Curious children: why do cats’ eyes glow in the dark? | Kiowa County Press

The same thing that makes their eyes shine helps cats see better in dim light.
Cletus Waldman/EyeEm via Getty Images

Braidee Foote, University of Tennessee

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Why do cats eyes glow in the dark? Chloe, 10, Barkhamsted, Connecticut

Cats and many other animals, including most dogscan reflect the light from their eyes. This is why cats’ eyes usually glow in photos taken in a dimly lit room or glow 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 watch for predators all night, or they mostly hunt at dawn and dusk. In fact, domestic cats can see in conditions that are only 16% as bright like what 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 lighting 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 a higher number of a specific type of light-sensitive cell in the back of their eyes than us. These cells, called rodscatches low level light.

Diagram of the eyes of a human, a lynx and a puma
Humans do not have a tapetum lucidum but cats, including lynxes and pumas do.
The open university, CC BY-SA

The tapetum lucidum

Besides having large pupils and lots of penises, 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 known as “eye 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.

A dog with bright eyes
Most dogs’ eyes glow in dark spaces when a light shines on them.
Tommy Greco, CC BY-SA

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.

like cats, the lemura small primate, and its close relative, the baby bush – also known as “night monkey” – also possess a super-reflective riboflavin-based tapetum.

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.

Bush babies' eyes are shining
This bush baby can probably see better at night than you.
Smartshots International/Moment via Getty Images

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.

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The conversation

Braidee FooteAssistant Clinical Professor of Veterinary Ophthalmology, University of Tennessee

This article is republished from The conversation under Creative Commons license. Read it original article.

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