Why do animal eyes glow in the dark? : NPR


The cat’s eyes shine thanks to its tapetum lucidum.

Christmas Zia Lee

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Christmas Zia Lee

The cat’s eyes shine thanks to its tapetum lucidum.

Christmas Zia Lee

The eyes that glow in the dark night make for a chilling tale for many. But why do some animals’ eyes glow at night?

“Many of the animals we see, especially those 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. This 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 relays information to the brain, Powell explains. But sometimes the light doesn’t hit the photoreceptor, so the tapetum lucidum acts like a mirror to bounce it back for a second chance.

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

And not all animal eyes shine 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 different amounts of pigment in the retina, and that can affect color.” Age and other factors can also alter color, so even two dogs of the same species can have eyes that glow in different colors.

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

“One of my favorites is the miniature schnauzer,” she says, whose eyes tend to glow turquoise. “It’s really beautiful.”


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