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.6 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.
Bird’s-eye view of an odor plume. The average wind direction is shown from top to bottom, but changes in direction lasting a few seconds cause the plume to “snake.” Eddies (caused by irregularities on the ground) cause the plume to swirl and break up into pockets.
In short, when dogs first pick up a trace of the odor they’re trying to locate, they can only guess roughly where it is coming from. This is where their wet noses become helpful. Technically known as the rhinarium, the “pebbled” area of specialized skin around the nostrils is crammed with pressure and temperature sensors. (The rest of the skin on their heads is rather insensitive generally.) Those wet noses are not only directly sensitive to the wind going by but also simultaneously cooled on the upwind side, giving them an instant readout of which direction to head in.
Yet because of the wind’s intrinsic vagaries, this tactic will almost certainly immediately take the dog out of the scent pocket. Inexperienced dogs will immediately switch to quartering, running mainly across the wind in an attempt to relocate the odor. As soon as they pick up the smell again, they will check its pace and revert to running upwind—until the scent is almost inevitably lost again, whereupon the whole process starts again. More experienced dogs—for instance, those who have regularly competed in gundog trials—will tend to go on running upwind for a few seconds after they have lost the odor, confidently expecting that they’ll soon run into another pocket of odor. If they don’t, they will then switch to the quartering tactic. A prolonged period of running without encountering any odor will cause most dogs either to give up or, if their motivation remains strong, to start a purposeful loop back downwind to the general area where they first detected the odor.
Once dogs get close to the source of the odor, they change their tactics. The average strength of the odor will be increasing at this point, but the dogs will probably be alerted more reliably by the sudden disappearance of the gaps of clean air between the pockets of odor. Now they will slow down abruptly, tail wagging furiously, and start to use their eyes to locate the target, because the olfactory information is no longer sufficiently detailed to be of much help. Only if they accidentally blunder on past the source and hence lose the scent will the dogs loop back around into the odor corridor and go back to using their noses.
Whether tracking or following an air trail, dogs can ramp up the sensitivity of their noses. Rapid sniffing makes the air entering the nose more turbulent, so that more of it comes into contact with the olfactory membranes. They can also change the airflow in their nostrils: By widening the nasal valve, dogs can send more information into the olfactory area. Dogs who are tracking along the ground need to travel quite slowly in order to maintain contact with the trail, and so can sniff all the time, at about six sniffs each second. They can also temporarily increase their rate of sniffing if they need to, up to twenty sniffs for each inhalation. Indeed, they may even be able to perform the saxophonist’s trick of breathing in through their noses, sniffing continuously, while simultaneously breathing out through their mouths. But dogs can’t be sniffing all the time, particularly while running—they would simply run out of air. When attempting to follow an air trail, as opposed to a track on the ground, dogs often move at a canter. At such a pace, they of course use up a great deal of oxygen, which requires them to breathe more heavily. Because of that, they have to make a trade-off between maximizing the amount of odor they can analyze and locating their target quickly. When gundogs follow a track laid on the ground, they sniff five or six times a second. However, they sniff only about twice per second when following a scent upwind, and even less frequently—once per second or so—when running crosswind trying to locate the scent plume.
Once inhaled by the dog, the contents of each sniff are analyzed by the olfactory system. The air in each sniff passes up the nasal passages and swirls around the turbinates, which are the scroll-shaped bones that carry the olfactory receptors. The receptors encode the nature and strength of the odor, and then pass on this information to the olfactory nerves and thence to the brain, where the sensation is generated and comparisons can be made with odors that have been sampled in the past.
Until recently, not much was known about how mammals detect and analyze odor, but now that the canine and other genomes have been sequenced, an increasingly detailed picture is emerging. The molecules that make up the odor—and there will be many different types in any natural odor—first have to be extracted from the air and passed on to the receptors. Because odor molecules move very slowly, the receptors need to be very close to the air; otherwise, it would take so long for the odor molecules to reach them that they wouldn’t be able to give the instantaneous response that a dog needs in order to keep up with its six sniffs a second. Thus in the dog’s nose, the receptors are just a few thousandths of a millimeter from the open air. Exposed like this, the receptors are very susceptible to damage. The olfactory equipment protects itself in two ways. First, it cleans, warms, and humidifies the incoming air by passing it over mucus-covered membranes before it can reach the olfactory epithelium itself. Second, the olfactory neurons, on which the receptors sit, are constantly renewed, undergoing replacement roughly every month.
Once absorbed by the olfactory epithelium, a specific odor triggers the corresponding receptors. The odor molecules diffuse through the mucus that covers the receptors’ exposed ends. In the dog, there are hundreds of types of olfactory receptors. All those of a particular type are connected to a single ball of nervous tissue about a tenth of a millimeter across, which in turn relays its information to the brain through a small number of nerves, the mitral cells. This combining together of information maximizes the capacity of the nose to detect tiny amounts of odor. In contrast to the eye, what matters isn’t where in the nose each molecule is being picked up but just how many receptors are
being activated at any one time, so it makes sense that all of the signals are gathered together before relaying the information to the brain.
The olfactory bulb in the brain then compares all the signals produced by specific receptors to generate much more nuanced information—much like our brains enable us to “see” millions of colors even though our eyes can detect only three. Clearly, since dogs can distinguish tens of thousands of different odor molecules even though they have only 800 or so types of receptor, there can’t be one receptor for each odor. Rather, a particular odor molecule binds to several different kinds of receptor, in some unique combination, and the “odor brain,” the olfactory bulb, combines the information together to decode what has been detected.
Finally, once the odor molecules have interacted with a receptor, special enzymes degrade them quickly; otherwise, the sensation of the odor would persist for too long. The receptor is then clean and ready to receive the next incoming molecule.
Dogs have yet another way of perceiving odors, and it’s one that we humans don’t share at all. Running between their nostrils and the roof of their mouths, just behind the front teeth, are a pair of fluid-filled tubes, the incisive ducts. From each of these runs a cigar-shaped, blind-ended tube called the vomeronasal or Jacobson’s organ. If you look behind your own incisors in the mirror, you won’t see anything—we don’t have incisive ducts or a functioning vomeronasal organ (VNO), although most other mammals (and reptiles) do. Like much of our olfactory sense in general, this organ disappeared in our distant ancestors, way back during the evolution of the higher primates.7
The purpose of the vomeronasal organ is not easy to determine. Both the VNO and its ducts are fluid-filled, so at first glance they would seem to be rather awkwardly placed to perceive odors. However, there is a muscular pump that can move the fluid in and out of the nose and down into the VNO, providing a possible mechanism for odor molecules to get from the outside world into the VNO. Potentially, scent molecules can first be absorbed into the saliva, or into fluid in the nostrils, and then be pumped to the VNO. The resultant delay means that the VNO cannot be of much use for detecting information that changes by the second, such as an airborne scent-trail, but should be adequate for an animal to analyze the scent of another member of its own species, which should remain constant. Thus the VNO is thought to be the sense organ that specializes in social odors, although not exclusively so: The roles of the VNO and the nose overlap considerably in this respect.
It’s still not clear precisely what dogs use their VNO for. Part of the problem is that there is no obvious external sign that indicates when a dog’s VNO is being brought into play. Cats and some other mammals—including the dog’s close relative, the coyote—make a characteristic facial expression when employing the VNO: The mouth is held slightly open and the upper lip is curled back, and the cat/coyote seems momentarily lost in thought. This expression is exhibited not when the animal is sniffing food (food odors are analyzed by the nose in cats and coyotes) but when it is sniffing a scent-mark left by another member of the same species. Although dogs don’t display the same expression, some dogs do chatter their teeth when they’re sniffing scent-marks, and others make a kind of chortling noise. These sounds may be indicators that the pump that transfers odors into the VNO is in use.
The canine VNO thus certainly does something. Most likely it is used to analyze social odors, which the dog either licks up or inhales, simultaneously activating the pump that carries them to the VNO itself. Odors can thus be analyzed twice—first, more or less instantaneously, by the nose and then a second time, in a more leisurely fashion, by the VNO. The highly detailed information that emerges can then be stored away in the brain for use in future social encounters. This is yet another example of the dog’s superiority over humankind in its ability to decode smelly information—but one that has thus far proved difficult for scientists to decipher, probably because we have difficulty appreciating a sense that we don’t possess ourselves.
Whether they’re perceived through the nose or the VNO, smells are very important to dogs, much more so than they are to us. Dogs don’t just use odor to decide what to eat and what not to: It’s their primary way of identifying people, places, and other dogs. Smell is their dominant sense, the one they use in preference to all their other senses, whenever they can.
Because odors are so complex, and because they differ depending upon the environment in which individual dogs live, it would be impossible for dogs to be born with the ability to recognize more than a handful of odors. Dogs therefore have to learn what each odor means. They start learning how to use their sense of smell even before birth, in much the same way that human babies learn the sound of their mother’s voice while still in the womb. Using ultrasound, scientists have observed puppies exercising their breathing muscles in the womb during the two weeks before they are born.
This “breathing” almost certainly allows the puppy to learn something about its mother’s characteristic odor, including the type of food that she’s eating most. In one experiment, pregnant bitches were given food flavored with aniseed throughout the last three weeks before they whelped.8 As early as a quarter of an hour after they were born, and before they had even begun to suckle, their puppies moved toward the smell of aniseed. The smell of vanilla, an odor that the puppies had never been exposed to, did not have the same effect, so clearly they were not simply investigating the aniseed because it was an unfamiliar smell. What caused this preference for aniseed? Presumably it had flavored the mother’s amniotic fluid, where the pups had “smelled” it before birth.
It’s not clear precisely why puppies need to learn their mother’s smell before they’re born; it would probably be equally helpful immediately after birth, since at that stage they are too helpless to move far from their mother anyway. (Prenatal learning is especially useful in animals, such as sheep, whose young are very mobile at birth and thus run the risk of getting separated from their mothers.) Perhaps dogs have simply retained this ability from their mammalian ancestors, even though it is not of much use to them now.
From the moment they’re born, puppies use odors to help them make sense of the world around them. Initially, this attention is focused primarily on the mother. Three days after she gives birth, the bitch produces a substance around her mammary glands that, in turn, is modified by bacteria on her skin to create an odor that helps her puppies locate her. It also seems to have a calming effect on them. Although the mechanism is somewhat obscure, the same substance appears to have a calming effect on adult dogs. Scientists are not sure whether this change in behavior is caused by an instinctive, “pheromonal” effect or is due to a memory of being protected by the mother, but the extracted odor has proved useful in the treatment of acute fears in dogs.
As soon as they are able to move around, dogs start sniffing anything and everything they come across—a behavior that continues for the rest of their lives. Many owners are embarrassed by their dogs’ proclivity for sniffing the crotch of every person they meet; many more train their dogs not to do this. Yet this is the dog’s first-choice method for identifying other animals, both human and canine. When two dogs meet in the park, their first and often only goal is to sniff each other. Sometimes they circle one another before diving in for a sniff. Sometimes one dog is so intent on sniffing the other that a chase ensues. However, eight times out of ten the objective of the encounter is to get olfactory information about the other dog.9
The odors in question evidently originate at both ends of the body. Sniffing is often focused around the ears, indicating that these may be the source of an individual-specific odor. But although the ears definitely contain odor-producing glands, little is known about what kind of information is contained in the odors they produce. Sniffing under the tail must pick up odors from the preputial (male) and vaginal (female) glands that add their contributions to the dog’s ubiquitous urine-marks.
Because each attempts to sniff the other first, the two dogs oft
en end up circling one another
However, the main target of canine sniffing seems to be the anal sacs. Located on both sides of the anus, as their name implies, these contain a pungent mixture of odors (mainly produced by microorganisms) that varies considerably from one individual dog to another. Perhaps because the anal sacs are usually closed, their odors don’t vary much from one week to the next, though they do change gradually over time-scales of a few months or so.10 They are therefore good candidates for a “signature” odor, albeit one that will, like all chemical signals produced by mammals, require relearning by recipients as it changes gradually over time. This is presumably part of the explanation for dogs’ insistent attempts to sniff the back ends of every other dog they meet; if the odor stayed consistent from one month to the next, they’d need to do it only very occasionally. Such sniffing, like urine-marking, has origins that go back to before domestication. Young male wolves have a similar fascination with sniffing this part of the body, and adult wolves sometimes actively invite other members of the pack to sniff them in this area, by standing stock-still with their tails held upright.
Scent inspections tend to follow predictable patterns. Some dogs, mostly males, go straight for the area under the tail, which produces an information-rich odor. Most females, and a few males, prefer to sniff the head of the other dog first, and then to move back—provided the other dog will let them. This difference between the sexes is thus far unexplained, but it is also true of wolves, so it may simply be an inherited tendency with little functional significance as far as modern dogs are concerned.
Dog Sense Page 29
