Who has not been startled by suddenly seeing a pair of glowing eyes in the dark? No matter how domesticated, your dog is essentially a hunter, and their eyes reflect this. That eerie glow is the reason they can see better at night.
Dogs eyes glow at night due to tapetum lucidum. To hunt at night, dogs need to be able to see more clearly than humans do, and nature provided the solution in the form of the tapetum lucidum. It is an iridescent structure that lies behind the retina, enabling dogs to see in dimmer light. This structure creates that glow in the dark.
Read on for an in-depth explanation of how dogs’ eyes work, how this differs from human eyes, and the role of the tapetum.
Why Dogs’ Eyes Glow in the Dark
Dogs have the tapetum lucidum in their eyes, just like several other animals, such as cattle, horses, deer, cats, and ferrets (source).
This tapetum lucidum is an iridescent structure behind the retina, which allows the animal to see better in dim light or the dark. It is this structure that glows in the dark when the light hits it at a certain angle. However, animals’ eyes can glow in different colors!
The tapetum contains different substances, such as zinc or riboflavin, and different amounts of pigment in the retina, which will affect the color of the glow, or the animal’s age can affect this.
The tapetum of cats reflects a bit more than those of dogs, and they will often glow bright green or yellow in the case of Siamese cats. Some animals, including dogs, will reflect a red glow, and in the case of miniature schnauzers, it will glow in a beautiful turquoise color.
How Dogs See
Dogs’ eyes function like a camera. As light enters through the pupil, the iris controls how much light will enter. As light passes through the clear cornea and lens, it is reflected onto the light-sensitive retinal layer. The retinal layer converts light into electrical signals to be passed via the optic nerve to the brain to be interpreted (source).
The eyeball is filled with vitreous humor, and a third eyelid protects the eye. The vitreous humor is essentially a nictitating membrane composed of thin, pinkish tissue.
Behind the retina lies the tapetum lucidum, which enables animals to see in dimmer light. The light is converted in the retina by the color-sensitive cones and the light- and motion-sensitive cones. An image is constructed from the signals passed in the brain.
.For additional information on dogs’ eyes, read our article, “Why Do Dogs Wink?“
How Are Dog Eyes Different from Human Eyes?
For over 200 years, humans have manipulated the domestic dog’s evolution by selecting traits that better serve our needs, such as hunting and retrieving and being less aggressive, allowing them to fit into society.
Other, more whimsical traits, such as making “puppy dog eyes,” remind us of ourselves or our children. Dogs can pull up their inner eyebrows by moving a little muscle called the levator anguli oculi medialis (LAOM). By moving this muscle, a dog’s face looks a little sad (source).
The Visual Streak
Right above the optic nerve is an area in the retina called the “visual streak,” a horizontal band with the highest concentration of rods and cones, which makes vision the sharpest in this area.
This visual streak varies greatly between different breeds, which would suggest that they experience the world differently from a visual perspective. It is related to the dogs’ head length, with the band being broader in longer heads and narrower and more circular in breeds with a short head, like pugs.
This is like the “area centralis” of vision in humans. Long-nosed dogs have blurry vision close-up. They have a wide visual streak and a wide field of view, which, combined with the tapetum lucidum, makes them better hunters. Short-nosed dogs make better lapdogs with sharper vision close-up.
The Retina, Cones, and Rods
The retina is located at the back of the eyeball and is the light-sensitive portion of the eye. The cones provide color perception and detailed sight in the retina, and motion and vision in dim light are detected by the rods.
However, dogs only have about one-tenth of the number of cones that humans have. They do not see all the colors that humans do, but their vision is comparable to that of a colorblind human or one with dichromatic vision (two color variations) instead of trichromatic vision.
Dogs cannot see green, yellow, orange, or red — they use brightness, position, texture, and smell to assist them with finding things (source).
Dogs’ superior night vision can be attributed to the tapetum, which allows them to see more clearly in dim light.
Peripheral Vision: Dogs vs. Humans
Dogs have increased peripheral vision compared to humans, which depends heavily on how the eyes are set. Humans and dogs are both predator species. Human eyes are set forward, and dogs’ eyes sit at a slight 20-degree angle. Most prey species have eyes on the side of their heads to see approaching predators.
The wider the field of vision, the greater the depth perception and binocular vision required for jumping, catching, or leaping, and other activities.
Humans with perfect vision have so-called 20/20 eyesight, which simply means we can distinguish letters that are 20 feet away. Dogs’ visual acuity is around 20/75, which means they must be 20 feet away to see as well as humans do at 75 feet.
Seeing-eye dogs are bred with exceptional eyesight. They must have better vision than other dogs to perform their duties, and Labradors are often used for this purpose. Labs can see moving objects better than stationary objects due to the number of rods in their eyes, so silent cues typically comprise sweeping movements.
Once you understand how dogs’ eyes are different from those of human beings, you will be able to see the world through their eyes, in less color but with wider fields of vision and not as acutely clear as we do. Their ability to see more clearly in dim light is also the reason their eyes glow in the dark, eerily so.