December is a time of darkness. The sun sets early, perfect for exploring the world at night without having to stay up super late. One of the coolest phenomena that happens at night is eyeshine.
It’s exciting when you see a pair of eyes shining back at you in the dark! How do they glow like that? Animals that spend a lot of time in the dark need to make use of what little light is available. At the back of their eyeballs is a reflective layer like a mirror called the tapetum lucidum. This means “bright tapestry” in Latin.
The tapetum lucidum is located right behind the retina. When any little bit of light enters the eye — from moonlight, headlights or a flashlight — the light goes in and bounces back off the tapetum lucidum so the eye gets a second chance to see everything.
Try this: Go into a dark room with a flashlight. Point it at the back of the room. Can you see a lot? OK, now put a mirror in the back the room. Aim your flashlight at the mirror. Does it illuminate the room a little better? That’s just like the tapetum lucidum at work.
But eyeshine is not for everyone. Have you ever seen your human friends’ eyes glow at night? They’d better not! Humans are diurnal, which means we are up mostly during the day. We don’t have to take advantage of the extra light at night. You’re also not going to see the glimmer of the bright tapestry in squirrels or pigs. Can you think of other animals that don’t have eyeshine?
There are plenty of animals that do have a tapetum lucidum, like cats, deer, rabbits, raccoons and foxes. Animals that have eyeshine are often nocturnal, or mostly awake at night, or crepuscular, which means they are most active at dawn and dusk. Underwater animals, including some fish, also use this cool physical trait to see better in mucky water. Can you name some other animals that you think do have eyeshine?
Eyeshine is not just one color. It can be blue, green, red, white and yellow. Sometimes people say that an animal only has one color, but that’s not strictly true. Eyeshine color can change a little depending on the angle you view it, the color of the light source and the types of minerals in the tapetum lucidum.
So while there is no hard and fast rule, here are some general eyeshine colors and their animals. Deer eyeshine is usually white. Rabbits and owls are often red. Foxes and domestic cats and dogs usually have green eyeshine, but cats can also have yellow or orange. Spiders are white or green. Blue is found in horses and some other mammals.
Keep in mind that eyeshine color can be different even within the same species. One great horned owl might be glowing back red eyeshine while another great horned owl will have orange.
Pay attention to the height of the eyes and how the animal moves, too. This might help you figure out if you are seeing a rabbit or an owl! Happy nighttime hunting, and enjoy the glow all around you. Just remember that if you are using a flashlight, don’t keep pointing it at a pair of eyes frozen in place. That hurts!
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