Scientists have established why reindeers' noses are so bright.
Researchers in the Netherlands and Norway have discovered that reindeer noses have 25 per cent more blood vessels than human noses.
The tongue-in-cheek investigation, published by the British Medical Journal in its Christmas edition used a hand-held microscope to examine the nasal lining of five healthy humans, two reindeer and a sixth person with a non-cancerous nasal growth.
It is the first time a scientific explanation has been offered for the glow that allows the world's most famous antlered herbivore to guide Santa's sleigh through the night before Christmas.
The tiny blood vessels provide plentiful oxygen-carrying cells and help control the body's temperature, showed their findings, which were backed by an infrared image of a reindeer after exercise.
"Rudolph's nose is red because it is richly supplied with red blood cells, comprises a highly dense microcirculation, and is anatomically and physiologically adapted for reindeer to carry out their flying duties for Santa Claus," the paper observes.
Rudolph's round-the-world feat has been closely scrutinised by physicists.
In order to deliver presents to children in around 100 million homes where the Father Christmas tradition is observed, he would have to travel at around 650 miles per second, they estimate.
At such speeds, the reindeer, Father Christmas and the sleigh would be vaporised by friction with the air, along with the gifts and any little elfish helpers who came along for the ride.
Rudolph would need to deploy an ion shield to protect them, or exploit loopholes in the space-time continuum so that they travelled between dimensions in order to deliver the presents on time.