Why do a cat's eyes shine in the dark?

If you've ever wondered why your cat's eyes seem to glow in the dark, or pondered how a hummingbird finds its way to certain nectar-filled flowers, what you are really asking is, do animals see the way we do? Though we might like to think that animals see the world with the same colors and shadows as human beings-although maybe from a different angle, perhaps high in the sky like an eagle, or under the sea like a shark-the truth is that animal vision is not like our own and differs greatly among animal species.

Glow in the Dark

Human beings are not like tigers, or housecats for that matter, whose eyes are superbly well adapted to seeing in the dark. One reason is that cats have more rods than cones in their retinas, unlike humans, making kitty's night and motion vision superior. (Rods are the receptors that the eye uses for nighttime viewing and sudden movement; cones are used during the daytime and process color information.)

Also, cats' pupils are shaped differently than those of humans (they are elliptical rather than round), which allows for a much larger pupil size. In fact, the most notable feature of nocturnal animals is the size of their eyes. The reason for this is that large eyes can collect more ambient light.

Fun fact: An owl's eyes fill over half of its skull!

As well, cats' eyes open and close much faster than do ours. And cats have a special membrane on the back of their eyes (called the tapetum lucidum, literally meaning "bright carpet") that increases the quantity of light caught by the retina. The tapetum collects and re-emits light back to the retina, giving the rods a second chance to absorb the image, thus maximizing their sensitivity to low light levels. As this light is reflected off the tapetum, the animal's eyes appear to glow.