Green light (530nm) is readily absorbed by both oxygenated and deoxygenated blood but bounces (scatters) off of the surrounding tissue. Shining green light inside the nose of reindeer and humans, the researchers were able to take detailed videos of the nasal circulatory system, tracking the rate of blood flow through individual blood vessels.The researchers wrote their own image analysis software that can determine the density of blood vessels and track velocity of red blood cells through the dense network of blood vessels.Despite the tongue-in-cheek premise of this study, researchers have not previously used this imaging technique to study nasal circulation. The team found that humans and reindeer noses both feature hairpin-shaped blood vessels and similar capillary networks. Most pertinent to their aim, they found that the density of blood vessels in reindeer noses is 25% greater than the density in human noses, leading to a rosy tint. \"These factors explain why the nose of Rudolph, the lead flying reindeer employed by Santa Claus to pull his sleigh, is red and well adapted to carrying out his duties in extreme temperatures,\" the authors conclude in their paper.The annual BMJ holiday issue is a great read-- check out other great findings from the holiday issues including: whether 27 is actually the dangerous age for musicians, how fast you have to walk to beat Grim Reaper , and the dreamed-of expose on tooth fairy malpractice.