Why Do Cats' Eyes Glow in the Dark?
Have you ever snuck to the fridge for a midnight snack only to find that a pair of glowing cat eyes have caught you in the act? You wouldn't be alone if you've chalked this superhero-like ability up to your cat's night vision. But, experts say, the actual function of glowing eyes is questionable.
"We can't ask cats to read an eye chart, so it's impossible to be certain as to their precise visual acuity," Lauren Jones, DVM, veterinary advisor for PetCoach says. But what we do know is that a cat's ability to see well in low light settings is due to the greater number of rods in their eyes (aka cells that help me, you, and cats see in low light settings). When it comes to glowing eyes, Jones says that it's not clear they play the most important role when it comes to seeing better in the dark.
What Does It Mean When a Cat's Eyes Glow in the Dark?
Glowing eyes, or eyeshine, is a normal phenomenon among nocturnal and crepuscular animals. The mechanism behind why a cat's eyes glow is pretty clear to the experts, but when it comes to how much it helps cats see—there may be a debate.
Sarah McCormack, DVM, Associate Veterinarian at Northwest Neighborhood Veterinary Hospital in Portland, Ore., explains that a thin reflective layer called the tapetum lucidum is responsible for the glowing eyes of a cat. Simply put, the tapetum lucidum acts like a mirror. It reflects leftover light back to receptors and emits a glow from the eye in the process.
"We think what the tapetum does is make dark conditions appear brighter," McCormack explains. Light particles that weren't absorbed by rods on their way to the back of the eye are reflected, providing the rods with a second chance of absorption.
The brightness of eyeshine is determined by the tapetum development and can vary between species, breeds, eye color, and even coat color, McCormack says. Siamese cats, for example, are known to have a poorly developed tapetum lucidum and you might guess they have poor night vision because of it. But, McCormack says, "We suspect so, but we can't ask them, so we rely on the observations of their owners and their visual acuity in the exam room."
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How Do a Cat's Eyes Work?
Just like our eyes, a cat's eyes work by taking in light through the pupil. This light travels to the back of the eyeball where it smacks into, and is absorbed by, photoreceptor cells (rods and cones). Here's where the real difference between a cat's eye and a human's eye becomes apparent.
For starters, humans have more cones. This means that we're able to see color better than a cat can. But cats make up for their slight color-blindness with more rods than us, allowing them to see better in low light.
Behind the rods and cones is where the tapetum lucidum is, and it's not present in humans. Here, unabsorbed light bounces off the reflective layer and scatters toward the front of the eye for one more chance at getting soaked up by a rod—and causing your cat's eyes to glow.
Depending on what you're up to at dawn (hunting mice or not), seeing better in low-light settings isn't always a plus. McCormack explains that when light bounces off the tapetum lucidum, it scatters in directions different than it entered. This causes the sharpness of an image to decrease. In fact, she says, a cat has to be seven times closer to an object to see it as sharply as we do.
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Why Does Only One of My Cats' Eyes Glow?
Jones says not to sweat it if you don't see both of your cat's eyes glowing at a particular angle, it's possible that light is simply entering the eyes differently. But, if there is a sudden change in your cat's eyeshine or if they have always lacked eyeshine in one or both eyes, McCormack says there could be something else going on.
If your cat has never presented with eyeshine, the tapetum could be absent or not fully formed in one or both of your cat's eyes. If the change in eyeshine is sudden, it could be a sign of infection, cancer, cataracts, or other corneal changes, McCormick says. So, it's best you make an appointment with your vet.