Rudolph and his Arctic reindeer kin have evolved a neat trick to help them contend with dark, sunless winters and bright summers: their eyes change color. In the winter, their irises take on a frosty blue, while in the summer they adopt a golden hue. These colors each promote a different type of visual advantage, either allowing them to see through the darkness or to keep from squinting in the sunny glare.
The bit that actually changes colour is the tapetum lucidum or “cat’s eye”—a mirrored layer that sits behind the retina. It helps animals to see in dim conditions by reflecting any light that passes through the retina back onto it, allowing its light-detecting cells a second chance to intercept the stray photons. The tapetum is the reason why mammal eyes often glow yellow if you photograph them at night—you’re seeing the camera’s flash reflecting back at you.
Reindeer eyes, by default, are gold. But during the long winter, their pupils dilate for months on end, Yong explains. All of this effort takes a toll on the reindeers’ eyes, which begin to swell and in turn exert pressure on tapetum.
This layer is mostly made up a collagen, a protein whose long fibres are arranged in orderly rows. As the pressure inside the eye builds up, the fluid between the collagen fibres gets squeezed out, and they become more tightly packed. The spacing of these fibres affects the type of light they reflect. With the usual gaps between them, they reflect yellow wavelengths. When squeezed together, they reflect… blue wavelengths.
The wintery blue, Yong writes, is about 1,000 times more sensitive to light than the summery gold. The latter color, on the other hand, helps in the summer by bouncing the majority of light off of the animals’ eyes, effectively acting like a pair of natural sunglasses. So when the Christmas decorations are broken out this year, take a moment to check and see if Rudolph is anatomically correct—he should be sporting a pair of baby blues that time of year.
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