Inside of a Dog: What Dogs See, Smell, and Know (17 page)

The order of operations is turned upside-down for dogs. Snout beats eyes and mouth beats ears. Given the olfactory acuity of dogs, it makes sense that vision plays an accessory role. When a dog turns his head toward you, it is not so much to look at you with his eyes; rather, it is to get his nose to look at you. The eyes just come along for the ride. You may be a recipient of an imploring gaze by a dog across the room right now. But can dogs even see what we do?

In many ways, the visual system of a dog—a subsidiary means to look at the world—is very much like our own. Its demotion behind other senses, in fact, may allow dogs to see details with their eyes that we overlook with ours.

One might well ask what a dog would even need eyes for. They can navigate and find food with their remarkable noses. Anything that needs closer examination goes right in the mouth. And they can identify each other through that sensory apparatus squished between their mouth and nose, the vomeronasal organ. As it turns out, they have at least two critical uses of their eyes: to complement their other senses and to see us. The natural history of the dog eye, seen in the story of their forebears, wolves, explains the context in which their vision evolved. It is a happy and transformative side effect that this has made them good watchers of human beings.

Just one element of the lives of wolves goes a long way to explaining the eyes they have evolved: eating. Most of their food runs away. Not only that, it is often camouflaged or living in the relative safety of herds. It is active—and thus findable—at dusk, dawn, or night. So wolves, like all predators, have evolved in response to their prey. As important as scent is, it cannot serve as the only indicator of the presence of prey, as air currents send odors on circuitous paths before they reach the nose. Odors are volatile: if smell lies on a surface, a sensitive nose can track it specifically; but if it is on the wind, the odor appears more like a cloud that could have come from one of a thousand sources. Rapidly moving prey outrun their own odor. Light waves, by contrast, are transmitted reliably through open air. So after catching a whiff, wolves use their sight to locate their prey. Many prey animals are camouflaged to blend in with their environment. This camouflage is betrayed in motion, however. So wolves are adept at spotting a change in the visual scene that indicates that something is moving. Finally, prey animals are often active at dusk or dawn, a compromise of lighting: easier to hide, harder to see. In response, wolves developed eyes that are especially sensitive in low light, and are especially good at noticing motion in that light.

Her eyes are deep pools of brown and black. It is hard to see which way they gaze, they are so dark—but it also makes any glimpse of her irises delightful, as though seeing inside her soul. Her eyelashes only became apparent when they grayed. Her eyebrows are also essentially invisible, but the effect of their moving—as with her head on the floor, to follow me walking across the room—is visible. In sleep, in dreams, her eyes scanned the world under their lids. Even closed, the lids reveal a bit of pink peeking out, as though she were keeping prepared to open her eyes at once should something important happen nearby.

But small differences between the eyes of humans and dogs immediately become apparent. First, our eyes are smack in the front of our face. We look forward, and images in the periphery fall away to darkness around our ears. While there is variation among dogs, most dogs' eyes are situated more laterally on their heads in the manner of other quadrupeds, allowing a panoramic view of the environment: 250–270 degrees, as contrasted to humans' 180 degrees.

If we look a little closer, we discover another key difference. The superficial anatomy of our eyes gives us away: it shows where we're looking, how we're feeling, our level of attention. While dog and human eyes are similarly sized, our pupil size—the black center of the eye that lets light in—varies considerably when we are in a darkened room or are aroused or fearful (expanding to up to 9 millimeters wide) or in the bright sun or are highly relaxed (contracting to 1 millimeter). Dogs' pupils, by contrast, are relatively fixed at about 3 to 4 millimeters, regardless of the light or the dog's level of excitement. Our irises, the muscles that control pupil size, tend to be colored to contrast with the pupil, blue or brown or green. Not so in most dogs, whose eyes are often so monochromatically dark that they remind me of bottomless lakes, repository of all attributions of purity or desolation we might ascribe to them. And the human iris sits amidst the sclera—the white—of the eye, while many (but not all) dogs have very little sclera. The anatomical sum effect is that we can always see where another person is looking: the pupil and iris point the way, and the amount of sclera revealed underlines it. Without a prominent sclera or a distinct pupil, the eyes of a dog don't indicate the direction of their attention nearly as much.

Closer still, and we begin to see serious species differences. Dogs manage to gather
more
light than we do. Once light enters the eye of a dog, it travels through the gel-like mass that holds nerve cells to the retina (we'll get to that in a moment), then through the retina to a triangle of tissue, which reflects it back. This
tapetum
lucidum,
in Latin "carpet of light," accounts for all the photographs you have of your dog with brilliant light shining out where their eyes should be. Light entering the dog's eye hits the retina at least twice, resulting not in the redoubling of the image but in a redoubling of the light that makes images visible. This is part of the system enabling dogs to have such improved night and low-light vision. While we might make out a match being brightly struck in the distance on a dark night, the dog could detect the gentle flame on the lit candle. Arctic wolves spend a full half of the year living in utter darkness; if there is a flame on the horizon, they have the eyes to spot it.

There are two slight changes in the canine retinae: in the distribution of photoreceptor cells and in the speed with which they operate. The former leads to their ability to chase prey, retrieve a tossed tennis ball, to their indifference to most colors, and to their inability to see something right in front of their noses. The latter leads to their being uninterested in daytime soap operas left on for them when their owners are out of the house. We'll look at these in turn.

Some of the most important things for humans to see are any other humans situated within a few feet of their face. Our eyes face forward, and our retinae have
foveae:
central areas with an extra abundance of photoreceptors. Having so many cells in the center of the retina means that we are very good at seeing things right in front of us in high detail, great focus, and strong color. Perfect for identifying that blob of color and form coming at you as your boyfriend or your mortal enemy.

Only primates have foveae. By contrast, dogs have what is called an
area
centralis:
a broad central region with fewer receptors than a fovea, but more than in peripheral parts of the eye. Things directly in front of a dog's face are visible to him, but they are not quite as sharply in focus as they would be for us. The lens of the eye, which adjusts its curvature to focus light onto the retina, doesn't accommodate to nearby sources of light. In fact, dogs might overlook small things right in front of their nose (within ten to fifteen inches), because they have fewer retinal cells committed to receiving light from that part of the visual world. You need no longer puzzle at your dog's inability to find the toy that he is nearly stepping on: he's not got the vision to take note of it until he takes a step back.

Breeds of dogs differ so much in their retinae that they see the world differently. The area centralis is most pronounced in those breeds with short noses. Pugs, for instance, have very strong areas centralis—almost fovea-like. But they lack a "visual streak," which dogs with long noses (and wolves) have. In Afghans and retrievers, for instance, the area centralis is less pronounced, and the photoreceptors of the retina are more dense along a horizontal band spanning the middle of the eye. The shorter the nose, the less visual streak; the longer the nose, the more visual streak. Dogs with the visual streaks have better panoramic, high-quality vision, and much more peripheral vision than humans. Dogs with the pronounced area centralis have better focus in front of their faces.

In a small but significant way, this difference explains some breed-based behavioral tendencies. Pugs are not typically so-called "ball dogs" but long-nosed Labrador retrievers are. Not because of their long noses per se. In addition to their ability to put their millions of olfactory cells to good use, Labradors are visually equipped to notice, say, a tennis ball traveling across the horizon, without having to shift their gaze. For a short-nosed dog (as for all humans of any nose length), a ball tossed to the side just disappears into the periphery if they don't follow it with their head. Instead pugs are probably better at bringing close objects into focus—say, the faces of their owners on whose laps they sit. Some researchers speculate that this relatively blinkered vision makes them more attentive to our expressions, and seem more companionable.

Dogs are not color-blind, as is popularly believed. But color plays a much less important role for them than it does for humans, and their retinae are why. Humans have three kinds of cones, the photoreceptors responsible for our perception of details and of colors: each fires to red, blue, or green wavelengths. Dogs have only two: one is sensitive to blue and the other to greenish-yellow. And they have fewer of even those two than humans do. So dogs experience a color most strongly when it is in the range of blue or green. Ah, but a well-scrubbed backyard pool must seem radiant to a dog.

As a result of this difference in cone cells, any light that looks to us like yellow, red, or orange simply doesn't look the same way to a dog. Consequently, they seem perfectly oblivious when you ask them to bring back grapefruits from the store and grow irritated when they bring back tangerines. Still, orange, red, and yellow objects might still look different to them: the colors have different brightnesses. Red may be seen by them as a faint green; yellow a stronger one. If they seem to be able to discriminate red and yellow, they are noticing a difference in the
amount
of light these colors reflect toward them.

To imagine what this might be like, consider the time of day when our color system breaks down: in the dusk right before night. If you're outside in a park, in your yard, or anywhere nature lives, take a look around you. You might notice the wash of exuberant green leaves above you subtly dulling to a more unassuming hue. You can still see the ground underfoot, but details—the distinctness of blades of grass, the layers of petals—are reduced. Depth of field squashes somewhat. I tend to stumble more than usual on protuberant gray rocks that blend with the earth. The reason for this loss of visual information is anatomical. Cones, clustered toward the center of the retina, are not sensitive to low light, so they don't fire as often at dusk or night. As a result, our brains get signals from fewer cells detecting colors. And the near world flattens out a little: we can still see that there is color, and we still detect lights and darks, but the richness of colors has fallen away; colors are grainier, less detailed. So it might be for dogs, even at noontime.

As they do not experience a great range of distinct colors, dogs rarely show color preferences. Your clashing choice of red leash and blue collar affects your dog not at all. But a deeply saturated color may get more attention from a dog, as will an object placed in a background of contrasting colors. It may be meaningful that your dog attacks and pops all the blue or red balloons left over after the birthday party winds down: they are most distinct among a sea of pastels.

Dogs make up for their dearth of cones with a battery of rods, the other kind of photoreceptor in the retina. Rods fire most in low-light situations and at changes in light densities, which is seen as motion. In human eyes, rods cluster at the periphery, helping us notice something moving out of the corner of our vision, or when the cones slow their firing at dusk or night. The density of rods in dogs' eyes varies, but they have as much as three times as many rods as we do. You can make that ball your dog is not seeing directly in front of him magically appear by giving it a little shove. Acuity greatly improves for close objects when they are bouncing.