Curious bit of evolutionary adaptation at work here - from Slate / Wild Things:
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.
And a possible explanation:
The eyes of an Arctic reindeer also do this, but because of the seasonal physical changes to the tapetum lucidum, the wintry blue eyes reflect 50 percent less light than the golden eyes of summer. Now, you might think less reflected light would mean worse night vision. But here’s the kicker: Scientists think the blue eye’s compressed structure actually scatters some of the light toward other photoreceptors on the sides of the eye. In essence, the blue eye may be an evolutionary adaptation to more effectively recycle light for an animal that has to survive several months of the year in inescapable darkness.
These findings were published in the Oct. 30 issue of the Proceedings of the Royal Society. The scientists stress that they “have not proved functional relationships” among the pressure change, the eye color change, the light reflection, and the visual benefits. But scientists are cautious as all get-out. Moreover, they write that the link is likely. They also found that blue-eyed winter reindeer had “significantly elevated visual responses compared with summer animals.” In other words, all the pieces of the puzzle are on the table—someone just needs to commission a study to put them together.
Delightful adaptation - just when we think we know it all...
