From Laura Klappenbach,
Your Guide to Animals / Wildlife.
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Does it surprise you that reindeer, who spend much time standing in snow, don't get cold feet? Likewise, dolphins, whose thin flippers glide constantly through cool water, seem to be just fine.

A special circulatory adaptation called countercurrent heat exchange enables both species to maintain the appropriate body temperature in their extremities. Countercurrent heat exchange is only one of many clever adaptations mammals have to help them to deal with variable temperatures.

All mammals are endothermic: they maintain and regulate their own body temperature. Living in widespread environments around the world, mammals face daily and seasonal fluctuations in temperatures and some--for example those living in harsh arctic or tropical habitats--face extreme cold or heat. To maintain their correct body temperature, mammals must be able to produce and conserve body heat in colder temperatures as well as dissipate exess body heat in warmer temperatures.

Heat production
The mechanisms mammals have for producing heat include shivering, cellular metabolism, and circulatory adaptations. At some point in our lives, we've all shivered. Shivering generates heat as muscles quickly contract and shake. Cellular metabolism is the chemical breakdown process that occurs within cells. This process releases heat and warms the body. Circulatory adaptations, such as countercurrent heat exchange introduced above, transfer heat from the core of the animal's body to the periphery by specially designed blood flow paths. Warmer blood from the interior of the animal flows to cold extremities, thereby moderating the temperature of the more exposed limb.

Disipating Heat
In warmer climates, excess body heat can accumulate and cause life-threatening problems for a mammal. Circulation near the skin's surface releases heat into the environment. Moisture from sweat glands or respiratory surfaces evaporates and cools the animal. Unfortunately, evaporative cooling is less effective in dry climates where water loss is also costly for mammals. In such situations, mammals often seek cover during the hotter daylight hours and resume active at night.

References

• Ricklefs, R.E. Ecology. W.H. Freeman and Company. New York, NY. 104 (1990).