A Strange Notion
A short while ago I attended day-long veterinary students’ “Behavior Club” meeting. It was truly something. What a line-up of speakers. As I milled around after the meeting, I struck up a conversation with one of our basic science researchers. A “mouse scientist”, so to speak, she began talking about her murine experiments. “I have just learned that mice can see ultraviolet light”, she said. Now all my experiments will be messed up because I did not allow for that.”
“What do they see in the UV range?” I asked.
“Oh, urine marks,” was the curt reply.
“What,” I exclaimed,” you mean mice see urine marks as well as smell them?”
“Yes, that seems to be the case,” she said dejectedly.
Then we got talking about dogs and cats. What if they could also see in the UV range? Then it struck me. I bet they can. I went digging around on the Internet for evidence to support this crazy notion.
Here’s what I came up with.
1. For animals that are crepuscular, that is, active mainly at dawn and dusk, like mice and dogs and cats, being able to see in the UV range would be super helpful as the wavelength of light shifts toward the UV range at these times of day. That is, it would be evolutionarily advantageous to be able to see in the UV range if that’s when you (they) are most active.
2. We humans have a UV filter in our corneas and lenses so UV light hardly reaches the light-sensitive retina at the backs of our eyes. Mice and dogs and cats do not have this filter mechanism so the UV light can get to the UV light-sensitive cells.
3. Urine of mice and dogs and cats fluoresces under UV light because it contains chemicals called porphyrins.
4. Dogs and cats don’t see well at the long wave length (red) end of the visible spectrum and are better able to distinguish colors at the blue-green and of the color spectrum. I.e. their color perception seems to be shifted toward the violet end of the spectrum – in the direction of the wavelength of UV light.
5. Small dogs sometimes do backward handstands when urine marking a hydrant or lamp post to overmark the previous larger dogs' marks. If the small dog doesn’t smell the whole height of the mark yet knows where he must reach, how does he do that? By seeing the mark fluoresce? Could be.
This is pretty exciting stuff. Not only is it fascinating to think of a dog (or cat) being able to see outside the visible spectrum (like they can hear outside our audible range), this skill, so useful in the wild, may have some relevance to dogs’ urine marking in the home. Think about it. If your dog leaves (to him) a visible mark in your home and all you do is eliminate or change the odor, you could be fighting a losing battle. This would explain why so-called appeasing pheromones barely work.
How to prove this theory is what is now on my mind. I can think of two ways of doing it; one behavioral (using false porphyrin marks and seeing if they are detected) and one anatomic (looking for a special type of photoreceptor in the retina of dogs eyes). If these photoreceptors are found, dogs (and cats) might then be reclassified as having a form of trichromatic (3 color) vision, rather than the dichromatic (2 color) vision with which they are currently credited. Dogs and cats seeing in the UV range is certainly is a strange thought but no stranger than snakes seeing in infrared thermal range of the spectrum – which they do. How useful would that be for the snake when hunting a warm little bundle of fur?
Dr. Nicholas H. Dodman
