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Plenty of animal eyes "glow" in the dark, but only one species has eyes that change color with the seasons.

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[ ♪ Intro ].

Shine a light into the woods at night, and you might see the glow of eyes staring back at you. But the eyes aren’t actually glowing—they’re reflecting light off a special layer of tissue found in some animals called a tapetum lucidum.

The color and shape of this ‘eye shine’ can tell you what’s peering at you from the darkness. But things are more complicated if you’re in the arctic regions of Norway. Because there, reindeer change the color of that part of their eye seasonally.

That’s right—the animals most famous for pulling Santa’s sleigh have eyes that change color to help them see in the darkness of winter. Animals that need to see well in low light, like cats, raccoons and even reindeer, all have tapeta lucida. They sit in back of the eyes right in between the outer layers of the eye and the retina—the part that actually “sees”.

These thin tissues act as reflectors, giving retina a second chance at absorbing light. Most animals have a particular color of eye shine which depends on the composition and structure of their tapetum lucida. But the Eurasian mountain reindeer is different—it’s the only one we know of that switches colors, changing from a summery golden yellow to a deep blue in winter.

And scientists think that’s a side effect of how their eyes have adapted to long periods of darkness. The arctic summers where they live have 24 hour days, and the winters include long stretches of night. The animals compensate for this in part by making their retinas more responsive to low light in winter.

And it’s believed they can see some wavelengths of UV light. But that’s not the most obvious adaptation. To see better when it’s so dang dark, a reindeer’s pupils open wide to let more light in—kind of like yours do if you go into a dark room.

But when a pupil stretches wider it also flattens the front of the eye ever so slightly, which increases the pressure inside the eye. Since their pupils are constantly wide open during winter, that pressure builds up, squeezing the tiny collagen fibers of the deer’s tapetum lucidum closer together. And this compression causes the color change.

The tightly-packed fibers strongly reflect shorter, blue wavelengths of light, while the more spaced out ones reflect longer, yellow ones instead -- a phenomenon known as Bragg's Law. Researchers can actually compress a dissected piece of the reindeer’s eye with a tiny weight and induce this same yellow to blue color change. But the color isn’t what’s really important to the animal.

The compressed fibers also scatter light sideways through the retina instead of reflecting it out of the eye, increasing the amount of light the retina can absorb. While summer eyes reflect more than 95% of the light shone into them, winter eyes reflect only 40%. And that likely means the reindeer can see in what seems like total blackness.

The only downside to this amazing feat is that, because light rays inside the eye are more scattered, everything also looks a little fuzzier. But that seems a small price to pay. I wish I could trade a little blurriness for night vision!

Thanks for watching this episode of SciShow! If you liked learning about reindeers eyes, you might like our episode where we explain what eyeballs are actually made of. [ ♪ Outro ].