You might not think that keeping cool is a problem for reindeer. After all, they live in cold northern regions, like Norway, Alaska and Siberia (and, of course, at least mythologically, the North Pole). But when they’re really active–running from a predator, say, or pulling Santa’s sleigh–then their nice, warm fur coat becomes a liability. They need a way to cool down their bodies, and especially their brains, before they overheat.
To find out how reindeer keep their cool, a group of researchers in Norway trained reindeer to trot on a treadmill at a speed of about 5 1/2 miles per hour (the scientists say that the animals appeared to enjoy the experience). While the animals were on the treadmill, the researchers varied the ambient temperature from 50 to 86 degrees Fahrenheit and measured the animals’ physiological responses, including respiration and blood flow. (The results of the study appear in the Journal of Experimental Biology.)
The researchers found that the reindeer employed three different tactics to cool down during their exercise. In the first stage, the reindeer increased their rate of breathing, from 7 breaths per minute to 260 breaths per minute, inhaling plenty of cool air through their noses. As the air passed through their nasal passages, water evaporated from their mucous membranes and cooled the blood in their noses. That cooled blood then passed into the reindeer’s body through its jugular vein and helped cool it down.
In the second stage, as the reindeer continued their trot and needed to get rid of more heat, they started to pant like dogs, opening their mouths and letting their tongues go floppy. “The tongue is large, vascularized and well circulated,” says the study’s lead author, Arnoldus Schytte Blix of the University of Tromsø. “They moisturize the tongue so you have evaporation which also takes heat away from the blood.”
It’s not until the reindeer’s brain reaches 102 degrees Fahrenheit that the animal employs stage three. At that point, cooled blood from the nose is diverted away from the body and into the head where a network of blood vessels act as heat exhangers, pulling out heat from the brain and into the blood and protecting the brain from dangerous overheating.
Which is a really good thing, because no one wants to wake up on Christmas morning to find a reindeer with heatstroke on the roof.