A Dog Duet—Part 1

I recently read two reports (sort of a duet) about a couple of current circumstances of dogs, that say something about both the qualities of canines as well as the closeness of humans and dogs—in ways that go well beyond the typical owner-pet relationship. I would argue that the old saw—that a dog is man's best friend—has a lot of truth to it. But these two reports go well beyond just the friendship concept. They tell us that there are deeper similarities; that in some ways humans and dogs share a couple of fascinating and relevant physiological qualities. Maybe we're more than best friends. Maybe we're even some kind of distant relative?

The first report described the fact that dogs are color-blind—far more than humans, but still in a physiologically similar way. Yet dogs are not really color blind; it's more that they cannot perceive the rich panoply of colors that we humans enjoy. How so? The eyes of both humans and dogs (as well as most animals) contain two kinds of light sensory receptors: cones and rods. Cones have evolved to provide visual acuity and also to respond to color, but they need rather bright light conditions to do so. Rods evolved to provide vision at low light levels and to detect motion; they do not respond to color. While human eyes contain a high proportion of cones (we see color), dogs' eyes have far fewer. Thus we see sharply, as well as perceive all the vivid hues of the artist's palette. Dogs—with more rods—have poor visual acuity but can see better than we do at night; especially being able to note motion, as their prey try to run away.

All mammals evolved to have both rods and cones in their eyes. Back when mammals were new to Earth's animal kingdom (100-200 million years ago), they were just little critters who were dominated by the much larger and more successful dinosaurs. Mammals were forced to skitter around mostly in the dark, to avoid being eaten or stepped on by the reigning dinosaurs. Needing to see well under low light levels, mammal eyes evolved lots of rods.

Think about what you see, when nighttime comes on. You can't see color—only shades of gray. There's not enough light to trigger your eye's cones, so the rods—being “color-blind”—take over. For various reasons, dog's eyes, unlike ours, have a preponderance of rods. Maybe their wolf ancestors continued their hunting activities at night. Humans, on the other hand, evolved to have a lot of cones in our eyes. We needed them—along with our ape cousins—to locate tasty and colorful fruit in our daytime foraging. Dogs mostly needed to see moving animals to chase in the dark. To compensate for their inability to distinguish colors, dogs evolved an exquisite sense of smell. With that wonderful nose they could better sniff out their food.

[As an interesting aside to dogs' and humans' eye differences, when a human is photographed with a flash, a phenomenon called “red-eye” is often observed. It is due to the light of the camera's flash being reflected off blood vessels at the back of the eye. Dogs' eyes have something called a “tapetum,” which is a mirror-like structure at the back of their eyes. When a dog is photographed with a flash, the light reflects off the tapetum and appears blue. We get red-eye, they get blue-eye.]

More on dogs and humans next time...