Darkness had fallen over the Vail Valley. As I watched the last light fade atop Bald Mountain, I heard a crash from my porch. Was it the huge black bear that frequented the nearby forest? In the light of my cell phone, I saw four yellow-green eyes light up on me. No, not a bear, just two remarkably large raccoons.
It’s always pretty exciting to see eyes glow in the dark. But what causes this eye glow? And why don’t all animals’ eyes reflect?
Eyeshine is caused by a reflective layer at the back of the eye called the tapetum lucidum. Latin for “light tapestry,” the tapetum lucidum is a layer of tissue behind the retina. This layer improves night vision by reflecting visible light back through the retina’s photoreceptors, allowing the light to stimulate photosensitive cells a second time. This double dipping contributes to the superior night vision of some animals, including nocturnal creatures and those that live underwater. Diurnal animals, including humans and squirrels, lack a tapetum lucidum.
Thus, eye shine is a visible effect of having a tapetum lucidum. And it’s a useful adaptation that allows animals to see at night or in low-light conditions, improving their visual sensitivity by up to 50 percent. There must be at least some light available – not total darkness – for the tapetum lucidum to work.
Eyeshine is available in a variety of colors – blue, green, red, white and yellow. Some sources say you can identify an animal based on the color of its eyes. However, since eye shine is a type of iridescencethe color varies depending on the angle you look at it, the color of the light source and the mineral content of the tapetum lucidum.
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Generally, mountain lions and bears have an eye glow that ranges from yellow to red. The eye shine of deer and elk is white, but the eye shine of moose tends to be red. Rabbits and pikas have red eyes. The blue glow of the eyes is seen in other mammals, including horses. Foxes and domestic cats and dogs typically have green eye shine, but cat eye shine can also be orange to red.
Eye color can vary by breed, and even within breeds. Speaking of domestic cats, the difference in eye color is caused by varying amounts of zinc or riboflavin in the tapetum lucidum. And blue-eyed cats, like Siamese, often lack tapetum lucidum.
When you detect a pair of eyes staring at you at night (hopefully not in your car headlights), what factors besides the color of the eye glow are helpful in identifying the owner of those eyes ? Helpful factors include:
- Eye height above ground
- Eye shine movement – jump, weave, leap, climb, fly
- Eye color, shape and size
- Pupil shape – predatory animals have vertically elongated pupils, while the pupils of prey animals tend to be horizontal.
For example, at night, black bears have large, round eyes, often yellow to orange (but sometimes red or green), almost pupilless, set close to the ground. Wild cat eyes typically have a heavy upper eyelid and a pupil perpendicular to the shape of the eye. The white eyes a few feet above the ground probably belong to a deer or an elk.
Our human eyes do not have a tapetum lucidum, but under certain conditions they exhibit what looks like a reflection. For example, the red-eye effect in photographs occurs when a flash of light illuminates the blood vessel-rich retina at the back of the eye. Another effect in humans and other animals that can resemble eye shine is leukocoria.a white sheen indicating abnormalities such as cataracts.
As Halloween approaches, you might know a little more about those weird eyes staring at you all night!
Frances Hartogh is a volunteer wildlife ranger for the Eagle Summit Wilderness Alliance.