Dog Sense (39 page)

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how any dog lover a picture of a cute dog, and you'll get an instant reaction. Show the same picture to a dog, and you probably won't get any reaction at all. (Unless it's your own dog, in which case you might get a puzzled expression that clearly signifies “Whatever are you up to?”)

Dogs may inhabit the same physical space as us, but they don't experience the world the way we do. We like to think that our version is “the” version, but it isn't. Like every other species, we pick up the information about the world that we need in order to survive, and we discard the rest. Or, more accurately, we pick up information that helped our primate and hominid ancestors survive. (We haven't lived our current lifestyles for nearly long enough for our senses to have been modified by evolution.) Dogs live in a world that's dominated by their sense of smell—one that is quite unlike ours, which is constructed around what we see.

It's easy to ignore the fact that we, too, get an edited version of what's going on around us. We can't “see” the light beam that comes out of our TV remote control, but it does consist of light—it's just that its wavelength is too long for our eyes to pick up. The mere fact that it's invisible to us doesn't mean it's not there. Hence it's worth reminding ourselves of what we do and don't pick up from our surroundings before going on to consider what dogs could tell us about what we're missing out on, if only they could talk.

First of all, we are color junkies, at least by comparison with most other mammals. Although we have only three types of cone—yellow-, green-, and violet-sensitive receptor cells (many animals have four, some even more), it's been estimated that our eyes can distinguish about 10 million different colors. (When I say “our,” strictly speaking I'm referring only to men; some women, possibly as many as half worldwide, have a fourth type of receptor in the yellow-green area and thus are likely able to distinguish between millions more shades of red, orange, and yellow than the rest of us can.)

Our ability to see all these colors has evolved only recently. Although reptiles (and birds) can see the full range of colors as well as ultraviolet, sometime during the course of the early evolution of mammals the ability to see both ultraviolet and red disappeared. It's possible that, because those early mammals were nocturnal, they needed the space on their retinas for rods—the receptor cells that are used in low-light vision, which are responsive only to black and white. The Old World monkeys and apes, most of which forage in daylight, “re-evolved” trichromatic vision about 23 million years ago, probably as a way of fulfilling the need to distinguish tender leaves and ripe fruits by their color alone.

What the eye can detect is only half the story: The brain still has to turn raw data into pictures. All the information gathered by our eyes is integrated together in the brain to form the three-dimensional color image that we consciously perceive as “seeing.” Although our brains can put together a 3-D image using the information from just one eye (try closing one eye and moving your head around very slightly), the most accurate and instantaneous information comes from our binocular vision. Our brains constantly compare the pictures coming in from each eye, using the small discrepancies between them to generate a full-color 3-D image. To make this process as efficient as possible, our eyes point in exactly the same direction. (This is unusual among mammals; even cats, with round flat faces like ours, have eyes that point slightly out to the sides, at about an eight-degree angle. By contrast, animals such as rabbits that primarily use their vision to detect approaching danger have eyes on the sides of their heads, sacrificing binocular vision completely in order to have the widest possible field of view.)

And so humans are extremely visual creatures. Scientists estimate that our brains receive about 9 million bits of information from our eyes every second—ten times as much as, for example, a guinea pig does. There are various theories as to why we evolved this ability; among them is the supposition that as primate society became more complex, the need to monitor everyone else in the group increased, resulting in especially detail-oriented visual acuity.

Although humans see more than most mammals do, we don't hear nearly as much; hearing is evidently not as important for primates as it is for many other mammals. Mice and bats can hear much quieter and much higher-pitched sounds than we can, and dogs and cats can hear pretty much everything we can, and much more besides. We're also not as good as most other mammals are at judging where sounds are coming from. Looking back to our evolutionary roots as hunter-gatherers, we can surmise that vision would have been much more useful than hearing for gathering edible plants and fruits—and for tracking game as well. However, our brains are probably much better than dogs' brains when it comes to distinguishing between very similar sounds, a skill we've evolved in order to decode speech.

It's our sense of smell that is really feeble compared to that of the rest of the animal kingdom (except birds). We can train ourselves to discriminate between different smells, provided they are strong enough for us to smell in the first place, but most of the odor information in the world around us simply passes us by. As a consequence, apart from a few professionals such as wine tasters and perfumers, we don't even have much of a language to describe the quality of odors.

Why is the human nose so insensitive? First of all, we have a tiny olfactory epithelium, which is the skin inside our nostrils that removes odor molecules from the air we breathe and sends messages to the brain about what they are. Secondly, the parts of the brain that deal with incoming odor information are greatly reduced in all the Old World primates and apes—and there has been, if anything, a further reduction during our own evolution. Thirdly, by comparison with almost all other mammals we have a very limited repertoire of odor receptors, which cuts down on the amount of subtlety we can extract from any particular
odor. We still have the relics of the genes that mice (for example) use to make the much greater range of receptors that they possess, but our versions of these genes don't work—indeed, they stopped working millions of years ago, during the evolution of the higher primates. As a result, although we can probably detect the same range of odors that mice can, we do so with less detail. And of course we also need much more of the odor to be present to smell anything at all.

The phasing-out of odor perception in humans roughly coincided with the evolution of three-color vision, and scientists believe that these changes are connected. The original primates were mainly nocturnal, like many mammals then and now, and they had the standard mammalian two-color vision. When our ancestors evolved three-color vision, this new ability was accompanied by a substantial enlargement of the visual cortex in the brain and a simultaneous shrinking of the areas that process olfactory information. (There appear to be limits on how much information any brain can process, and so enlargement of one area is often accompanied by shrinkage of another.) Then, as the apes and man evolved, the brain enlarged further, becoming a processor of social information—especially information gathered visually. In the process, the “ancient” olfactory part of the brain became buried underneath the cerebral cortices.

Thus the version of the world as perceived by mankind is rather atypical—even among mammals in general. Humans have highly refined color vision, reasonable night vision (which most of us rarely use), average hearing, and an utterly puny sense of smell. Dogs, by contrast, have poor color vision, good night vision, excellent hearing, and a very sensitive and sophisticated sense of smell. Mankind has exploited these differences throughout the domestic dog's history, valuing the dog's sensitive nose especially as an aid to hunting. However, pet dogs are often so anthropomorphized that it's easy for their owners to ignore these differences and to treat their dogs as if they perceived the same world that we do.

The visual world that dogs inhabit is similar to ours in many ways—indeed, close enough that the differences are rarely apparent and present no problems for the dogs themselves. Dogs can see slightly more
than we can during the night, slightly less in daylight. With one notable exception—namely, perception of color—their visual abilities are not so different from ours that their subjective world and ours would look substantially different. Thus whatever we can see, they are likely to see also, if in slightly less detail.

For dogs, like most mammals, it is more important to see all the time rather than to see particularly well, in order to remain vigilant to danger. We humans have sacrificed some of our ability to see in the dark in order to be able to see in color and in great detail during the day; presumably our tree-dwelling ancestors had few nocturnal predators. For this reason, dog's eyes are adapted to see much better in the half-dark than we do, but less well (though perfectly adequately) in bright light.

In order to be more efficient at night, dogs' eyes contain a structure that ours don't. Most dog owners who walk their dogs at night will have noticed that their eyes shine when a flashlight is pointed at them. This is due to a reflective layer of cells behind the retina called the tapetum, which almost doubles the sensitivity of the eyes in low light. Furthermore, dogs' eyes are connected to their brains differently than ours. We have a staggering 1.2 million nerve fibers in our optic nerves, which allows us to resolve a lot of detail, provided there is enough light. Dogs have a mere 160,000 connections—and unlike our optic nerves, theirs are connected to multiple rods or cones and can be triggered if any one of these receives a scrap of light. This enables dogs to see at lower light levels than we can, but their perception of detail is inevitably reduced, by a factor of about four, since their brains have no way of knowing which particular one of a bundle of light-sensitive cells has been triggered. Put another way, perfect vision in humans is described as 20:20 whereas dogs can manage only 20:80 at best. Some may have worse eyesight than this but we don't know for sure, as it has proved difficult to design an eye test for dogs that's as sensitive as those that opticians use on us. (Wolves, incidentally, have rather clearer vision than dogs do; it's possible that dogs are descended from a wolf more nocturnal than those that survive today.)

Dogs' eyes also produce a wider picture than ours—a picture that is less centered on straight-ahead vision. They can see more of their surroundings without moving their heads. The average dog's field of view
is about 240 degrees, compared to our 180 degrees—so they can see at least a limited amount of what's going on behind them. Each eye points about 10 degrees off the center-line of the muzzle, so there is a considerable area of overlap to the front, and dogs do use this to produce true binocular vision. Some breeds probably have better binocular vision than others; indeed, since dogs' eyes are on either side of their noses, it's not surprising that the degree of overlap is less in breeds with long muzzles.

But while dogs' field of vision is larger than ours, their close vision isn't nearly as good. Most dogs can't focus much closer than thirty to fifty centimeters from their noses, even when they're young (though, as with humans, that minimum tends to increase as they get older). Once they get their snouts within a foot or so of anything they're interested in, other senses take over—especially their keen sense of smell.

The most notable difference between canine vision and our own, however, lies in the limited range of colors that dogs can see. Like most mammals, they have only two types of color-sensitive receptor cells (cones) and so can see only two primary colors—blue-violet and yellowish-green.
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Of course, like all mammals they can distinguish many different colors based on the relative strength of the signals coming from these two, but the absence of the yellow cone (which humans have) means that they can't distinguish red from orange or orange from yellow. There is also a gap between the colors that their two types of cones are sensitive to, such that dogs see turquoise as grey. Nevertheless, scientists who have measured dogs' colour vision found that they were more attentive to color than most other mammalian species (apart from ourselves and our trichromatic relations, of course), so it is likely that dogs do sometimes use color in their everyday lives.

Thus, for instance, a dog running around in a park in the daylight will see much of what we see, with some small differences. More, because they can see to the sides of their heads as well as straight ahead; but also less, in the sense that the leaves on the trees and the grass will be rather similar muted shades of grayish-green, and red and yellow flowers will be similar to each other in color. As the dog's carnivorous ancestors did not need to pick out the ripest fruits or the most tender leaves, the latter deficiencies were most likely of no particular consequence
to them, and probably matter little to dogs today. As darkness falls, however, the dog's superior night vision comes into play, enabling dogs to continue running about happily in the undergrowth long after their owners need a flashlight to find their path.

Despite these minor differences, the degree of overlap between the dog's visual world and ours means that misunderstandings of what they can and cannot see rarely cause problems. Anyone trying to train dogs to respond to visual cues based on their color would do well to avoid using both red and orange, but this suggestion is unlikely to apply to more than a handful of specialist trainers—most everyday training uses sound and movement cues. Their general lack of interest in color probably explains why few dogs display much interest in watching television—though the poor quality of TV sound (as far as they are concerned) may also play a part.