Cats and many other animals, including most dogs, can reflect 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, as they forage or have to watch for predators throughout the night, or they mainly hunt at dawn and dusk. In fact, domestic cats can see in conditions that are only 16% as bright as 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 dim light, a cat’s pupils can become up to 50% larger than human pupils. They also have more of a specific type of light-sensitive cells in the back of their eyes than we do. These cells, called rods, capture 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 “shining or shiny tapestry.” The tapetum lucidum is also known as “eyeshine”.
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 that 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.
The feline tapetum lucidum is special because its reflective compound is riboflavin, a type of B vitamin. Riboflavin has unique properties that amplify light at a specific wavelength that cats can see well, greatly increasing sensitivity from the retina to dim light.
In cats, the tapetum most often glows yellow-green or yellow-orange, but the color varies, as do their irises – 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 farmed sheep and goats.
Tapetum lucidum is also useful for fish, dolphins and other aquatic animals, as it helps them see better in murky, dark waters.
In land animals, the tapetum is in the upper half of the eye behind the retina because they need to see what is on the ground better. But in aquatic animals, the tapetum takes up most of the eye, because they need to see everything 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 superreflective tapetum made from riboflavin.
Even though many animals have bright eyes, some small domestic dogs lack this trait. Most animals with blue eyes and white or light coats have 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 dim light.
That’s because all that light bouncing around when it reflects off the tapetum can make what they see look a little blurry. So 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.
Written by Braidee Foote, Clinical Assistant Professor of Veterinary Ophthalmology, University of Tennessee.
This article first appeared in The Conversation.