Why Cats’ Eyes Glow in the Dark
Cats and many other animals, including most dogscan reflect the light from their eyes. As a result, cats’ eyes usually glow brightly in photos taken in a dimly lit room or glow when illuminated in the dark by a flashlight or car headlights.
Bright-eyed species have evolved to see better in low light conditions, because they either feed or have to watch predators all night, or they mainly hunt at dawn and dusk. In fact, domestic cats can see in conditions that are only 16% as bright like what humans need to see.
Cats are able to accomplish this because their pupils—the openings that appear black in the middle of their eyes that widen and narrow in response to lighting conditions—are special. Pupils function like windows, with larger ones letting more light into the eye. And in the dark, a cat’s pupils can become up to 50% larger than human students. They also have a higher number of a specific type of light-sensitive cell in the back of their eyes than us. These cells, known as rodscaptures low intensity light.
The tapetum lucidum
Besides having large pupils and lots of shafts, cats have something people don’t have: a tapetum lucidum, a Latin medical term that translates to “luminous or shiny tapestry.” The tapetum lucidum is also calledeye shine.”
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 which, like a mirror, reflect light back to the retina. This gives the retina a second chance to absorb more light.
The feline tapetum lucidum is particular because its reflective compound is riboflavina type of B vitamin. Riboflavin has unique properties that amplify light to a specific wavelength that cats can see well, greatly increasing the retina’s sensitivity to low light.
In cats, the tapetum most often glows yellow-green or yellow-orange in color, but the color varies, as does their Iris – the colored part of their eye, which can be green, yellow, blue or gold. Tapetal color variation is not unique to cats and can be found in many species.
The eyes of other animals also shine
Many other animals that need to see at night have a tapetum lucidum. This includes predators and prey, from wild foxes to farm animals. sheep and goats.
The tapetum lucidum is also useful for fish, dolphins and other aquatic animals, as it helps them see better in murky, dark water.
In terrestrial animals, the tapetum is found in the upper half of the eye behind the retina, because they need to see what is on the ground as well as possible. But in aquatic animals, the the tapetum takes up most of the eyebecause they need to see everything around them in the dark.
Even though many animals have bright eyes, some small domestic dogs lack this trait. Most animals with blue eyes and white or light coat also lost this trait.
So don’t worry if your dog’s or cat’s eyes don’t shine. The list of other species without tapetum lucidum includes pigs, birds, reptiles and most rodents and primates, including humans.
Is there a downside?
Unfortunately, animals with a 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 look a little blurrier. So a cat must 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.
Written by Braidee Foote, Clinical Assistant Professor of Veterinary Ophthalmology, University of Tennessee.
This article was first published in The conversation.