Dear Doctor: What Dogs See
Q I've always understood that dogs don't see as many colors as people, or don't see colors as brightly, but I read recently that they have ultraviolet vision and can see colors that aren't even on our radar. How can both things be true?
Dear Mr. Schain,
A You’re right that dogs don’t see colors as we do. The ability to perceive the full range of color that people see is due to the fact that our eyes have three types of cones — receptors for color vision. Dogs have only two types of cones, which lessens their color perception. That’s why, although dogs can distinguish yellow and blues fairly well, they see red (and orange) as yellow, and green appears to them as white. In other words, their world consists pretty much of yellow, blues, and greys. A red ball in green grass wouldn’t stand out to a dog the way it would to you. (It’s similar to the way someone who is red/green colorblind sees.)
But evidence has come to light that dogs’ eyes take in ultraviolet waves that ours don’t, enabling them to perceive certain things through sight that we cannot. That said, it’s not necessarily about color. It’s more about the ability to distinguish between things in various lights that we cannot.
It goes back to the fact that in dogs, ultraviolet light waves go right through to the retina, where the eye can make use of them. In people, on the other hand, ultraviolet rays are absorbed by the lens near the outer surface of the eye. They don’t penetrate any further.
Researchers in the United Kingdom led by Ron Douglas made the finding when they examined the lenses of 38 different species of mammals, including our canine friends. The upshot of their discovery: when ultraviolet light passes through the lens of a dog (or cat, hedgehog, reindeer, horse, or rabbit, to name a few of the other species examined) and reaches his retina, his brain is able to translate, so to speak, what it is seeing.
For a reindeer, for instance, that might mean being better able to perceive “the white fur of predatory polar bears within a snowy landscape,” posit the researchers, while for a rodent it could enhance the visibility of urine trails. But much of what sensitivity to ultraviolet waves does is increase visual sensitivity in low light — something that would be important to dogs because they are crepuscular. That is, they are naturally most active at dawn and dusk and need to be able to see grey rodents running through brown fields before the sun has really risen (and after it has started to set).
There’s a tradeoff for that ability. Dogs and other mammals don’t have the visual acuity we do; image quality is not as good. The picture is fuzzier in part because of the entry into the eye of the UV rays. A dog’s best vision is somewhere between 20/50 and 20/100, not 20/20, as humans have. That’s why you can trust a dog to catch a squirrel in a dimly lit field, but you wouldn’t want him to have the car keys. (Another bonus for people is that not having UV rays reach their retinas protects their eyes better over the long term.)
The researchers note that more importance is sometimes ascribed to animals’ UV sensitivity than may be called for, that in the scheme of an animal’s senses, absorption of UV light waves does not translate to a huge difference between us and them. ”Perhaps,” they write, “the reason why there is a tendency to attribute some special importance to UV sensitivity is simply that humans are not able to see it.”
“Being a cat owner,” Dr. Douglas said to us in private correspondence, “I know that they are basically nuts and do things that defy explanation, reacting to things that appear not to be there. However, I doubt this has anything to do with UV and is more likely a response to things we are not aware of, such as sounds at different frequencies.”