Your dog has a visual ability that humans do not.
Research suggests that your dog may be able to see things that are completely invisible to you.
If you look at the size, shape, and general structure of a dogs eye it looks very much like the human eye. For that reason we have a tendency to guess that vision in dogs is much like that in humans. However science has been advancing and we are learning that dogs and humans dont always see the same thing and dont always have the same visual abilities. For example, although dogs do have some color vision (click here for more about that) their range of colors is much more limited in comparison to humans. Dogs tend to see the world in shades of yellow, blue, and gray and cant discriminate between the colors that we see as red and green. Humans also have better visual acuity, and can discriminate details that dogs cannot (click here to read more about that).
On the flip side, the dogs eye is specialized for night vision and canines can see more under dim lighting than we humans can. Furthermore, dogs can see motion better than people. However a study published in the Proceedings of the Royal Society B* suggests that dogs may also see a whole range of visual information that humans cannot.
Ronald Douglas, a professor of biology at City University London and Glenn Jeffrey, a professor of neuroscience at University College London, were interested in seeing whether mammals could see in the ultraviolet light range. The wave lengths of visible light are measured in nanometers (a nanometer is one millionth of one thousandth of a meter). The longer wave lengths, around 700 nm, are seen by humans as red, and the shorter wavelengths, around 400 nm, are seen as blue or violet. Wavelengths of light which are shorter than 400 nm are not seen by normal humans, and light in this range is called ultraviolet.
It is well known that some animals, such as insects, fish, and birds, can see in the ultraviolet. For bees this is a vital ability. When humans look at certain flowers they might see something which has a uniform color, however many species of flowers have adapted their coloration so that when viewed with ultraviolet sensitivity the center of the flower (which contains the pollen and nectar) is a readily visible target making it easier for a bee to find. You can see that in this figure.
In human beings the lens inside the eye has a yellowish tint which filters out the ultraviolet light. The British research team reasoned that certain other species of mammals might not have such yellowish components in their eyes and therefore might be sensitive to ultraviolet light. It is certainly the case that people who have had the lens of their eye removed surgically because of cataracts often report a change in their vision. With the removal of the yellowish lens such individuals can now see in the ultraviolet range. For example, some experts believe that it was because of such a cataract operation the artist Monet began to paint flowers with a blue tinge.
In the current study a broad range of animals including: dogs, cats, rats, reindeer, ferrets, pigs, hedgehogs and many others, were tested. The transparency of the optical components of their eyes was measured and it was found that a number of these species did allow a good deal of ultraviolet light into their eyes. When the eye of the dog was tested they found that it allowed over 61% of the UV light to pass through and reach the photosensitive receptors in the retina. Compare this to humans where virtually no UV light gets through. With this new data we can determine how a dog might see a visual spectrum (like a rainbow) in comparison to a human and that is simulated in this figure.
The obvious question to ask is what benefits the dog derives from its ability to see in the ultraviolet. It may have something to do with having an eye that is adapted so that it has good night vision, since it appears that those species who were at least partially nocturnal had lenses capable of transmitting ultraviolet, while those who functioned mostly in the daylight did not. However it is also the case that certain types of information can be processed if you have ultraviolet sensitivity. Anything that either absorbs the ultraviolet or reflects it differentially would thus become visible. For example in this figure we have an individual on whom we have painted a pattern using a sunscreen lotion (which blocks ultraviolet). The pattern is not visible under normal conditions, but when viewed in ultraviolet light it becomes quite clear.
In nature there are a number of significant things which might become visible if you can see in the ultraviolet. Of interest to dogs is the fact that urine trails become visible in ultraviolet. Since urine is used by dogs to learn something about other dogs in their environment, it may be useful to be able to spot patches of it easily. This might also be of assistance in wild canines as a method of spotting and trailing potential prey.
In certain specific environments sensitivity to the ultraviolet part of the spectrum can provide an advantage to an animal that hunts in order to survive, such as the ancestors of our dogs. Consider the figure below. You can see that the white coloration of an arctic hare provides good camouflage and makes the animal difficult to spot against a snowy background. However such camouflage is not as good when used against an animal with ultraviolet visual capacities. This is because the snow will reflect much of the ultraviolet light while white fur does not reflect the UV rays as well. Thus to the UV sensitive eye the arctic hare is now much more easily seen since it appears as if it is a lightly shadowed form, rather than white against white, as can be seen in the simulation below.
If visual sensitivity in the ultraviolet does provide certain advantages to an animal like a dog, then perhaps the question we should be asking is why other animals, like humans, would not benefit as well from having the ability to register ultraviolet light. The answer seems to come from the fact that there are always trade-offs in vision. You can have an eye that is sensitive in low levels of light, such as the dogs eye, but that sensitivity comes at a cost. It is the short wavelengths of light (those that we see as blue, and even more so, those shorter yet wavelengths that we call ultraviolet) which are most easily scattered as they enter the eye. This light scattering degrades the and makes it blurry so you cant see details. So dogs who evolved from nocturnal hunters may have maintained their ability to see ultraviolet light because they need that sensitivity when there is little light around. Animals who function in the daylight, such as we humans, rely more on our visual acuity to effectively deal with the world. So we have eyes that screen out the ultraviolet in order to improve our ability to see fine visual details.
We have been talking about the first study which has dealt with this aspect of canine vision and its results were a surprise to many of us who never expected that dogs might have this added form of visual sensitivity. Obviously further research is needed to determine how dogs really benefit from this ability. I doubt that it was an evolutionary development which simply allows dogs to have greater appreciation for the psychedelic posters which became so popular in the 1970s — you know those posters that were created by using inks that fluoresced under a “black light” or ultraviolet light source. But only through future research will we know for sure.
Copyright SC Psychological Enterprises Ltd. May not be reprinted or reposted without permission
* Data from: R. H. Douglas, G. Jeffery (2014). The writer spectral transmission of ocular media suggest ultraviolet sensitivity is widespread among mammals. Proceedings of the Royal Society B, April, volume 281, issue 1780.
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Fun Facts About Dogs and Color:
Dogs do better at agility training when the weave poles, tunnels, jumps, and boards are painted in colors they can easily discern.
Dogs see 20/75, which makes them quite nearsighted.
Turns out, dogs have fewer cone receptors than humans — which means they cant see as many colors. Human cones can detect 3 colors: red, green, and blue.
Alexandra Horowitz — author of “Being a Dog” — told us that its difficult to know exactly what colors a dog sees, but its probably similar to what we see at dusk.
The reason lies within the eye. In the eye are light receptors called cones and rods. Cones help us distinguish different colors, while rods help us see in dim light.
Dog cones can only detect 2 colors. No one is certain what those 2 colors are. Some experts think it could be blue and yellow.
Dog eyes have more rods than humans, which means they can see much better at night. Dogs also have a layer of eye tissue that humans lack called the tapetum lucidum, it reflects light into the retina.
Can Dogs See Color? – How a Dog’s VISION Works
Owners who want to better understand their canine companions must recognize that dogs see the world from a different visual perspective. The differences begin with the structure of the eye. We have a good idea what dogs see because we know the make-up of the retina of a dog’s eye.
The retina is the light sensitive portion of the eye. This structure is located in the back of the inside of the eyeball. The retina contains two types of light sensitive cells; rods and cones. Cones provide color perception and detailed sight, while rods detect motion and vision in dim light. Dogs have rod-dominated retinas that allow them to see well in the dark. Along with superior night vision, dogs have better motion visibility than humans have. However, because their retinas’ contain only about one-tenth the concentration of cones (that humans have), dogs do not see colors as humans do.
Dogs see like a color-blind human. Many people think that a person who is red / green color blind cannot see any color, but there are variations of color blindness. Most people have vision that is trichromatic (three-color variations). People who are red / green color blind are dichromatic (two color variations). Dogs’ retinas can distinguish two colors. These colors are blue-violet and yellow. Dogs can also differentiate between shades of gray. Dogs are unable to recognize green, yellow, orange, and red.
Dogs use other cues (such as smell, texture, brightness, and position) rather than relying solely on color. Seeing-eye dogs, for example, may not distinguish between a green or red stoplight; they look at the brightness and position of the light. This, along with the flow and noise of traffic, tell the dog that it is the right time to cross the street.
How a dog’s eyes are set determines the field of view as well as depth perception. Prey species tend to have eyes located on the sides of their head. This gives the animals an increased field of view and allows them to see approaching predators. Predator species, like humans and dogs, have eyes set close together. Human eyes are set straight forward while dog eyes, depending on the breed, are usually set at a 20 degree angle. This angle increases the field of view and therefore increases the peripheral vision of the dog.
Increased peripheral vision compromises the amount of binocular vision. Binocular vision occurs where the field of view of each eye overlaps. Binocular vision is necessary for depth perception. The wider-set eyes of dogs have less overlap and less binocular vision (thus less depth perception). Dogs’ depth perception is best when they look straight ahead. This is not an ideal situation as their nose often interferes. Predators need binocular vision as a survival tool. Binocular vision aids in jumping, leaping, catching, and many other activities fundamental to predators.
In addition to having less binocular vision than humans have, dogs also have less visual acuity. Humans with perfect eyesight are said to have 20/20 vision. This means that we can distinguish letters or objects at a distance of 20 feet. Dogs typically have 20/75 vision. What this means is that they must be 20 feet from an object to see it as well as a human standing 75 feet away. Certain breeds have better visual acuity. Labradors, commonly used as seeing-eye dogs, are bred for better eyesight and may have vision that is closer to 20/20.
If you’re silently standing across the field from your dog, don’t expect him (her) to recognize you. He’ll recognize you when you do some sort of motion particular to yourself. He (she) may also recognize your presence by his outstanding sense of smell and / or hearing. Because of the large number of rods in the retina, dogs see moving objects much better than they see stationary objects. Motion sensitivity has been noted as the critical aspect of canine vision. Much of dog behavior deals with posture and appropriateness. Small changes in your body posture mean a lot to your dog. Dog owners need to modify training based on this fact. If you want your dog to perform an action based on a silent cue, we suggest using a wide sweeping hand and arm motion in order to cue your dog.
When dogs go blind, owners often wonder if the dogs’ quality of life has diminished to the point where they are no longer happy. Humans deal well with being blind, and humans are much more dependent on their eyes than are dogs. Blind dogs lead happy lives as long as they are comfortable. The owner may need to make some adjustments in the pet’s environment. Some of these adjustments include fencing the yard, taking leashed walks, and not leaving unusual objects in the dog’s normal pathways. Obviously, most blind dogs cannot navigate stairs very well. When blind dogs are in their normal environment, most people don’t know they are blind.