Inside of a Dog: What Dogs See, Smell, and Know (10 page)

The sniff is the great medium for getting smelly objects to the dog, the tramway on which chemical odors speed up to the waiting receptor cells along the caverns of the dog nose. Sniffing is the action of inhaling air, but it is more active than that, usually involving short, sharp bursts of drawing air into the nose. Everyone sniffs—to clear the nose, to smell dinner cooking, as part of a preparatory inhale. Humans even sniff emotively, or meaningfully—to express disdain, contempt, surprise, and as punctuation at a sentence's end. Animals mostly sniff, as far as we know, to investigate the world. Elephants raise their trunk into the air in a "periscope sniff," tortoises slowly reach and open their nostrils wide in a sniff, marmosets sniff while they nuzzle. Ethologists watching animals often take note of all these sniffs, for they may precede an attempt to mate, a social interaction, aggression, or feeding. They record an animal as "sniffing" when it brings its nose close to—but not touching—the ground or an object, or an object is brought close to—but not touching—the nose. In these cases, they are assuming that the animal is in fact inhaling sharply—but they may not be able to get close enough to the animal to see the nostrils moving, or the tiny vortex of air that stirs the area in front of the nose.

Few have looked closely at exactly what happens in a sniff. But recently some researchers have used a specialized photographic method that shows air flow in order to detect when, and how, dogs are sniffing. They have found that the sniff is nothing to be sniffed at. In fact one could make the case that it is neither a single nor a simple inhalation. The sniff begins with muscles in the nostrils straining to draw a current of air into them—this allows a large amount of any air-based odorant to enter the nose. At the same time, the air already in the nose has to be displaced. Again, the nostrils quiver slightly to push the present air deeper into the nose, or off through slits in the side of the nose and backward, out the nose and out of the way. In this way, inhaled odors don't need to jostle with the air already in the nose for access to the lining of the nose. Here's why this is particularly special: the photography also reveals that the slight wind generated by the exhale in fact helps to pull more of the new scent in, by creating a current of air over it.

This action is markedly different from human sniffing, with our clumsy "in through one nostril hole, out through the same hole" method. If we want to get a good smell of something, we have to sniff-hyperventilate, inhaling repeatedly without strongly exhaling. Dogs naturally create tiny wind currents in exhalations that hurry the inhalations in. So for dogs, the sniff includes an exhaled component that helps the sniffer smell. This is visible: watch for a small puff of dust rising up from the ground as a dog investigates it with his nose.

Given our tendency to find so many smells disgusting, we should all celebrate that our olfactory system adapts to an odor in the environment: over time, if we stay in one place, the intensity of every smell diminishes until we don't notice it at all. The first smell of coffee brewing in the morning: fantastic … and gone in a few minutes. The first smell of something rotting under the porch: nauseating … and gone in a few minutes. The sniffing method of dogs enables them to avoid habituation to the olfactory topography of the world: they are continually refreshing the scent in their nose, as though shifting their gaze to get another look.

I crack open her window in the car—just enough to fit a dog-sized head (remembering the time she threw herself completely out the open window after that squirrel hitchhiking on the side of the road). Pump props herself up on the armrest and pokes her muzzle out of the car as we race along in the night. She squints her eyes tight, her face is streamlined in the wind, and she projects her nose deep into the rushing air.

Once a smell has been vacuumed in, it finds a receptive welcome from an extravagance of nasal tissue. Most purebreds, and nearly all mutts, have long muzzles in whose noses are labyrinths of channels lined with special skin tissue. This lining, like the lining of our own noses, is primed to receive air carrying "chemicals"—molecules of various sizes that will be perceived as scents. Any object we encounter in the world is cast in a haze of these molecules—not only the ripe peach on the counter but the shoes we kick off at the door and the doorknob we grasp. The tissue of the inside of the nose is entirely blanketed with tiny receptor sites, each with soldiers of hairs to help catch molecules of certain shapes and pin them down. Human noses have about six million of these sensory receptor sites; sheepdog noses, over two hundred million; beagle noses, over three hundred million. Dogs have more genes committed to coding olfactory cells, more cells, and more
kinds
of cells, able to detect more kinds of smells. The difference in the smell experience is exponential: on detecting certain molecules from that doorknob, not single sites but combinations of sites fire together to send information to the brain. Only when the signal reaches the brain is it experienced as a scent: if it is we doing the sniffing, we'd say
A-ha! I smell it.

More than likely, though, we won't smell it. But the beagle will: it's been estimated that their sense of smell may be millions of times more sensitive than ours. Next to them we are downright anosmic: smelling nothing. We might notice if our coffee's been sweetened with a teaspoon of sugar; a dog can detect a teaspoon of sugar diluted in a million gallons of water: two Olympic-sized pools full.*

What's this like? Imagine if each detail of our visual world were matched by a corresponding smell. Each petal on a rose may be distinct, having been visited by insects leaving pollen footprints from faraway flowers. What is to us just a single stem actually holds a record of who held it, and when. A burst of chemicals marks where a leaf was torn. The flesh of the petals, plump with moisture compared to that of the leaf, holds a different odor besides. The fold of a leaf has a smell; so does a dewdrop on a thorn. And
time
is in those details: while we can see one of the petals drying and browning, the dog can smell this process of decay and aging. Imagine smelling every minute visual detail. That might be the experience of a rose to a dog.

The nose is also the fastest route by which information can get to the brain. While visual or auditory data goes through an intermediate staging ground on the way to the cortex, the highest level of processing, the receptors in the nose connect directly to nerves in specialized olfactory "bulbs" (so shaped). The olfactory bulbs of the dog brain make up about an eighth of its mass: proportionally greater than the size of our central visual processing center, the occipital lobes, in our brains. But dogs' specially keen sense of smell may also be due to an additional way they perceive odors: through the vomeronasal organ.

What specificity of image the name "vomeronasal" conjures up! Evoking the displeasure of getting a good sniff of fresh vomit, the "vomer" is actually a description of the part of the small bone in the nose where the sensory cells sit. Still, the name seems somehow fitting for an animal that is notorious for coprophagia (feces eating) and that may lick another dog's urine off the ground. Neither act is vomitous for dogs; it's just a way of getting even more information about other dogs or animals in the area. The vomeronasal organ, first discovered in reptiles, is a specialized sac above the mouth or in the nose covered with more receptor sites for molecules. Reptiles use it to find their way, to find food, and to find mates. The lizard who darts out its tongue to touch an unknown object is not tasting or sniffing; it is drawing chemical information toward its vomeronasal organ.

These chemicals are pheromones: hormonelike substances released by one animal and perceived by another of the same species, and usually prompting a specific reaction—such as readying oneself for sex—or even changing hormonal levels. There is some evidence that humans unconsciously perceive pheromones, perhaps even through a nasal vomeronasal organ.*

Dogs definitely have a vomeronasal organ: it sits above the roof (hard palate) of the mouth, along the floor of the nose (nasal septum). Unlike in other animals, the receptor sites are covered in cilia, tiny hairs encouraging these molecules along. Pheromones are often carried in a fluid: urine, in particular, is a great medium for one animal to send personalized information to members of the opposite sex about, say, one's eagerness to mate. To detect the pheromones in that urine some mammals touch the liquid and do a distinctive, mortifying, lip-curling grimace called
flehmen.
The face of a flehmening animal is notably unlovable—but it is the face of an animal who is on the hunt for a lover. The flehmen pose seems to hurtle the fluid toward the animal's vomeronasal organ, where it is pumped into the tissue, or is absorbed through capillary action. Rhinos, elephants, and other ungulates flehmen regularly; so do bats and cats, which have their own species variations. Humans may have vomeronasal organs, but we do not flehmen. Neither do dogs. But a regular observer of dogs will notice an often very intense interest in the urine of other dogs—sometimes an interest which lures them right … up … into … wait, gross! Stop licking that! Dogs may lightly lap up urine, especially urine of a female in heat. This could be their version of flehmen.

Even better than flehmen is keeping the outside of the nose nice and moist. The vomeronasal organ is probably why a dog's nose is wet. Most animals with vomeronasal organs have wet noses, too. It is difficult for an airborne odor to land squarely on the vomeronasal organ, since it is situated in a safe, dark interior recess of the face. A hearty sniff not only brings molecules into the dog's nasal cavity; little molecular bits also stick onto the moist exterior tissue of the nose. Once there, they can dissolve and travel to the vomeronasal organ through interior ducts. When your dog nuzzles against you, he is actually collecting your odor on his nose: better to confirm that you're you. In this way, dogs double their methods of smelling the world.

When Pump got her nose into a good smell in the grass—when she really dug her nose deeply into the earth—I came to know what was going to happen next. She'd hop around, resniff the smell from different angles, then take a tentative swipe at it, upending a dollop of turf. More deep sniffing, some licking, smushing her nose into the ground—and then the climax: an unrestrained dive into the smell, nose first, throwing her whole body down after it, and wriggling madly back and forth.

What, then, do these noses enable the dog to smell? What does the world look like from the vantage of a nose? Let's start with the easy stuff for them: what they smell of us and of each other. Then we might be ready to challenge them to smell time, the history of a river stone, and the approach of a thunderstorm.

We needn't even touch objects for them to smell of us: as we move, we leave behind a trail of skin cells. The air is perfumed with our constant dehumidifying sweat. Added to this, we wear in odor what we've eaten today, whom we've kissed, what we've brushed against. Whatever cologne we put on merely adds to the cacophony. On top of this, our urine, traveling down from the kidneys, catches odorous notes from other organs and glands: the adrenal glands, the renal tubes, and potentially the sex organs. The trace of this concoction on our bodies and our clothes provides more uniquely specific information about us. As a result, dogs find it incredibly easy to distinguish us by scent alone. Trained dogs can tell identical twins apart by scent. And our aroma remains even when we've left, hence the "magical" powers of tracking dogs. These skilled sniffers see us in the cloud of molecules we leave behind.