Why do the eyes of cats, dogs and other animals light up in the dark?

© Laveol / Wikimedia Commons

Like two gems shining at night, cats, dogs or even foxes will surprise you at night and pierce you with their gaze! Cats and many other animals can actually reflect light from their eyes. This is why their eyes usually light up in pictures taken in a dimly lit room or light up when illuminated in the dark by a flashlight or car headlights.

The cat, the lord of the night

Species with glowing eyes have evolved to see better in low light, either because they forage for food, have to see predators all night long or mainly hunt in darkness, dawn and dusk. Among these species, we often think of the cat.

In fact, domestic cats can see under conditions that are only 16% of the brightness required by humans. Cats achieve this thanks to the specificity of their pupils, these black openings located in the center of the eyes, which expand and narrow in relation to the brightness. Pupils act as windows where the larger ones let more light into the eye. And a cat’s pupils can grow up to 50% larger than a human’s in low light. Cats also have more of a certain type of light-sensitive cells in the back of their eyes than we have. These cells, called rods, capture light at low intensity.

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Their secret? that wallpaper lucidum

In addition to having large pupils and many rods, cats have something humans do not have: a wallpaper lucidum, a Latin medical term translated into shining or glowing tapestry. that wallpaper lucidum is also known as dark circle. It is located at the back of the eye, behind the retina, where a thin layer of tissue receives light, converts it into an electrical signal and sends this signal to the brain to interpret the image.

that wallpaper lucidum of the cat is composed of cells with crystals that, like a mirror, reflect light towards the retina. This gives the retina a second chance to absorb more light. that wallpaper lucidum is special because its reflective compound is riboflavin, a kind of vitamin B. Riboflavin has unique properties that amplify light at a certain wavelength that cats can see well, which greatly increases the sensitivity of the retina in low light.

In cats is wallpaper usually 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. The color variation of wallpaper is not unique to cats and can be seen in many species.

Other animals can also shine!

Many other animals to see at night have one wallpaper lucidum. This includes both predators and prey, from wild foxes to sheep. that wallpaper lucidum is also useful for fish, dolphins and other aquatic animals as it allows them to see better in dark and turbid water.

In terrestrial animals it is wallpaper are in the upper half of the eye, behind the retina, because they need to see better what is on the ground. But in aquatic animals it is wallpaper occupy most of the eye as they have to see everything around them in the dark. Like cats, the lemur, a small primate, and its close relative, the bush baby, also have one wallpaper super reflective made of riboflavin.

Although many animals have bright eyes, some small domestic dogs lack this feature. Most animals with blue eyes and white or light fur have also lost this trait. So do not worry if your dog’s or cat’s eyes do not shine! The list of other species is missing wallpaper lucidum includes pigs, birds, reptiles and most rodents and primates, including humans.

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