The color of Rudolph's nose has long been the talk of the town. But the color of his eyes may be more interesting. Because Arctic reindeers' eyes change color with the seasons—from gold in the summer to deep blue in the winter.
Changes to a tissue layer in the eye called the tapetum lucidum are responsible for the color change, the purpose of which seems to be to change the sensitivity of the retina to different wavelengths of light. That’s according to a study in Proceedings of the Royal Society B. [Karl-Arne Stokkan et al., Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer]
The shift in wavelength depends on the spacing of the collagen fibers in the tapetum lucidum. The smaller the spacing, the better the retina is at picking up light at shorter wavelengths, including into the near ultraviolet.
By extending the reindeers’ visual range, this unique adaptation allows them to see better in the continuous darkness of winter in the Arctic reindeers' habitat.
Perhaps Rudolph with his nose so bright wasn’t needed to guide Santa’s sleigh after all. He could have used any reindeer with its eyes so blue.
[The above text is a transcript of this podcast.]