You’ve seen it: the yellow glare from your cat’s eyes as she passes a beam of light in a darkened room. That yellow glare might seem creepy, but her “eye shine” is a sign of her distinct advantage over a human being in a low-light area.
Cat Vision vs. Human Vision
Even domestic cats are nocturnal predators. Their large, luminous eyes are specifically designed to help them hunt at night. Their daytime vision isn’t terrific -- imagine near-sighted Aunt Martha reading her menu without her glasses when you go out to eat -- but at night they can see the smallest prey moving even from a distance. In order to see so well at night, the cat’s eye has a wider pupil and a greater curve of the lens and the cornea. The cat’s eye also has a reflective surface called the tapetum lucidum, which reflects light back through the retina and causes the yellowish-green or red “eye shine.” Although your cat's actual eye color doesn't change -- Puss still has the green, orange or blue eyes she was born with -- the wider pupil she has in low light reveals more of the tapetum, and the reflected light takes on the color of that surface.
The tapetum lucidum is essentially a colored mirror behind cats' retinas. Human beings, other primates and certain other animals like squirrels, horses and kangaroos lack this mirror. The light reflected by the tapetum helps the cat see better at night or in any dimly lit environment. Different species have different colored tapetums, including pink and blue. The eyes of animals that lack a tapetum also reflect light. The “red eye” that shows up in flash photography, usually when you are trying to preserve memories of sentimental family moments, is a reflection of the light off the blood vessels contained in the human retina.
Most cats have a tapetum lucidum that reflects a light that appears to be either yellow or green. The differences in the eye shine color reflected are caused by varying amounts of zinc or riboflavin in the iridescent tapetum itself. The amount of light reflected and the color of the light being reflected varies. Even cats within the same breed can have different reflected eye colors. Cats have riboflavin in their tapetum, unlike dogs who have zinc.
Cats with blue eyes, whether white-coated or Siamese, typically lack a tapetum lucidum. Like human beings, their eyes produce the red eye effect. Some Siamese cats do have eyes that apparently reflect a yellow light, but they are the only breed in which most members lack a tapetum.
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