The conversation asks kids to send in questions they’d like an expert to answer. Chloe, 10, wants to know why cats’ eyes glow in the dark. A veterinary ophthalmology expert explains.
Cats and many other animals, including most dogs, can reflect light from their eyes. This is why cats’ eyes usually shine brightly in photos taken in a dimly lit room, or sparkle when illuminated in the dark by a flashlight or car headlights.
Species with glowing eyes have evolved to see better in low light conditions because they feed or have to search for predators all night, or they mostly hunt at dawn and dusk. In fact, house cats can see in conditions that are only 16% as bright as people need.
Cats accomplish this because their pupils—the openings that appear black 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 dim light. They also have higher numbers of a specific type of light-sensitive cell 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’s 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 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 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.
Braidee Foote, Clinical Assistant Professor of Veterinary Ophthalmology, University of Tennessee. This article first appeared on The conversation.