, Alaska, during the winter.