Reindeer are well adapted for the icy cold of an Arctic winter. Their thick, insulating fur keeps the chill out when they are resting, but it can also prevent heat from escaping when the animals are active. Researchers from Norway tackled the question of how reindeer avoid overheating when they exercise in a recent issue of the Journal of Experimental Biology.
Lead author Arnoldus Blix from the University of Tromsø, Norway, and colleagues recorded the brain temperatures, breathing rates, and blood flow of reindeer at rest and during activity. The activity was a brisk trot on a treadmill - it turns out reindeer are amenable to training and could be taught to exercise on treadmills in low temperatures, mimicking outdoor activity.
At first, the reindeer increased their breathing rate, from seven breaths per minute to 260 breaths per minute, effectively panting through their nose. The animals inhale cold air through their noses, evaporating moisture from the mucus membranes in their nasal sinuses and cooling the blood there before it travels to the rest of their body, via the jugular vein, and keeps their temperature down.
With continued exercise and generation of heat, the reindeer switched to another cooling strategy: panting with their mouths open and tongues hanging out. Like dogs, reindeer have highly vascularized tongues. Exposure to the cold air evaporates water on the tongue, resulting in more heat loss from the blood.
When the reindeer's brain reached a high of 39°C, the animals switched to their third tactic. They activated a system that selectively cools the brain by diverting cold venous blood from the nose away from the body and up into blood vessels in the head. There, the cooled blood enters a network of heat exchanging blood vessels and helps cool the warmer arterial blood before it reaches the overheated brain.
Blix and colleagues propose that reindeer use this three-pronged cooling strategy to regulate body and, especially, brain temperature during heavy exercise. A combination of panting, first through the nose and later through the mouth, and eventually selective brain cooling, seems to keep reindeer calm and cool in their extreme environment.