Glowing eyes in cats and dogs – Pet tip 119

Have you ever been frightened at night when looking in your backyard, seeing a pair of glowing eyes glaring back at you? As you quickly flick on the porch light, your fears subside as you realize that it is just your beloved pet cat! It may look like your pet’s eyes are glowing in the dark, but technically they are just reflecting light back from the surrounding environment. Curious about why your pets eyes are capable of reflecting light in this manner, and why your own eyes are not capable of this function – read on!

The cat, dog, and many other animals, have structural differences in their anatomy that allows for this function of reflecting light. The white part of your pet’s eye, called the sclera, acts as a protective coating and helps it keep its shape. Beneath the sclera, the material that reflects light is found on another layer called the choroid. The choroid is a membrane that absorbs the light that is not reflected. On the opposite side of the substance that reflects light in the eye, is the retina. The retina is the inner coating of the eye and is responsible for forming images that are sent to the brain for interpretation. Finally, the actual section that reflects the light is found on the choroid, between the choroid and the retina. It is called the tapetum lucidum, and has a shiny blue appearance in its natural state. If reading this description has boggled your eyeballs, envision these layers in the eye as a mini peanut butter and jam sandwich in your eye. It may sound strange, but here’s an explanation: the top layer of bread is the sclera. You spread your jam on the top slice of bread (sclera). The jam is the choroid. The bottom layer of bread is the retina. You spread your peanut butter (tapetum) on the bottom slice of bread (retina). Then you put the two pieces of bread together and get your sandwich, from the outside of the eye to the inside: sclera (bread), choroid (jam), tapetum (peanut butter) and bread (retina). This arrangement of layers is at the back portion of the eyeball, light shines in front the front of the eyeball to reach these layers. It is an efficient (and tasty!) way to maximize light absorption.

Take a moment to breathe, and let’s ponder the function of these structures a little more. You are now aware that the tapetum (remember, the reflective surface) acts as a mirror, reflecting light. But where exactly does it reflect the light? Since we are discussing the back end of the eye, light coming into the eye reaches the retina first. The retina is thin like tissue paper. This allows for light to pass through the retina to the tapetum to be reflected back to the retina! This results in a greater quantity of the light entering the eye that can be used by the retina for image processing. This is not as beneficial in conditions with ample light, but possesses endless benefits in the nighttime. This is why cats and other animals do not bump into walls in the middle of the night as we do. As long as there is a small amount of light entering their eyes from the environment, they can use this light much better than we can to see in the dark by way of the tapetum.

When our pets are utilizing this mechanism we see the light reflecting back to the retina as a glow that can range in colour from blue to yellow. Fascinatingly, it seems that the colour of a dog’s coat is loosely connected to the colour of their eye glow. Black coated dogs tend to have a green glow, while dogs and cats under six months will often have a blue glow. Young animals of different coat colour usually have this same reflective light colour in their eyes because they are still maturing.

That’s all fine and dandy, but wait! Why is it that some humans get red eye, or their eyes appear to glow when a light is flashed in their eyes in a picture? Although this may appear to be the same as what is occurring in your furry friends, the cause of the red glow is because humans are lacking the tapetum. Specifically, what you are seeing with red eye is the choroid, which contains numerous blood vessels containing red blood. Interestingly, some of our feline companions that have little pigmentation show the same red colour glow even though they still have and make use of their tapetum.

The tapetum is an amazing adaptation of animals, which humans can only marvel at. It allows for animals to see very efficiently in conditions of low light, and is responsible for the glow that we often see in our pet’s eyes when a bright light is shone on them. Next time you are bumping around in the middle of the night trying to find your way to the light switch, have comfort in knowing that at least your cat is having great satisfaction watching you.

By Laura Platt – Pets.ca writer