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Rudolf may have had a glowing red nose, but real Arctic reindeer have eyes that shine a different hue depending on the season — deep blue in the cold, dark winter, and golden in the summer. 

For more than a decade, no one could explain the color difference.  Now, a conducted by Glen Jeffery, from the University College London, suggests a reason behind this mysterious seasonal change.

Jeffery examined the Arctic reindeer’s tapetum lucidum, a special membrane at the back of the animal’s retina that acts like a mirror to reflect light and helps improve night vision.  Such “eye shine,” or “cat’s eye,” is found in a number of animals that travel at night, from house cats and raccoons to opossums, crocodiles and lemurs. 

When the tapetum changes color, it reflects different wavelengths of light, which vary by season. In the summer, the golden tapetum casts most of the light back out of the eye. On the flip side, during the winter’s darkness, the deep blue tapetum bounces less light out of the eye.  Instead, it scatters the light inside the eye, allowing it to be absorbed by photosensitive eye cells, which according to Jeffery, probably improves a reindeer’s chances of seeing moving predators in the dark.

Scientists are still unsure what causes the color change, but Jeffrey theorizes that it might have something to do with a shift in pressure within the eyeball. In the winter, pressure increases in the eye because of pupil dilation, preventing fluid from draining naturally. This compresses the tapetum, which in turn, makes the tissue reflect shorter wavelengths, notably the blue light common in Arctic winters.  This could be the key to a reindeer’s striking blue eyes come Christmas.

Rudolf would be so jealous.