Cooling the Body

During periods of cold weather and reduced food supply, small warm-blooded animals will de-

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Warm-blooded animals that live in cold environments, such as bears, have thick fur coats to help retain their natural body heat. (PhotoDisc)

crease their body temperature to a few degrees above their surrounding temperature to reduce heat loss and conserve bodily energy stores. This can be a hibernation lasting throughout the winter months or a period of torpor lasting for the night. This lower body temperature, however, is not a decrease in body temperature as observed in coldblooded animals, but a new, highly regulated body temperature attained by lowering the animal's metabolic rate. It is a very important strategy for energy conservation and survival.

To maintain a steady body temperature, warmblooded animals depend on a balance between heat produced by metabolism and heat lost to their surroundings. When the ambient tempera ture exceeds body temperature, an animal will gain heat instead of losing heat to the surroundings. This situation can then elevate body temperature. Body temperature, however, is held constant by heat loss through evaporation of water from the animal. Water, in changing phases from liquid to vapor, requires energy (heat), and this represents the only mechanism available to an animal if the surrounding temperature is greater than body temperature.

Two mechanisms for cooling the body by increasing water loss are panting and sweating. Zebras, gazelles, and bison are examples of closed-mouth panters. These animals rapidly move air through the nostrils, which cools the tissues lining their upper airways (nasal cavities). Dogs are the best example of open-mouth panters; they move air rapidly over the moist tongue to remove heat. In contrast to panting, sweat glands secret a hypotonic salt water over the skin surface. The sweat evaporates and cools the body surface. Both panting and sweating can reduce an animal's bodily water reserves, resulting in dehydration. Thus, increasing water intake during periods of hot weather is especially important.

Camels are known for their tolerance of hot environments. Although they are closed-mouth panters, they also allow their body temperature to rise during the day by 3 to 4 degrees Celsius. This helps the camel tolerate a hot environment in two ways. By increasing its body temperature, the camel will gain less heat from the environment because there is less of a difference between the surroundings and the camel's temperature. Second, the camel can reduce the amount of panting and water loss needed to keep a steady body temperature. A camel stores body heat during the hot day and loses this heat during the cool desert night. The camel's humps are not stores of water, but fat. This makes the camel's skin thinner, which allows for better heat loss at night.

—Robert C. Tyler See also: Cold-blooded animals; Fur and hair; Hibernation; Mammals; Thermoregulation.

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