Modes of thermal protection in polar bear cubs- at birth and on emergence from the den ARNOLDUS SCHYTTE BLIX AND JACK W. LENTFER Institute of Arctic Biology, University of Alaska, Fairbanks 99701; and US National Fish and Wildlife Laboratorv Anchorage, Alaska 99503 II 1 BLIX, ARNOLDUS SCHYTTE, AND JACK W. LENTFER.MO&S of therms/ protection in polcrr bew cubs-at birth and OIZ wwrge~~~ from the &I. Am. J. Physiol . 236 (1): R67-R74, 1979 or Am. J- Physiol.: Regulatory Integrative Comp. Physiol. 8 1): R67-R74, 1979. -At birth in late December the polar bear is small (700 g), uninsulated, and helpless. It probably has a modest capacity for metabolic heat production and depends on the female and a snow den in which it is born for thermal protection. The microclimate of an artificial polar bear den was investigated at Point Barrow, AK, and the temperature therein found to stay around 0°C provided a heat source (200 W) equivalent to an adult polar bear was introduced. When the bears desert the den in early April the cub has grown to about 10 kg and has a well-developed fur insulation, but almost no subcutaneous fat. The cub has a high resting metabolic rate (4.6 W-kg I), which is supported by the fat polar bear milk. Its lower critical temperature is about -30°C, and an ambient temperature of -45°C results in only a 33% increase in metabolism. The cub can tolerate a wind chill of 2.3 kW hrn? without apparent stress or drop in rectal tempera- ture. If the cub is immersed in ice water rectal temperature drops 11°C in 30 min. It is concluded that the cub can tolerate extremely low temperatures in air due to fur insulation and high metabolic heat production, but is unable to cope with the chill of ice water for any prolonged period of time. u?-SUS rYlClFitiF71 US; temperature regulation; cold tolerance; metabolism; wind chill POLAR BEARS (Ursus maritimus) give birth in snow dens in December, when outside temperatures often are below -40°C. Occasionally, polar bears have given birth in captivity, and the newborn cub has been described as small (600-800 g), blind, and sparsely haired (9). Knowledge of the natural den microclimate is scanty. Temperatures of -lO°C (6) and -18°C (7) have been reported from dens in Canada. Temperatures well below zero (-5 to - 13°C) have also been reported by Belikov t 1) from the Soviet Union. Although these records hardly reflect the microclimate near the newborn cub they undoubtedly point to a rather cold environment. We recently had the unusual opportunity to examine two newborn polar bears and found it very unlikely that they can cope with severe cold stress. Because true polar bear den temperatures are very difficult to obtain, we examined the microclimate of an artificial den of typical polar bear construction. Quite in contrast to the low temperatures recorded by Belikov and Harington (1, 6, 7), we found that the temperature of the den probably stays near freezing provided the female is inside. In Alaska the polar bear will usually desert the den by early April, when ambient temperature occa- sionally drops to -30°C. This implies that the cub on emergence might be subject to a thermal gradient similar to the one experienced at birth. Consequently, the present report also deals with the modes of temper- ature regulation that are employed during this second critical period in polar bear life. The tolerance to differ- ent environmental stresses in this species has also been given some consideration l MATERIALS AND METHODS Three polar bear cubs (U. maritimus) were used in this study. Two cubs were born quite unexpectedly (27 Decem- ber> at an ambient temperature of -45°C in an outdoor uninsulated cage that provided only protection from the wind. One (female) died 2 h after birth, while the other (a male) survived for 2 days at the same ambient temperature. Both cubs were frozen immediately post- mortem and kept in a deep freezer until the start of the present investigation. After thawing the cubs were measured and weighed. The weight-to-surface area relationship of their bodies was estimated by way of integrated circumference measurements and weighing. Morphological character- istics potentially relevant to temperature regulation were visually examined. Fat and skeletal muscle (m. psoas and m. latissimus dorsi) were excised and prefixed in 2% glutaraldehyde for 3 h. The samples were then kept in 0.1 M cacodylate buffer for l-5 days, postfixed for 2 h in 1% OsO,, dehydrated in ascending grades of ethanol, and finally embedded in plastic (13). Sections were cut in a Sorvall Mt-2 ultramicrotome and exam- ined in a JEM-GAS electron microscope. The hair den- sity of the fur of one cub was determined as number of hairs/cm” under a light microscope. One cub was captured off Point Barrow, probably within a few days after emergence from the den (29 March). The cub, a male, was kept in an indoor pen at about -5°C for the next 10 days. It was fed four times a day with a 1:l mixture of whipping cream (30% fat) and milk. It was able to drink from a bowl immediately upon capture l The cub was kept for 3 days before any experiments took place. In this period it grew very tame and it was programmed to stay awake during the day and sleep in 0363~6119/79/OO00-oooO$O1.25 Copyright 0 1979 the American Physiological Society H67 by 10.220.32.247 on July 31, 2017 http://ajpregu.physiology.org/ Downloaded from