Why do cat eyes glow in the dark?
Cats and many other animals, including most dogs, can reflect light from their eyes. That’s why cats’ eyes usually glow brightly in photos taken in a dimly lit room or shine when illuminated in the dark by a flashlight or car headlights.
Species whose eyes glow have evolved to see better in low light because they feed or need to watch for predators at night, or do most of their hunting at dawn and dusk. In fact, domesticated cats can see in conditions that are only 16% brighter than people require.
Cats can see at night because their pupils, the black-like openings in the middle of their eyes that widen and narrow in response to light conditions, are special. Pupils function like windows, with larger ones letting more light into the eye. And a cat’s pupils can become up to 50% larger than human pupils in low light conditions. They also have a greater number of a specific type of light-sensitive cells in the back of their eyes than we do. These cells, called rods, capture low-level light.
Humans do not have a tapetum lucidum, but cats, including bobcats and cougars, do.
The tapetum lucidum
In addition to having large pupils and lots of rods, cats have something that people don’t: a tapetum lucidum, a Latin medical term that translates to “bright tapestry “. The tapetum lucidum is also known as “eye glitter.”
It is located at the back of the eye behind the retina, a thin layer of tissue that receives light, converts the light into an electrical signal and sends this signal to the brain to interpret the image.
A cat’s tapetum lucidum is made up of cells with crystals that, like a mirror, reflect light back to the retina. This gives the retina a second chance to absorb more light.
Feline tapetum lucidum is special because its reflective compound is riboflavin, a type of B vitamin. Riboflavin has unique properties that amplify light to a specific wavelength that cats can see well, which greatly increases the retina’s sensitivity to dim light.
In cats, the tapetum usually glows greenish-yellow or orange-yellow, but the color varies, as do their irises, the colored part of their eyes, which can be green, yellow, blue or gold. Tapetum color variation is not unique to cats and can be found in many species.
The eyes of other animals also glow
Many other animals that need to see at night have a tapetum lucidum. That includes predators and prey alike, from wild foxes to sheep and farm goats.
Tapetum lucidum is also useful for fish, dolphins and other aquatic animals, because it helps them see better in murky, dark water.
In terrestrial animals, the tapetum is located in the upper half of the eye behind the retina, because they need to see better what is on the ground. But in aquatic animals, the tapetum occupies most of the eye, because they need to see around them in the dark.
Like cats, the lemur, a small primate, and its close relative, the bush baby, also known as the “night monkey,” also have a super-reflective tapetum made of riboflavin.
Although many animals have eye glitter, some domesticated small dogs lack this trait. Most animals with blue eyes and white or light-colored fur have also lost this trait.
So don’t be alarmed if your dog or cat’s eyes don’t sparkle. The list of other species without tapetum lucidum includes pigs, birds, reptiles and most rodents and primates, including humans.
Are there any drawbacks?
Unfortunately, animals with tapetum lucidum sacrifice some visual acuity for their ability to see in the dark.
This is because all that light bouncing around when it reflects off the tapetum can make what they see a little blurrier. Therefore, a cat needs to be seven times closer to an object to see it as clearly as a person would in a brightly lit place.
But don’t worry, I’m sure your cat would rather see clearly at night than read a book.
