Scientists have solved one of the biggest Christmas mysteries - why Rudolph's nose is red.

They say that the colour is down to a high amount of red blood cells, which help to protect the nose from freezing weather conditions and to regulate brain temperature. Both of these are essential for pulling Santa's sleigh in extreme temperatures, such as those found at the North Pole.

It is already known that tiny blood cells in the nose - known as micro-vessels - have a number of important jobs, such as regulating temperature and controlling inflammation. However until now, these have not been studied in great detail.

Dutch and Norwegian scientists decided to investigate this further. They used a hand-held video microscope to assess the noses of five healthy people and two reindeers.

They found that the density of blood vessels carrying a high concentration of red blood cells was 25% higher in the reindeers.

They also noted that the reindeers had a high density of mucous glands in their noses, which help ‘maintain an optimal nasal climate during changing weather conditions and extremes of temperature, as well as being responsible for fluid transport and acting as a barrier'.

Furthermore, infrared thermal images confirmed that reindeers' noses are red.

"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 team said.

Details of these findings are published in the British Medical Journal.