Energetic costs of the winter arboreal microclimate: The gray squirrel in a tree

Heated taxidermic mounts of the gray squirrel were used to analyze the thermal environment of a small arboreal endotherm. Changes in the standard operative temperature (T_es) calculated from the temperatures of heated and unheated mounts agreed well with the power consumption (M-E) of mounts on the ground and on the wind-ward side of a 48-cm diameter tree trunk. As wind speed (u) rose and sky solar radiation (Q_r) decreased, the windward side of the tree trunk became an increasingly more stressful thermal environment than the leeward side of the trunk or the ground, producing M-E differences of more than 30%. Although the M-E of a ground mount and a limb mount 4 m in the air are dependent on Q_ras well as u, the ratio of the two value of M-E is independent of Q_r, poorly predicted by u and well predicted by u^1/2.