Why Do Animals' Eyes Glow In The Dark?

Eyes that glow in the pitch-black night make for many a scary tale. But why do some animals' eyes glow at night?

"A lot of the animals we see, especially the ones that go out at night, have a special, reflective surface right behind their retinas," says Dr. Cynthia Powell, a veterinary ophthalmologist at Colorado State University. That light-reflecting surface, called the tapetum lucidum, helps animals see better in the dark.

When light enters the eye, it's supposed to hit a photoreceptor that transmits the information to the brain, Powell explains. But sometimes the light doesn't hit the photoreceptor, so the tapetum lucidum acts as a mirror to bounce it back for a second chance.

A large number of animals have the tapetum lucidum, including deer, dogs, cats, cattle, horses and ferrets. Humans don't, and neither do some other primates. Squirrels, kangaroos and pigs don't have the tapeta, either.

And not all eyes animals' glow the same color. Powell says this is due to different substances — like riboflavin or zinc — in an animal's tapetum. "Also," she says, "there are varying amounts of pigment within the retina, and that can affect the color." Age and other factors also can change the color, so even two dogs of the same species could have eyes that glow different colors.

Cats often have eyes that glow bright green, though Siamese cats' eyes often glow bright yellow. Cat tapeta also tend to reflect a little bit more than dogs, Powell says.

"One of my favorites are miniature schnauzers," she says, which have eyes that tend to glow turquoise. "It's really beautiful."

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