Thermoregulation in resting and active polar bears

Summary Polar bears (Ursus maritimus) regulate their body temperatures both physiologically and behaviourally proportional to their level of activity while within the thermoneutral zone. Core temperatures (T _c=36.9±0.5°C at rest) varied with the 4^th power of walking speed for the two subadult (220 kg) bears tested, whereas subcutaneous temperatures (T _sk=35.3±2.2°C at rest) were closely correlated withT _c but also varied with wind speed (v _a) and ambient temperature (T _a). Radiative fur temperatures (T _r) were closely correlated withT _a and negligibly withT _sk. Predictive equations for these temperature relationships were derived by regression analysis. Maximum rates of heat storage (S _max) were above that predicted from the literature implying that the polar bear is an energetically costly walker. Radiative heat losses of a resting polar bear amount to between 36–67% of the metabolism and assuming a respiratory heat loss of 7–10%, convective heat losses (by difference) would thus range from 33–64%. When walking, the cooling of the fur surface by forced convection and the pendulum effect of the moving legs of the bear lead to estimated convective heat losses on the order of 75% of the heat production while radiative losses are reduced to 13–22%. Increasing wind speeds enhance further this reciprocal effect.