Arctic Reindeer Change the Color of their Eyes for the Long, Dark Winter
There, at the corner of your yard, where the woods creep up and night obscures the seeming safety of suburbia—a pair of glowing eyes hovers … watching … waiting. The hair on the back of your neck stands at attention, your muscles tense, and deep within your brain a thought emerges clear as day. Those eyes are obviously attached to the business end of a ravenous beast—and your life will be over within the minute.
These iridescent orbs are caused by “eyeshine,” light reflected off a thin layer of tissue in the backs of some creatures’ eyes to let them see better at night. Humans lack this tissue, called the tapetum lucidum, and the night vision that comes with it, which is probably why our adrenaline gland is always trying to warn us those eyes belong to a direwolf instead of more likely creatures—like wolf spiders in the grass. Luckily, we’ve invented flashlights and crossbows to compensate. But many animals cannot afford such luxuries—like the Arctic reindeer (Rangifer tarandus), an animal that lives in lands bathed in darkness for months on end. And unlike you and me, wolves are a very real threat to a reindeer.
In summer, Arctic reindeer eyes are a golden hue. In winter, they turn deep blue. So far as researchers know, this seasonal shift is unique. Neither horses, nor house cats, nor any other mammals with tapetum lucidum are known to do this.
When the sun dips below the horizon for two full months of the Norwegian winter, darkness causes the reindeer’s eyes to become near-constantly dilated. We know from glaucoma studies that dilation increases intraocular pressure, or the fluid pressure within the eye. In turn, increased pressure causes the collagen fibers of the tapetum lucidum to scrunch together—changing the wavelength of light it reflects back.
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http://www.slate.com/blogs/wild_things/2013/10/30/arctic_reindeer_change_eye_color_blue_eyes_help_them_see_during_the_long.html
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