Say you’re out walking late at night, and you come across a pair of glowing dots in the distance. Maybe they’ll blink at you, or they’ll just watch quietly. Don’t get too spooked – that’s just an animal looking back at you.
It may certainly feel creepy but rest assured that there are no ghostly forces behind those uncanny lights. The reason behind this phenomenon is much simpler: It’s the result of evolutionary mechanisms which allow certain species to see better in the dark.
First, there’s one thing to clarify. When you see these animals at night, their eyes don’t actually “glow,” as in, producing light by themselves. Technically, their eyes are reflecting light from other sources, whether it’s natural starshine or your bright flashlight.
When light passes through our eyes, it hits our retinas, composed of cells called rods or cones. Rods are useful in darker settings, acting as receptors of light. Cones constitute much of our daytime vision, letting us perceive the world in detail and color.
How does seeing at night differ? Eye structures and functions in the animal kingdom vary far and wide, but one commonality that night-loving vertebrates share is the “tapetum lucidum” in their eyes.
Latin for “shining tapestry,” the tapetum looks the part. A cross-section of an eyeball may reveal an iridescent sheet: turquoises and blues and golds swirling like some galactic oil spill.
Biologically, the tapetum lucidum is a mirror-like surface located behind a vertebrate’s retina. Its job is to catch errant photons which photoreceptors didn’t manage to absorb the first time, reflecting those light particles back onto the retina a second time.
As a result, animals see better at night by double-dipping their light. They can process the light coming inward through their eyes, but also get the information as it goes back outwards. Our primary evidence of the tapetum lucidum at work is called “eyeshine,” also known as the eerie, perceived glow of animal eyes at night.
Unfortunately, humans are among the diurnal or daytime species that don’t have or need tapetum lucidum. Pigs, kangaroos, and most birds don’t possess it either.
The closest we can get to eyeshine is through red eyes. We’ve all dealt with eerie red pupils that ruin an otherwise perfectly good photo. That red flash isn’t because we have tapetum lucidum, but rather because the bright, unexpected camera light bounces off the blood-rich backs of our eyes, igniting them crimson.
Among species that do possess biological reflectors, eyeshine varies greatly in color, from a dog’s blues to an owl’s reds. You could try to guess as to what species you’re looking at based on its color of eyeshine.
Here are more animals, some familiar and others unexpected, that illuminate the dark with brilliant evening eyeshine.
Read More: The 5 Senses Animals Have That Humans Don’t
Eyes of a black cat closeup. (Credit: SeeVera/Shutterstock)
Cats come from a large family, ranging from the tiger to the domestic housecat, meaning that tapetum color ranges too. According to researchers, cat tapeta, which cover most of their eyes, can change color as they mature from kittens – but their eyeshine most often are in the yellow-green or yellow-orange range.
Domestic cats, much like their fellow, bigger family members, are carnivores, and therefore find prime hunting time in the dawn or dusk. With the tapetum lucidum and many rods, cats are extraordinarily sensitive to any amount of light. However, a smaller supply of cones means perception in the daytime is limited. So while cats can see six to eight times better than their human counterparts can in the dark, they’re quite a bit near-sighted in the day.
Read more: Are Dogs and Cats Colorblind?
Four cute baby raccoon sitting on a deck at night. (Credit: Tony Campbell/Shutterstock)
It might be a pain to hear them rummaging through your dumpsters or scurrying across your rooftop – but you must admit the panda-esque patterns and round, dark eyes of the raccoon are endearing.
If you catch a racoon rifling through your garbage at night, you’d likely see bright yellow lights reflected at you. The brighter a vertebrate’s eyeshine, the higher the concentration of rods in their eyes. This means that raccoons, like cats, at the very least aren’t great at distinguishing between colors – even if we don’t know for sure that they’re totally colorblind.
Fighting reindeers in the village of the tribe Saami near Tromso, North Norway, Europe. (Credit: Nowaczyk/Shutterstock)
The reindeer of the Arctic reflect their environments in more ways than just seeing. Researchers found that when winter drenches the atmosphere in a deep, pervasive blue, reindeer tapetum also darkens into a rich, reflective ultramarine. This is a stark change from the typical gold-turquoise mix of reindeer tapetum in the summer.
This change is likely another adaptation that allows reindeer to survive in the lengthy, dark winters of the Arctic, where twilights can last up to 24 hours. When living in such consistent darkness, every photon counts.
Reindeer so far are the only mammal with the ability to change their tapeta color with the seasons. But it’s not a permanent change: When the snow melts and summer returns once more, the tapetum lucidum of the reindeer transforms back into the iridescent membrane it once was.
A close up of a Caribbean Reef Shark (Credit: Greg Amptman/Shutterstock)
Mammals aren’t the only vertebrates who give off eyeshine. In the sea, where waters churn and grow murky, light really has only one way to go: down.
Ultimately, the historic misconception that sharks have poor vision, supplemented by a keen sense of smell, doesn’t exactly hold up, at least in the dark. In dim settings, sharks are about 10 times more sensitive to light than humans are.
Sharks, particularly species who operate in the deep zones of the ocean where sunbeams rarely penetrate the water, rely on their tapetum lucidum. Among a myriad of other sensory adaptations, they take advantage of every scattered photon.
For sharks that also frequent the shallower parts of the sea, their tapeta can darken during the day to deal with the brighter conditions – sort of like donning a pair of sunglasses.
American Alligator Reflection in Water (Credit: Connor Howe/Shutterstock)
If you find yourself atop a boat, coasting across a swampy land, you may encounter a pair of bright orbs floating above the rippling waters, emitting vivid red.
Spotlighting, or cataloging the eyeshine of crocodiles is actually one way researchers in Australia have been monitoring the health and numbers of salt and freshwater crocodiles.
At night, surveyors can stand on boats and cast bright lights across the water surface in a zigzagging pattern. When their lights snag onto crocodilian eyeshine, visible from even long distances in ideal conditions, then it’s a success.
Boophis madagascariensis during night in Madagascar. Night frog is sitting in dark. Small green frog with pointed mouth. (Credit: Martin Prochazkacz/Shutterstock)
The eyeshine of a wide-eyed frog might appear greenish in the dark – and that’s one indicator that tells researchers they’ve spotted these amphibians out and about at night. Like how spotlighting is used to check on crocodile populations, the technique is also used to scout out frogs.
In fact, a research team at the University of Rhode Island has recently proven that the Eastern spadefoot frog is not as rare as scientists have previously thought, using spotlighting and eyeshine.
Scientists from the Natural History Museum, have discovered a connection between frogs that are active during the day (diurnal frogs) and the development of lenses in their eyes. These lenses help them filter out UV light, making their vision clearer.
Read more: Meet 10 of the World’s Most Adorable Frogs
Adult Female Wolf Spider of the Family Lycosidae (Credit: Vinicius R. Souza/Shutterstock)
With hunting habits like the furry and formidable predator we all know and love, the wolf spider lives up to its namesake. The tapetum lucidum of these night hunters endows them with extraordinary night sight, allowing them to scuttle across all sorts of habitats and digest their victims by injecting them with enzymes strong enough to turn their prey into a smoothie.
If you’re fortunate enough – or maybe not, depending on your arachnophobia levels – to live in the neighborhood of one of these hunters, shining your flashlight to the ground can uncover the brilliant pinpricks of this arachnid.
Does that make these spiders even creepier-and-crawlier? It’s all in the eye of the beholder. Witnesses of wolf spiders have described their eyeshine as diamonds, fallen stars, or even fairy lights, scattered and glistening across the ground.
Read More: Meet 5 of the Biggest Spiders in the World