Dog Sense (40 page)

Our restricted hearing, compared to that of dogs, can lead to situations where they are inconvenienced or, at worst, even suffer. Dogs' hearing is significantly more sensitive, and more versatile, than ours is. Their low-frequency hearing has a range similar to ours, but they can hear high-pitched sounds that we can't hear at all. We refer to such frequencies as “ultrasound”—although, if they could, dogs would describe us as having high-frequency deafness. Cats, who can hear even higher-pitched sounds than dogs can, would presumably describe
them
as having high-frequency deafness.
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It's easy to forget that dogs can hear sounds that we can't. Some researchers into canine responses to sounds have used ordinary audio playback equipment, apparently unaware that this is designed to mimic what we humans hear and thus doesn't reproduce the high-frequencies that are presumably an important part of all sounds as far as dogs are concerned. Unsurprisingly, therefore, dogs sometimes react to “live” sounds but don't necessarily react to recordings or broadcasts (TV, for example) that are, to our ears, virtually identical. The equivalent experience for us would presumably be something like the difference between FM radio and longwave AM (which doesn't reproduce high frequencies).

It's not clear why dogs (or wolves) would ever have needed to hear such high-pitched sounds; this ability is probably a legacy from their
smaller canid ancestors. Foxes, who can hear the ultrasonic squeaks made by mice and other small rodents, use their high-frequency hearing to locate these animals when hunting. But wolves do not routinely seek out such small prey. “Silent” dog whistles make use of the high-frequency sounds that dogs can hear and we can't, but they are something of a gimmick: Whistles that produce at least some sound audible to human ears are much easier for us to control. (How can you tell when a silent whistle isn't working?) Dogs are, however, very skilled at distinguishing between quite similar sounds, probably using mainly high-frequency information. For instance, although research into how dogs discriminate between different types of barks is still in its infancy, there is little doubt that they can extract a great deal of detail from what they hear, as well as being able to detect very quiet sounds.

Dogs also have much more sensitive ears than ours, and as owners we should be attuned to this difference. Within their optimum frequency range, their hearing is approximately four times more sensitive than ours is. This means that dogs' hearing probably becomes damaged when they are subjected to the din encountered in some noisy kennels (which can be unpleasant enough even for us cloth-eared humans). Because of our own insensitivity to ultrasound, we are likely to be unaware of the discomfort that dogs must experience when subjected to noises that contain a lot of high-frequency sound, such as the banging of metal gates or the scrape of metal buckets on concrete floors.

In their sense of smell dogs are miles ahead of us humans. And it's us humans who are unusually insensitive, not the other way around. Compared to other Carnivorans, dogs are about average. For example, grizzly bears have even more sensitive noses than dogs do, allowing them to find food underground even in the dead of winter. Nevertheless, dogs possess a unique combination of trainability and olfactory ability, one that we humans have made extensive use of throughout history—and indeed are finding new uses for almost every day.

It is hard to convey how sensitive dogs' noses are without getting into some almost incomprehensibly large numbers. They can detect some odors, probably most, at concentrations in the parts per
trillion
. By comparison, humans generally detect odors in the range of parts per million
to parts per billion—a sensitivity between 10,000 and 100,000 times lower than that of dogs. Dogs' noses are as responsive as they are because they possess a very extensive olfactory epithelium, the surface that traps odor molecules and then analyzes them. Although the area of this surface varies from breed to breed, the German shepherd's, at 150–170 square centimeters (roughly the area of a CD cover, spread over a labyrinth of bony structures called turbinates), is typical—and over thirty times larger than ours. And between 220 million and 2
billion
nerves, a hundred times more than in our own noses, link the epithelium to the dog's brain. Why so many? Not only is the area of epithelium larger in the dog, but also the receptors are packed in much more densely on the dog's epithelium than they are on ours. So that dogs can process all this information, their olfactory cortex—the part of the dog's brain that analyzes smells—is roughly forty times bigger than ours.
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Dogs can also pick up much more detailed information from odors, because they have a greater diversity of olfactory receptors than we do. So far, more than 800 functional olfactory receptor genes have been identified in the dog genome (along with 200 “pseudogenes” that don't appear to make receptors, although they probably did at some time in the dog's evolutionary past). Each gene codes for a slightly different receptor, each of which is sensitive to a slightly different shape in the odor molecules. Most odors trigger many of these receptors, and the brain compares the relative strength of all the signals it receives in order to characterize each odor. Humans have a range of receptors similar to that of dogs, but with fewer of each type. The implication is that everything that dogs can smell, we can too, but with less detail extracted. We also need a much higher concentration of a given odor before we can detect anything at all. Humans can discriminate between thousands of different odors. Dogs' much greater diversity of receptors suggests that they can detect a great many more.

In practice, the range of smells that dogs can detect seems almost limitless, judging by the proliferation of odor detection tasks they are asked to perform. Traditionally, mankind has exploited the dog's nose in locating food, from the tracking of game to the detection of delicacies such as truffles. More recently, dogs' keen sense of smell has been used to detect various types of cancer (melanomas as well as ovarian
and bladder tumors) and impending epileptic seizures in humans. Dogs are able to smell pests such as the nematodes that can infest sheep as well as the bedbugs that can infest humans. And they have even been put to use in conservation efforts; for instance, they are employed to sniff out illegal exports of sharks' fins and sea cucumbers in the Galapagos. Scientists have likewise drawn on them to map populations of rare South American maned wolves and bush dogs (based on the odor of their feces).

Dogs are much more capable than humans of discriminating between very similar scents. For example, they can distinguish between the odors of nonidentical twins living together and those of identical twins living apart. (However, they seem unable to reliably distinguish between the odors of identical twins living together.)
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In short, dogs can identify us using not only odor cues derived from the environment that we live in (e.g., the food we eat and the fabric conditioner on our clothes) but also genetically based factors that contribute to our characteristic individual odors. Only when the genes are the same, and the environments also, do dogs begin to get confused. Dogs' acuity at distinguishing particular human odors is now being used in several countries, including the Netherlands and Hungary, as a way of linking criminals to crime scenes.

Because we make so little use of our sense of smell, we have to exercise considerable imagination in order to understand how dogs experience this unfamiliar world. Odors don't behave in the same way that either beams of light or sound waves do; they are much less predictable than either. The rate at which they get into the air varies with temperature, humidity, and the kind of surface that they're coming from. Moreover, the speed with which—and direction in which—odors travel are much more haphazard than is true of either light or sound. Yet these factors don't matter much to us, and indeed rarely impinge upon our consciousness at all, because we use our visual sense to find our way around. Dogs, by contrast, have by necessity developed strategies to glean useful information from the odors that they rely on to locate objects of interest—whether those are scent-marks left by other dogs, potential food items, or odors that we have specifically trained them to find.

Finding interesting odors isn't as simple or as instantaneous as gathering visual information. When we go somewhere new—let's say we enter a room we haven't been in before—we look about and check our surroundings. Because light travels in predictable straight lines, it's immediately obvious if there are any parts of the room that we can't see; for example, we know without having to think about it that we can't see into cupboards if the doors are closed, or behind screens or large pieces of furniture. Unfortunately for dogs, smells don't travel nearly so predictably as light does. They spread very slowly of their own accord, by molecular diffusion, but the distances involved in this process are so minute (no more than a few centimeters) that they are relevant only to small insects like ants that live in the thin “boundary” layer of still air close to flat surfaces.
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For an odor to travel any distance greater than this, it would have to be transported by air movements, and these are very erratic.

To understand what a dog experiences, imagine opening a food cupboard and not being able to tell instantly whether something you were looking for was on a shelf inside, in a rack on the back of the door, or on the work-surface beneath. Try turning the lights out and locating a spice jar by its odor alone. We humans
do
retain some vestigial ability to navigate by smell, but it's a slow and cumbersome process. Even when we can smell something, tracing the source of the odor is rarely straight-forward. Air movements are just too unpredictable, especially indoors. That's why, if you watch dogs for a while, you'll realize that they spend a lot of time and energy looking for visible indicators of likely places to find an interesting odor. How do they know where to sniff first? Presumably this is largely a matter of experience, though if they're leaving their own scents for others to find, they'll either leave them somewhere obvious (on the proverbial lamppost, say) or leave a visible indicator (such as the “tramlines” that some dogs scratch in the dirt pointing to where they've just urinated).

If there are no visible cues, then dogs just have to use their legs to work out where the smell is coming from. If there's not much air movement, then they will run around sniffing, working out by trial and error where the smell is strongest. In situations where the odor is coming
from a point source, this strategy is usually successful, sooner or later. But if it isn't, the dog can become very confused and frustrated. There is an apocryphal tale of a trained narcotics-detection dog who went crazy inside a container full of oriental furniture. There was a strong odor but he was completely unable to identify where the smell was coming from. As the story goes, it turned out that the whole consignment had been lacquered with cannabis resin—so the container itself was the source!

Dogs are particularly adept at following scent-trails, such as those left by other animals, or deliberately by people, as in drag-hunting. Dogs will follow a trail by zigzagging to and fro. They find the edges of the “corridor” of odor coming off the trail and, if they lose it, will head back in the opposite direction, back across the invisible trail. As well as keeping to the trail, dogs have to make up their minds about which direction it's going in. It's likely that wherever possible they use visual cues, such as the flattening of grass or undergrowth in the direction of travel. However, some dogs, though probably not all, seem to be able to follow a trail in the correct direction even if the visual cues are misleading. In one set of experiments, human subjects were persuaded to walk backward across a grassy field so that if the police dogs who then followed the tracks used heel-to-toe cues to determine direction, they would go the wrong way.
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They didn't, instead heading in the direction the persons had traveled. (It is possible, however, that they were picking up on the detail of which way the grass had been flattened, rather than simply the heel and toe impressions—even though the trails were by then an hour old.)

Dogs often face the problem of finding the source of an odor when there is no track along the ground. Outdoors, there is usually some kind of breeze to carry the odor to the dog—but breezes are not very predictable when it comes to carrying odor. You might think that odor travels in a straight line downwind from the source, but in fact it spreads out sideways as it travels downwind, resulting in a conical distribution with the source at the apex. Nevertheless, at any one instant the distribution of odor would look, from above, more like a snake—one that is solid in some places and thin and wispy in others. The reason is that, as the wind blows over a surface, friction between the two
causes eddies to develop. Some are several meters across, causing the snaking effect; others are smaller, causing the snake to spread out or bunch up. As a result, a dog who is standing still, even directly downwind of the smell's source, will be outside the odor plume for a longer time than within it; conversely, a dog who is downwind but actually well away to one side will occasionally receive a burst of odor as the snake wiggles particularly violently.