What do you think of this fact?
Your eye color is set for life. Arctic reindeer have a different deal: part of their eyes switches color with the seasons.
A mirror that changes color
Behind the retina, reindeer have a reflective layer called the tapetum lucidum. It bounces light back through the retina, giving the light-sensitive cells a second chance to catch it. In 2013, scientists from UCL in London and the University of Tromsø in Norway, led by Glen Jeffery, described in the journal Proceedings of the Royal Society B what happens to it over the year. In summer, the tapetum of Arctic reindeer is gold. In winter, it is deep blue.
Many of the eyes came from Sami reindeer herders, from animals killed in summer and in winter. According to Live Science, Jeffery said: “No one has ever seen anything like this in a mammal before, let alone such a large shift.”
Pressure in the eye
The Arctic serves up extremes: according to National Geographic, about three months of constant daylight and three months of constant darkness. In the dark winter, reindeer pupils stay wide open for months. The team thinks this probably raises the pressure inside the eye. That squeezes the collagen fibers in the tapetum closer together, and tighter spacing reflects bluer light.
The golden summer layer reflects a lot of light back through the retina. The blue winter layer reflects far less light back out of the eye and scatters more of it onto the light-sensitive cells. That helps in the dim winter light, at the cost of some sharpness. National Geographic reports that the team measured the blue retinas as at least 1,000 times more sensitive to light than the golden ones. A vision scientist quoted there calculated, though, that the reflective layer alone could at most double sensitivity, so that number still needs explaining.
Bonus fact: The same team had found earlier that Arctic reindeer can see ultraviolet light. According to UCL, the blue winter reflection likely favors that UV sensitivity.
Same reindeer, same eyes, two colors a year. Santa’s team comes with built-in seasonal settings.