The Science Behind Rudolph's Red Nose and Colour-Changing Eyes

It’s one of the greatest Christmas mysteries, a perplexing festive puzzle: Why does Rudolph the red-nosed reindeer have a very shiny nose?

Wonder no more, because science is here to settle the matter. A team at Lund University in Sweden have captured the reindeer glowing-nose phenomenon on film, using a thermographic camera.

It’s further proof of an explanation published last year in the Christmas edition of the BMJ by scientists from Norway and the Netherlands. They described their objective:

“To characterise the functional morphology of the nasal microcirculation in humans in comparison with reindeer as a means of testing the hypothesis that the luminous red nose of Rudolph, one of the most well known reindeer pulling Santa Claus’s sleigh, is due to the presence of a highly dense and rich nasal microcirculation.”

Motherboard's Derek Mead explained how the researchers in that study tested the hypothesis with reindeer and human participants, a treadmill, and some cocaine. Ultimately, they found that the reindeer’s noses had 25 percent more blood vessels than the humans’. They concluded that the blood vessels in Rudolph’s nose “help to protect it from freezing during sleigh rides and to regulate the temperature of the reindeer’s brain, factors essential for flying reindeer pulling Santa Claus’s sleigh under extreme temperatures.”

That might have been a bit of lighthearted Christmas research, but it’s the sort of thing scientists at Lund University take very seriously. They’ve got a whole research group looking into mammals and their noses, and are looking into such questions as why dogs’ noses are so cold when reindeer’s are so warm.

Meanwhile, another reindeer facial feature has come to scientific attention this year: their eyes. A study published by British and Norwegian researchers in the Proceedings of the Royal Society B at the end of October found that Arctic reindeer’s eyes change colour at different times of the year. In summer, they’re gold, but in winter, they’re blue.

In what sounds like a scene from a horror film, the Norwegian researchers mailed Glen Jeffery, a neuroscientist at University College London, a package of ten reindeer eyes collected in the summer (bought from local herders) and ten collected in the winter. “When I opened them, I had the biggest shock I've ever had in science,” said Jeffrey. I'm not surprised—I'd probably be shocked to open up a parcel full of eyes—though what disturbed Jeffrey so much was in fact their colour. “The winter ones were clearly blue and the summer ones were clearly gold.”

The reason for the colour difference is a change in what’s called the tapetum lucidum, a reflective part of the eye that’s generally associated with night vision in nocturnal creatures. It’s what makes cats’ eyes glow when you see them in the dark.

The researchers wrote that, to their knowledge, it was the first time a change in colour had been observed in this part of a mammal’s eye. They suggested the reason for the change was linked to the light conditions in the Arctic: In winter it’s constantly twilight for several months, while in summer it’s constantly light.

The report explained that the reindeer’s summer gold eyes are better at general vision than their winter blue eyes, but the blue ones are much more sensitive to light. “The most parsimonious explanation is that these factors are functionally linked to provide an adaptive mechanism to a challenging environmental light that is completely novel among mammals,” they concluded. “It remains to be seen whether other Arctic mammals have adopted similar strategies.”