Why do cats' eyes glow in the dark?

Humans do not have a tapetum lucidum but cats, including lynxes and pumas, do. The Open University, CC BY-SA^[10][11]

The tapetum lucidum

In addition to having large pupils and lots of rods, cats have something people don’t: a tapetum lucidum, a Latin medical term that translates to “bright or shining tapestry^[12].” The tapetum lucidum is also known as “eyeshine^[13].”

It’s located in the back of the eye behind the retina^[14] – a thin layer of tissue that receives light, converts the light to 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^[15] 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^[16], a type of vitamin B. Riboflavin has unique properties that amplify light to a specific wavelength^[17] that cats can see well, which greatly increases the sensitivity of the retina to low light.

In cats, the tapetum most often glows yellow-green or yellow-orange, but the color varies, just like their irises^[18] – the colorful part of their eye, which can be green, yellow, blue or golden^[19]. Variation in tapetum color is not unique to cats and can be found in lots of species^[20].

Most dogs’ eyes will glow in dark spaces when a light shines on them. Tommy Greco, CC BY-SA^[21]

Other animals’ eyes glow too

Many other animals that need to see at night have a tapetum lucidum. That includes predators and prey alike, everything from wild foxes to farmed sheep and goats^[22].

The tapetum lucidum is also useful to fish, dolphins^[23] and other aquatic animals, because it helps them see better in murky, dark water.

In land animals, the tapetum is found in the top half of the eye^[24] behind the retina, because they need to see what is on the ground best. But in aquatic animals the tapetum takes up most of the eye^[25], because they need to see all around them in the dark.

Like cats, the lemur^[26], a small primate, and its close relative, the bush baby – also known as a “night monkey^[27]” – also have a superreflective tapetum made with riboflavin.

Even though a lot of animals have eyeshine, some small domesticated dogs lack this trait. Most animals with blue eyes and white or light-colored coats^[28] have also lost this trait.

So don’t be alarmed if your dog’s or cat’s eyes don’t glow. The list of other species without a tapetum lucidum^[29] includes pigs, birds, reptiles and most rodents and primates – including humans.

This bush baby can probably see better at night than you can. Smartshots International/Moment via Getty Images^[30]

Is there a downside?

Unfortunately, animals with a tapetum lucidum sacrifice some visual acuity^[31] for their ability to see in dim light.

That’s because all that light bouncing around as it reflects off the tapetum can make what they see a little fuzzier. So, a cat needs to be seven times closer^[32] to an object to see it as sharply 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.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com^[33]. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

Authors: Braidee Foote, Clinical Assistant Professor of Veterinary Ophthalmology, University of Tennessee

Read more https://theconversation.com/why-do-cats-eyes-glow-in-the-dark-179531

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