Why do cats and dogs' eyes glow in the dark?
Ever taken a picture of Fluffy only to recoil in horror at their demonised portait, glowing yellow eyes staring straight into your soul? Cats and dogs among other animals, both experience a curious "glowing" of the eyes when in the dark and a torch is shone on them or when a camera light flashes.
You might remember from science class that the human eye contains sensory receptors called rods and cones. Cones are responsible for detecting different colours and function best in the light. The rod receptors in our eyes allow us to see in low light settings however rods cannot pick up colours. Cats and dogs have a much higher concentration of rod receptors in their eyes which allows them to see much better in the dark compared to humans.
Unlike humans, cats and dogs have an additional layer of tissue in their eyes called a Tapetum Ludicum. This tissue layer sits behind the retina and reflects light back through the retina to hit the photoreceptors (rods and cones) a second time. This reflection gives the light a second chance to hit the animal's photoreceptors which in turn helps to enhance their night vision. This reflection is the "glowing" that we see when we accidentally shine a bright light on our pet in a dark environment.
Why do different animals have a different colour shine?
The mineral concentrations in the Tapetum affect the colour that is reflected back through. Depending on the levels of Zinc or Riboflavin present in the Tapetum tissue, the animal's eyes will reflect different colours from yellow or green to even violet.
It has been found that most blue eyed cats and dogs are actually lacking a tapetum membrane and in single flash photos you will notice that the "reflected" colour is red. Without the mirror like function of the tapetum, the flash is actually picking up the red colour because of the blood vessels in the eye which is what also happens to humans.
No need to call an exorcist next time you see the glowing yellow eyes through your front window, it's just Fluffy's Tapetum Ludicum shining back at you.
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