Rudolph’s nose may be bright, but it won’t be frostbit

Have you ever worried about reindeer getting frostbite? I used to think about this a lot, which tells you something about my personal life. It is confusing because when I watch National Geographic specials, the reindeer are all just out there in the open, and it gets pretty cold up at the North Pole. Maybe Santa keeps his reindeer in a special barn or something. But still, flying around for hours on Christmas Eve must put them at risk for hypothermia.

But ever since I went to school and learned biology I don’t worry about it anymore. Knowledge can be a wonderful thing. Education isn’t just about a fatter paycheck. No, there are peace of mind, understanding, clear thinking and finer sensitivities to be had. And there’s the opportunity to remain out of the work force for several more years.

The reason I’m not worried anymore is because I learned that reindeer are designed for their environment in a most unique way. There are only two groups of animals that can maintain their own body heat: birds and mammals. Most mammals’ body heat is found in their body mass. Appendages have smaller surface areas per volume, so appendages tend to radiate heat. That is why when you exercise in the heat, your hands and feet tend to swell. The blood is shunted out to the body’s periphery to radiate excess heat through fingers, toes, ears and nose.

In the winter the opposite can happen. The body tries to conserve heat by keeping it in the core of the body. This slows the circulation to the extremities. That is why your ring slips off your hand when your hands are cold. There is less blood flowing into the fingers. That is also why your fingers, toes, ears and nose are the first to get cold, and sometimes frostbitten. The body decides to lose them instead of heat from the core of the body. It represents a sort of biological triage I guess.

The reason reindeer don’t lose their rings in the winter is because of the way their circulatory system is structured. The arteries and veins leading to and from the feet are parallel to each other, but with the artery on the outside of the two. The arteries are insulated from losing heat to the outside by a layer of fat and the hair on the animals’ legs. This keeps the heat for warming the appendages. Some heat is lost, of course, so venous blood is somewhat cooler than the arterial blood. This means cool, venous blood is approaching warmer arterial blood. The heat is transferred into the vein preventing hypothermia.

If the two vessels ran in the same direction, the best that could ever be achieved is a 50 percent transfer of heat. At that point, the heat would be equal in the two vessels. But when the two vessels run in opposite directions, the efficiency can be extremely high. No matter how hot the arterial blood, or cold the venous blood, warm blood will always be approaching cooler blood. The length of the pipeline dictates how efficient the transfer will be. That may be one reason reindeer have such long legs.

This kind of system is called a counter-current system, and it is prevalent in the animal world. Fish gills, used for extracting oxygen, propel the water across them in the opposite direction of the blood flow. Birds have a unique lung where oxygen is extracted as it flows both in and out through separate passages. Again the blood flow is in the opposite direction. Humans even have counter-current systems in their kidneys.

I wanted to explain all this so that on Christmas Eve you can rest easy concerning Santa’s reindeer. Nature has provided them with an efficient, counter-current mechanism to retain heat in the face of the North Pole winter.

Knowledge is such a liberating thing.

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Gary McCallister is professor of biology at Mesa State College.


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