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.
The tapetum lucidum reflects light back toward the retina like a mirror, in effect giving the eye a second chance to use whatever light is available—be it stars, moon, or a distant streetlamp. More powerful light sources, like a flashlight or spotlight, mean more powerful reflections—which is why a raccoon in your garbage appears to have the eyes of a radioactive hellhound.
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.
Furthermore, one group of reindeer never made the shift from golden eyes to blue, despite the fact that Norwegian winter was very much upon them. The researchers observed this group at the University of Tromsø, which is situated near town and is in proximity to permanent urban lighting. The eyes of these animals were green—apparently a transition color suitable for neither the full light of summer nor the full dark of the true, on-like-Donkey-Kong winter typical of rural Norway.
In another experiment, the researchers removed the golden tapetum lucidum of summer reindeer and laid them out for observation. They then applied pressure with a small weight and watched the samples immediately turn deep blue. When the weight was removed, the blue ebbed, and the gold slowly returned—though the eye slabs lay disembodied on a slide.
Whether you live near Artic reindeer or not, that ought to give you a few things to think about the next time you see a pair of eyes in the night. You know, before you go all fight-or-flight over a ferret.