Do you find it surprising that reindeer, which spend much of their time standing in snow, don't get cold feet? Or that dolphins, whose thin flippers are gliding constantly through cool water, still manage to pursue very active lifestyles? A special circulatory adaptation known as countercurrent heat exchange enables both of these animals to maintain the appropriate body temperature in their extremities, and this is only one of the many clever adaptations mammals have evolved over the last hundred million years to help them to deal with variable temperatures.
Mammals Are Endothermic
All mammals are endothermic-that is, they maintain and regulate their own body temperature, no matter the external conditions. (Cold-blooded vertebrates, like snakes and turtles, are ectothermic.) Living in widespread environments around the world, mammals face daily and seasonal fluctuations in temperatures and some-for example, those indigenous to harsh arctic or tropical habitats-have to deal with extreme cold or heat. To maintain their correct internal body temperature, mammals must have a way to produce and conserve body heat in colder temperatures, as well as dissipate excess body heat in warmer temperatures.
The mechanisms mammals have for producing heat include cellular metabolism, circulatory adaptations, and plain, old-fashioned shivering. Cellular metabolism is the chemical process that constantly occurs within cells, by which organic molecules are broken down and harvested for their internal energy; this process releases heat and warms the body. Circulatory adaptations, such as the countercurrent heat exchange mentioned above, transfer heat from the core of the animal's body (its heart and lungs) to its periphery via specially designed networks of blood vessels. Shivering, which you've probably done some of yourself, is easiest to explain: this crude process generates heat by the rapid contraction and shaking of muscles.
If An Animal Gets Too Warm
What if an animal is too warm, rather than too cold? In temperate and tropical climates, excess body heat can accumulate quickly and cause life-threatening problems. One of nature's solutions is to place blood circulation very near the surface of the skin, which helps to release heat into the environment. Another is the moisture produced by sweat glands or respiratory surfaces, which evaporates in comparatively dryer air and cools the animal down. Unfortunately, evaporative cooling is less effective in dry climates, where water is rare and water loss can be a real problem. In such situations, mammals, like reptiles, often seek protection from the sun during the hotter daylight hours and resume their activity at night.
The evolution of warm-blooded metabolisms in mammals wasn't a straightforward affair, as witness the fact that many dinosaurs were apparently warm-blooded, some contemporary mammals (including a species of goat) actually have something akin to cold-blooded metabolisms, and even one type of fish generates its own internal body heat.