Why Do Cats' Eyes Glow in the Dark?

Our feline companions exhibit different behaviors, but one of the most unsettling is when they sit staring through the darkness, and their eyes turn into flashlights. What is the reason behind that strange glow? First, science teaches us that it is not something exclusive to cats, and second, it can also explain the differences in their color.

Anyone who decides to adopt a cat for the first time needs to go through a certain adaptation process. Schedules, hygiene, feeding, behavior, communication... some humans basically have to learn from scratch, and assimilate very strange details, to the point where they give us a good scare. The most recurring example is when the cat sits in the dark to observe “something”, and its eyes glow as if they had a kind of “biological backlight”.

The Glowing Eyes of Cats

The mystery is associated with the formidable night vision they possess. Personally, I have seen them (after the fact, obviously) catch crickets, cockroaches and rats even on the darkest nights, and that performance comes from several factors, among which the so-called tapetum lucidum stands out.

Experts describe the tapetum lucidum as a special layer located immediately behind the retina, and its goal is to give the cells a second chance at photoreceptor stimulation. In other words, it is a mirror that reflects light back to the retinal cells, and on certain occasions, that light escapes through the pupils, causing our cats to imitate Scott Summers (?).

The color of the glow depends on the compounds that form the tapetum lucidum, and there are important differences between animals. In fish, guanine predominates, producing a white glow. In sheep, we find collagen, which can give a green or blue glow. And among cats, we find riboflavin (vitamin B2) with zinc. The amount of zinc is what establishes their tendency toward yellow, blue, or green.

In short, the glow in cats' eyes is a product of their natural functioning. Like the vast majority of diurnal animals, humans do not have a tapetum lucidum. The “beta version” in the human eye is generated by the famous red-eye effect, and it requires something as fast and powerful as a camera flash.