Just in time for the holidays, researchers in the Netherlands and Norway published a study showing that some fraction of reindeer actually do have red noses, due to "an extremely dense array of blood vessels, packed into the nose in order to supply blood and regulate body temperature in extreme environments." Joseph Stromberg summarizes the findings:
They discovered that the reindeer had a 25% higher concentration of blood vessels in their noses, on average. They also put the reindeer on a treadmill and used infrared imaging to measure what parts of their bodies shed the most heat after exercise. The nose, along with the hind legs, reached temperatures as high as 75°F—relatively hot for a reindeer—indicating that one of the main functions of all this blood flow is to help regulate temperature, bringing large volumes of blood close to the surface when the animals are overheated, so its heat can radiate out into the air.