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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 dog's 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 don't always see the same thing and don't 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 can't 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 dog's 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 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.

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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.

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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.

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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.

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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 dog's 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 image and makes it blurry so you can't 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.

Stanley Coren is the author of many books including: Gods, Ghosts and Black Dogs; The Wisdom of Dogs; Do Dogs Dream? Born to Bark; The Modern Dog; Why Do Dogs Have Wet Noses? The Pawprints of History; How Dogs Think; How To Speak Dog; Why We Love the Dogs We Do; What Do Dogs Know? The Intelligence of Dogs; Why Does My Dog Act That Way? Understanding Dogs for Dummies; Sleep Thieves; The Left-hander Syndrome

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* Data from: R. H. Douglas, . 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.