WHY DO REINDEER EYES CHANGE COLOR?
During summer reindeer eyes are a lovely golden color, but in the winter they change to deep blue. Not the whole eye, of course: The change occurs in the tapetum lucidum, a layer of reflective tissue located behind the retina. Humans don’t have it, which is one of the reasons we don’t see too well at night. Why does it change color in reindeer? Because their typical habitat lies at 50 degrees north latitude or higher, where the longest days and nights are at least 16 hours long during their respective solstices, rising to 24 hours at extreme latitudes. Thanks to this color change, reindeer eyes reflect a high percentage of sunlight during summer but far less in the winter. Their blue eyes are roughly 1,000 times more light-sensitive than the golden eyes of summer. That’s because the mirrored layer reflects light that passes through the retina back onto it, thus enabling reindeer to see far better than humans can in dim light. On the other hand, because much more light bounces off the golden eyes, they are effectively a pair of natural sunglasses. Incidentally, the tapetum is the reason many mammals’ eyes glow yellow in the dark, though only reindeer have been known to display the seasonal color shift.