Staying warm in a cold climate
Mammals and birds are homeothermic animals, maintaining a constant body temperature. In cold weather, they must either reduce heat loss or increase heat production. Reindeer, for example, have developed highly effective winter fur to insulate their bodies. Thick guard hairs with air-filled cavities protect woollen underfur that prevents the movement of air against their skin. Their legs are less insulated than their torso, and are maintained at a lower temperature. The nasal cavities are similarly cooled to reduce heat loss from breathing. As large animals, reindeer also have a smaller surface area to body mass ratio, which further reduces heat loss. By means of these various adaptations, reindeer can tolerate ambient temperatures at or below -30°C without needing to increase heat production.
Reindeer calves face a particular challenge at birth. They are typically born in early spring when snow still covers much of the landscape and cold rains are common. At birth, the calf may experience a drop in temperature of 50-60°C. Of necessity, newborn reindeer are well developed and can soon stand and walk. They have an effective capacity for regulating their body temperature, largely by activating the heat production of brown fat, a mechanism known as non-shivering thermogenesis.
Brown fat is a unique mammalian tissue specially adapted to generate heat. Its color is caused by a rich network of capillaries and cell organelles, called mitochondria, where and around vital organs such as the kidneys and heart. The heat produced by the various sites of brown fat is carried by the blood to the rest of the body. Brown fat plays an important role in the cold resistance in many newborn Arctic mammals, including muskox and harp seal pups. Brown fat is also present in adult cold-adapted small mammals and hibernators. In reindeer, the brown fat is most active shortly after birth and declines rapidly during the first month of life, gradually turning into a tissue that resembles white fat.
Several mammalian species in the Arctic evade the problems of cold and scarcity of food by spending the winter in torpor. They store large quantities of white fat during summer and autumn and use them as their energy source during winter. Species such as bear and badger spend their winter sleeping under the snow. Their body temperature is usually decreased by 4-5°C. Smaller mammals such as ground squirrels, bats and mice hibernate by decreasing their body temperatures much further, near to the temperature of their surroundings. They may periodically wake up from hibernation and excrete waste products from their body. These small hibernators have well-developed thoracic brown fat that is essential in warming the heart when awakening from hibernation.
Päivi Soppela, Arctic Centre, University of Lapland, Rovaniemi, Finland. From the CAFF publication, Arctic Flora and Fauna, www.caff.is