Helping Santa to keep cool

Santa Claus is well adapted to living in the sub-zero temperatures of the North Pole: He has a good supply of insulating fat and his plump physique and ample facial hair reduce the surface area exposed to the elements. Additionally, Santa's fashionable and widely-imitated attire would meet the cold-weather quality control specifications of your mother!

All these things help Santa adapt to the cold. But how will Santa cope with the heat when he travels to hot climates with his sleigh full of toys? Luckily, physiologists can now help Santa resolve this problem.

Lisa Leon, a physiologist at the US Army Research Institute of Environmental Medicine, investigates how the body maintains its "set point" of 98.6 degrees Fahrenheit, under widely varying environmental temperatures. Specifically, Leon studies what happens at the molecular and cellular levels to provoke the physiological changes that protect us from overheating (hyperthermia).

Advice to Santa: Be cool!

Leon knows what Santa should do first: Remove his jacket and hat, take off his boots and dangle his bare feet over the sled as he flies southwards to warmer regions. In doing so, he will expose greater body surface areas to the air. The more surface area exposed, the greater the loss of heat, which is why we curl up when we want to stay warm and stretch out when we want to cool down. Similarly, mice like to expose their furless tails, feet, noses and ears to help dissipate heat.

How does this work, from a physiological perspective? Leon says the body's surface blood vessels open up (vasodilate) when the core temperature rises beyond its set point. At the same time, the internal core blood vessels constrict, shunting the warmer blood from inside the body to the skin, where the cooler surface air draws away heat from the blood.

Do sweat it out

Even after Santa removes his jacket, his primary physiological response to heat - sweating - is likely to begin. Sweat, comprised of water and electrolytes such as salt, is a great conductor of heat and cold. That's why sweat is less effective in humid climates, where it evaporates more slowly, or not at all. (Under really high humidity, the sweat will simply roll off the skin and not help to cool.)

It is also why submersion in cold water cools the body down much faster than air of the same temperature and explains why Santa hardly ever goes swimming in the North Pole winter.

When the blood vessels dilate and move the warm blood to the skin surface, the sweat picks up the heat, which is then removed from the body as the sweat evaporates. Some animals that don't sweat, such as mice, lick their fur to cool down. As the saliva evaporates from their fur, it also whisks away heat.

Santa's reindeer will take a slightly different approach. They will open their mouths, dangle their tongues and pant when they get hot, thus exposing greater surface area to the air and using evaporation to draw away heat from their bodies, says Perry S Barboza, who studies reindeer at the Institute of Arctic Biology at the University of Alaska in Fairbanks.

So, please understand that those panting sounds from Rudolf and his gang on the roofs in the warmer climates are unavoidable.

What to leave out for Santa?

Now behavioural changes, such as removing a coat, are the first line of defence to ward off overheating. These changes are usually easy to make and they have an advantage. But when the body copes with the heat by using physiological processes such as sweating, there is always the risk of exhausting the response and suffering heat stroke as a result.

There are other problems too. Physiological responses eventually break down. For example, sweating can lead to significant fluid loss, which can draw water from the blood cells, resulting in thicker blood that creates a strain on the heart.

So give Santa a break: Residents of cold climates may offer pie and cookies, which will speed up his metabolism and help keep him warm. They might also leave some liquid refreshments, because the cold can evaporate moisture out of the body. But residents of warmer climates should leave plenty of water or sport/energy drinks, including a generous array of various liquid refreshments on the roof!