Why Do Animals' Eyes Glow In The Dark? Eyes gleaming out of the pitch-dark night make for many a scary tale. But what's behind that pair of glowing eyes, and why don't humans' shine?

Why Do Animals' Eyes Glow In The Dark?

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Welcome back to All Things Considered. I'm Andrea Seabrook.

(Soundbite of an animal howling)

SEABROOK: In the opening pages of "White Fang" by Jack London, two men huddle around the fire in the midst of frozen wilderness. A wall of darkness presses them from every side.

(Soundbite of an animal howling)

SEABROOK: London writes, there was no suggestion of form in the utter blackness. Only could be seen a pair of eyes gleaming like live coals. Henry indicated with his head a second pair and a third, a circle of the gleaming eyes had drawn about their camp. Oh, eyes gleaming in the pitch-black night.

But why do animals' eyes glow at night? It's this week's Science Out of the Box.

(Soundbite of music)

SEABROOK: Dr. Cynthia Powell is a veterinary ophthalmologist. She's at Colorado State University. And, Dr. Powell, why do critters' eyes do that when you shine, say, your headlights into them?

Dr. CYNTHIA POWELL (Department of Clinical Sciences College of Veterinary Medicine and Biomedical Sciences, Colorado State University): Well, a lot of the animals that we see, especially the ones that go out at night, have a special reflective surface right behind their retinas that functions to reflect light back out so that they can actually see better at night.

SEABROOK: And what's this thing called?

Dr. POWELL: It's called the tapetum lucidum.

SEABROOK: And how does it help them see better?

Dr. POWELL: Well, when light enters the eye, it's supposed to hit a photoreceptor, and the photoreceptor is what transmits information to your brain so that you can interpret what you're seeing. And sometimes that light doesn't hit a photoreceptor. And so, to give it a second chance to hit that photoreceptor, it hits the tapetum lucidum, which acts as a mirror, and then that mirror effect bounces that photon back and gives it a second chance to hit the photoreceptor.

SEABROOK: Do all animals have a tapetum lucidum?

Dr. POWELL: A great number of animals do, but humans don't, and some other primates and squirrels tend not to have them and red kangaroos and the pig.

SEABROOK: OK. So, deer, I would imagine, having seen their eyes glow, have them?

Dr. POWELL: Yes, they do.

SEABROOK: Dogs, cats?

Dr. POWELL: Cattle, horses, ferrets, a great number of animals have this reflective surface.

SEABROOK: I mean, I was just thinking, my cat got out the other night, and I shined a flashlight under the car and saw his eyes glowing, and it was this gorgeous green, like an emerald.

Dr. POWELL: Yeah. Cats often have a very bright green. Siamese cats often have very bright yellow. And cat tapetums do tend to reflect even a little bit more than dogs'.

SEABROOK: So all these animals have this thing, the tapetum lucidum. Why do their eyes glow different colors?

Dr. POWELL: Well, there are different structures and different tapetums, and they can have riboflavin in there. They can have zinc in their tapetums. And because of the different substances, they can reflect different colors. Also, there are varying amounts of pigment within the retina, and that can affect the color and also age of the animal, and there are other different factors that might affect it.

SEABROOK: So if you have two dogs, say, same species, they could have different colored glows, night glows?

Dr. POWELL: Oh, absolutely. I've seen yellow and orange and blue, and one of my favorites are miniature schnauzers that tend to always have a turquoise color. It's really beautiful.

SEABROOK: Dr. Cynthia Powell is an associate professor of veterinary ophthalmology. She shines her light into the eyes of the K9s of Colorado in Fort Collins at Colorado State University. She joined us from KUNC in Greeley. Well, thanks so much for clearing these things up for us.

Dr. POWELL: Well, you're very welcome.

(Soundbite of an animal howling)

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