Why Do Cat’s Eyes Glow In The Dark
The glowing eyes of cats at night can be shocking or even scary if you don’t expect them. According to Ancient Egyptians, cats could capture the glow of the setting sun and keep it safe in their eyes until morning. Today, we know that the eyes of many nocturnal animals glow in the dark because they reflect the light.
Why Do Cats’ Eyes Glow In The Dark?
Although all eyes reflect the light, cats’ eyes, instead of large pupils and rods, have a special structure called tapetum lucidum that makes them glow. This tapetum lucidum, meaning “shining layer” in Latin, is made up of crystal cells, like a tiny mirror in the back of their eyeballs, that helps these animals see very well at night.
How Does It Work?
When the light enters a cat’s eye, it may take some routes:
- Part of the light directly hits the retina. The retina is a layer at the back of the eyeball containing cells, which trigger nerve impulses that pass via the optic nerve to the brain.
- Some light, however, will miss the photo receptors, pass past the retina and hits the tapetum lucidum, which reflects light back, increasing the light available to the photoreceptors. This is why cats can see better in the dark than humans.
- The last part of the light bounces off the tapetum lucidum misses the retina and bounces back out of the cat’s eyes. This reflected light is what we see when a cat’s eyes glow.
Do Other Animals Have A Tapetum Lucidum?
Many other animals that need to see at night, in order to survive, have a tapetum lucidum. That includes predators and prey alike, many aquatic animals that need to see better in dark water, and small primates. Those are:
- deer
- canids and felids
- sheep, goats, cattle, horses
- ferrets
- lemurs and their close relative, the bush baby
- marsupials
- fruit bats
- a lot of fishes, sharks, and dolphins
- owls and a few other nocturnal birds
- alligators
In land animals, the tapetum lucidum is found in the top half of the eye behind the retina, while in aquatic animals it takes up most of the eye, as they have to see all around them in the dark.
However, some small domesticated dogs, cats, and other animals with blue eyes and white or light-colored coats have lost this trait. So don’t be alarmed if your dog’s or cat’s eyes don’t glow.
Downsides Of Tapetum Lucidum?
However, tapetum has also a few cons. Animals with a tapetum lucidum sacrifice some visual sharpness for their ability to see in dim light. This is because the light that’s bouncing back off the tapetum can make what they see a little fuzzier. So, a cat needs to be a lot closer to an object to see it as sharply as a human would in a bright place.
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