Incredible Facts About Animal Eyes That Will Change the Way You See the Natural World

Ever wonder what a horse sees when it turns its head? Or how a gecko can read a moonlit yard as if it were noon? In the eyes of animals, evolution has produced some of the strangest, most efficient tools of all. Some can't move at all. Some can see colors we don't even have names for. And at least one animal can regrow a whole eye if it loses one. Here are the incredible animal eye facts proving that nature's eyesight is a lot weirder—and smarter—than our own.

The eye is a tradeoff. Every eye on Earth possesses distinct characteristics. A creature that has to spot a hawk overhead trades depth perception for a wider field of view. A hunting animal in total darkness forfeits color vision for raw light sensitivity. In nature, no single eye is best; instead, there are thousands of solutions, each shaped by what an animal needs to eat, avoid as prey, or find in the dark. That's why it's so much fun to compare them.

There's a reason the Eagles top most lists. Their eyes are thought to be four to eight times sharper than a human's, thanks mainly to the fact that their retinas pack in a far greater number of cone cells that sense light. A fovea is a deep pit that acts as an almost built-in zoom lens, allowing an eagle to pick out a rabbit from a height from which most humans couldn't even see the ground clearly.

In general birds are wired for sharp vision. Where a human retina packs in about 200,000 photoreceptors per square millimeter, many birds squeeze in about a million. Add to that color vision that often includes four or five types of cones—rather than our three—and you begin to understand why a hawk can track a mouse from a hundred meters up while you're squinting at a street sign.

The tapetum lucidum, a mirror-like layer located behind the retina, enables cats to see in the dark. It bounces light back through the retina for a second pass. This phenomenon is also why a cat's eyes glow when you shine a flashlight on them at night. Catfish use the same trick underwater, giving them an advantage in murky, low-light rivers.

Nocturnal geckos take this phenomenon to the extreme. Their eyes are believed to be about 350 times more sensitive to color in the dark than human eyes—so a gecko can likely still tell the colors in starlight, which is almost impossible for us. Geckos do not blink; they lick their eyes with their tongues because they have no eyelids.

Horses see nearly all of the world at once. They have eyes on the sides of their heads, which means they have almost 360-degree vision. Perfect for a prey animal that needs to see a predator coming from almost any direction. The tradeoff is two small blind spots, right in front of the nose and directly behind the tail, where a horse can't see anything without turning his head.

Owls do the reverse. Their eyes are so large that there is no room left in the skull for eye muscles, so an owl's eyes are essentially much more fixed in place—unable to swivel at all. An owl's neck can swivel about 270 degrees to compensate, powered by extra neck blinks and a blood-supply system that can cope with that sort of twist without cutting off circulation.

Chameleons do the same thing, but in an entirely different way. Each eye moves independently, so a chameleon can watch for a predator behind it while hunting an insect in front of it—at the same time. Scientists have only recently learned how this unique vision works. The optic nerve behind each eye is coiled up like an old telephone cord, so it has enough slack to allow the eye to swing freely in almost any direction without the nerve tearing.

Think your vision is cool? The mantis shrimp would like to have a word. Human eyes have 3 types of color receptors. Mantis shrimp have around a dozen of them, as well as the ability to see polarized and ultraviolet light. Researchers believe they may be using polarized light almost like a private messaging system between shrimp, invisible to almost every other species around.

Reindeer have their color trick, but it's seasonal, not built-in. In fact, the reflective layer behind a reindeer's eye changes color with the seasons, from gold in the summer to white in the depths of the Arctic winter, altering the amount of light reflected back and helping the animal cope with months of near-total darkness.

The colossal squid is believed to have the largest eyes of any living animal, measuring more than 27 centimeters across, or the size of a soccer ball, by some estimates. Only an eye of such stupendous size could capture the faintest traces of bioluminescent light drifting past in crushing ocean depths where almost no light penetrates.

But relative to body size, the record goes to the tarsier. This tiny primate has eyes so large that each is about the same volume as its brain. Those eyes are so large that they have limited mobility within their sockets, necessitating that a tarsier swivel its entire head to observe its surroundings, much like an owl.

The next time you see a cat's eye flash in your headlights or an owl turns its head almost all the way around, you're watching millions of years of evolution solving the very same problem—how to see well enough to survive—in a totally different way every time. Want to know more about how different species are built to survive? Try the random animal generator to see another animal with its own set of strange and brilliant adaptations.