By Dan Zarlenga, Missouri Department of Conservation
Photo caption: The tapetum lucidum in the eyes of a whitetail deer stare back from the dark of night. Photo by Joesboy via iStock.
You’re venturing out during a dark night in October. It’s the month of Halloween. Your mind may be swirling with thoughts of haunted houses, ghost stories heard around the campfire, or the latest scary movie you’ve seen. You stop. Something in the back of your mind tells you to shine your light, over there. And suddenly in the darkness are two eerie eyes shining back at you! What’s happening?
Fortunately, you don’t have to fall back on ghost stories and scary movies to explain these glowing orbs. All you need to do is keep your eyes on science.
Many species of wildlife in Missouri display eye shine when exposed to a bright light at night. These include deer, racoons, coyotes, owls, frogs, and spiders. No need to fear though; it’s just a special adaptation that helps them see in the dark.
The cause behind this mysterious glow is a structure within the eye called the tapetum lucidum. All these creatures have this special layer in the back of their eye that reflects light. The tapetum lucidum does not produce any light itself. Rather it bounces the reflected light it receives back into the retina like an optical amplifier, reinforcing and brightening the image, giving the retina a second chance to see it.
The photon feedback loop is nature’s version of night vision. While we may only see the blackness of night, these creatures see the world at night.
This special power is vital to animals who conduct most or all essential functions of life during the night, like moving about, foraging for food, hunting, or trying to avoid being hunted.
The color of the shine varies with the species. The tapetum lucidum of a whitetail deer, always wary of predators, reflects a greenish glow when light hits it. The eyes of night scavenging racoons cast a yellow or amber reflection. Nocturnal canine hunters like red foxes and coyotes pierce the darkness with a sheen of pale yellow-to-white.
Owls rely largely on sound to home in on their prey, but eyesight is still important to them. The tapetum lucidum in some manifests like two smoldering orange-red coals. Hunters of the night also, the eyes of frogs and toads will likewise glow red when hit by a flashlight. And who hasn’t seen the ethereal, emerald glow of wolf spiders staring up from the dewy ground!
It’s easy for us night-blind humans to envy the magic power which enables these creatures to penetrate the blanket of darkness. But consider this. While the extra reflection created by the tapetum lucidum improves night vision for these animals, it reduces their ability to discern sharpness and detail compared to human vision. These nocturnal denizens excel at detecting movement and contrast in the dark, but
they lack the fine focus that we have by day. This would make them blind to reading words, examining intricate textures, or doing complex work like making tools.
So, next time your flashlight beam catches glowing eyes that watch you in the October dark, it’s not supernatural. It’s real nature. And it’s revealing a superpower of nocturnal adaptation!