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.1 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.
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 lowfrequency 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.2
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, theref
ore, 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.3
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.)4 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.5 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 understa
nd 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 straightforward. 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!
Dog Sense Page 28
