Dogs see better in the dark,: that's a fact. Well, it would be more accurate to say that dogs see better in dim light rather than pitch-black darkness. While us humans are capable of seeing better in bright light, dogs surely beat us when it comes to low-light conditions. This likely explains why you may be walking in the dark palpating your surroundings, while Rover moves around with confidence courtesy of his eyesight and whiskers (which by the way, complements his eyesight in dim conditions).
A History as Crepuscular Hunters
Dogs are known for having a history as crepuscular hunters, meaning that, in their past, their ancestors used to be active during dusk and dawn when critters were most vulnerable.
The term crepuscular though may not be entirely accurate considering that canids may have hunted as well occasionally at night, courtesy of the strong light emitted by a full moon, and they may have also hunted during the day when the conditions were overcast.
Regardless, it's assumed that dogs must be capable of seeing in light that is five times dimmer than what a human can see, explains Paul Miller, clinical professor of comparative ophthalmology at University of Wisconsin-Madison in an article for Science Daily. This is quite impressive, yet a dog's vision in low light is not as good as the dim light vision seen in cats.
Hunting at twilight certainly required certain specific features that are not seen regularly in diurnal animals (animals mostly active during the day.) While dog noses were great at capturing scent, one must consider that prey was fast moving and often camouflaged.
This means that fast moving prey outrun their own odors, causing dogs to rely on their sight after catching a whiff, explains Alexandra Horowitz in the book "Inside of a dog, what dogs see, smell and know."
Hunting in dim conditions therefore required the evolution of specific adaptations in response to their needs to becoming successful predators. These adaptations mostly included the way a dog's pupils, retinas, and a special mirror-like structure known as the "tapetum lucidum" were designed to be the way they are.
Now, you can't always have the best of both worlds in nature. Good crepuscular vision required sacrificing other features that were not much necessary. The trade-offs for good vision at night and a keen perception of movements, is that dogs have less visual acuity, and therefore, are not capable of seeing minute details, and they don't have full color vision as humans do.
What Big Eyes You Have...
Look at any nocturnal animal and you will likely notice what big eyes they have. Owls, tarsiers and cats are some examples. In these animals, their eyes are very large and they have pupils that are capable of opening wide, often covering the entire front of the eye.
Now consider that dogs have a history of being crepuscular, which means that don't have pupils as large as nocturnal animals, but they are sure larger than the pupils of humans. If you look carefully, you most likely will notice the large pupil with just a little bit of iris encircling it.
The function of large pupils is to allow more light to strike the retina which comes handy in low-lit conditions. A dog's pupils are better capable of enlarging in low light. This ability, once again, played an important role in a dog's' ancestors as they were hunting at dusk and dawn.
A dog's corneas (the clear domed layer forming the front of the eyes) are also quite large and so are the lens. Both of these features come handy as well in helping dogs see well in dim light.
And Look at the Number of Rods!
A dog's retina is composed by rods and cones which are special cells that carry out several important functions.
The rods are sensitive to dim light, but do poorly in distinguishing fine details or in perceiving colors. Rods cells also help dog eyes in detecting and following movement.
Cones, on the other hand, can detect colors well, but they fail in allowing to see in low light conditions.
Compared to the retinas of people, dog retinas are equipped with a large density of rods compared to cones. This is likely because humans had a deeper need for distinguishing colors due to the fact that a lot of a human's evolution depended on distinguishing ripe fruits.
Dogs on the other hand, depended on meals that would run away and camouflage. Fortunately, movement helped prey stick out from the dim light and failed their attempts at camouflaging.
The Power of the Tapetum Lucidum
The tapetum lucidum, a mirror-like layer found just behind the retina and in front of the blood vessels, is there to help nocturnal and crepuscular animals make the most out of small amounts of light. The term tapetum lucidum derives from the Latin words for "carpet of light."
What happens here is that, light that enters the eye, hits the retina, reflecting off the tapetum, and therefore, giving the cells of the retina (photoreceptor cells) a second chance to sense it. This almost doubles the sensitivity of a dog's eyes in low light, points out John Bradshaw in the book "Dog Sense: How the New Science of Dog Behavior Can Make You A Better Friend."
It is also due to this structure that dog and cat eyes shows that typical eerie shine seen when a glare of a flashlight flash of a camera or a car's headlight hit this area. This is referred to as "eyeshine." The green color seen is simply due to the layer of shiny cells that composes the reflective tapetum-lucidum.
It is ultimately thanks to this structure that dogs are so adept in having good low-light vision. Humans lack a tapetum lucidum although human eyes can generate a weak reflection from the fundus (the back of the eye which is covered with blood vessels) causing what's known as the "red eye effect" in pictures.
Interestingly, Mother Nature's invention of the tapetum lucidum is so ingenious that humans have borrowed the idea to manufacture raised pavement markers as a safety feature for night-time drivers.
Now That You Know...
As seen, dogs can see quite well in dim light which explains why your dog can be still romping around happily at dusk while you need to carry a flashlight.
Pay attention to your dog's night vision. Older dogs tend to develop vision issues as they age. Cataracts, nuclear sclerosis, or the presence of a mass around the area of the optic nerve may cause reduced vision in elderly dogs. On top of this, cognitive dysfunctions may cause old dogs to become confused at night when it is dark.
Younger dogs aren't spared from eye conditions as well. Some dog breeds are prone to developing a condition known as progressive retinal atrophy (PRA). This is an inherited condition affecting both eyes that is progressive and ultimately leads to blindness. What happens is that this disease of the retina leads to the rod cells dying.
This condition has been found in over 100 dog breeds and affects puppies in their first 2 to 3 months of age, and in the later onset form it affects dogs in the ages of 2-5 years.
Affected dogs develop behavioral issues that at first may appear odd to dog owners. Since the rod cells are responsible for causing dogs to see in dim light, it is not unusual for dog owners to report that their dogs have started to act anxious at night, refusing to enter dark rooms or bumping into objects.
Owners of affected dogs may also notice their dog's pupils being more dilated and their eyes becoming more reflective when light shines on them, explains veterinarian Dr. Cheryl Yuill. If you notice anything unusual with your dog's vision, always best to consult with a vet.