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Why do cats’ eyes glow in the dark? Chloe, age 10, Barkhamsted, Connecticut
Cats and many other animals, including most dogs, can reflect light from their eyes. That’s why cats’ eyes will usually shine brightly in photos taken in a dimly lit room or glow when illuminated in the dark by a flashlight or a car’s headlights.
Species whose eyes glow have evolved to see better in low light because they either forage or need to look out for predators throughout the night, or they do most of their hunting at dawn and dusk. In fact, domesticated cats can see in conditions that are only 16% as bright as what people require.
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 operate like windows, with bigger 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-sensing cell in the back of their eyes than we do. These cells, called rods, catch low-level light.
Humans do not have a tapetum lucidum but cats, including lynxes and pumas, do.
The Open University, CC BY-SA
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.” The tapetum lucidum is also known as “eyeshine.”
It’s located in the back of the eye behind the retina – 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 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 vitamin B. Riboflavin has unique properties that amplify light to a specific wavelength 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 – the colorful part of their eye, which can be green, yellow, blue or golden. Variation in tapetum color is not unique to cats and can be found in lots of species.
Most dogs’ eyes will glow in dark spaces when a light shines on them.
Tommy Greco, CC BY-SA
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…