Inside of a Dog: What Dogs See, Smell, and Know

by Alexandra Horowitz

If we want to understand the life of any animal, we need to know what things are meaningful to it. The first way to discover this is to determine what the animal can perceive: what it can see, hear, smell, or otherwise sense. Only objects that are perceived can have meaning to the animal; the rest are not even noticed, or all look the same.

In the dog umwelt beds have no special functional tone. Dogs sleep and rest where they can, not on objects designated by people for those purposes. There may be a functional tone for places to sleep: dogs prefer places that allow them to lie down fully, where the temperature is desirable, where there are other members of their troop or family around, and where they are safe. Any flattish surface in your home satisfies these conditions. Make one fit these criteria, and your dog will probably find it just as desirable as your big, comfy human bed.

researchers of wild canids—wolves, coyotes, foxes, and other wild dogs—report that puppies lick the face and muzzle of their mother when she returns from a hunt to her den—in order to get her to regurgitate for them. Licking around the mouth seems to be the cue that stimulates her to vomit up some nicely partially digested meat. How disappointed Pump must be that not a single time have I regurgitated half-eaten rabbit flesh for her. Furthermore, our mouths taste great to dogs. Like wolves and humans, dogs have taste receptors for salty, sweet, bitter, sour, and even umami, the earthy, mushroomy-seaweedy flavor captured in the flavor-heightening monosodium glutamate. Their perception of sweetness is processed slightly differently than ours, in that salt enhances the experience of sweet tastes. The sweet receptors are particularly abundant in dogs, although some sweeteners—sucrose and fructose—activate the receptors more than others, such as glucose. This could be adaptive in an omnivore like the dog, for whom it pays to distinguish between ripe and non-ripe plants and fruit. Interestingly, even pure salt doesn’t kick-start the so-called salt receptors on the tongue and the roof of the mouth in dogs the way it does in humans.

wolves do have features that made them terrific candidates for artificial selection. The process favors a social animal who is behaviorally flexible, able to adjust its behavior in different settings. Wolves are born into a pack, but only stay until they are a few years old: then they leave and find a mate, create a new pack, or join an already existing pack. This kind of flexibility to changing status and roles is well suited to dealing with the new social unit that includes humans.

For dogs, socialization among humans is natural; not so for wolves, who learn to avoid humans naturally. The dog is a member of a human social group; its natural environment, among people and other dogs. Dogs show what is called with human infants “attachment”: preference for the primary caregiver over others. They have anxiety at separation from the caregiver, and greet her specially on her return.

The key to dogs’ success living with us in our homes is the very fact that dogs are not wolves. For instance, it is high time we revamp the false notion that our dogs view us as their “pack.” The “pack” language—with its talk of the “alpha” dog, dominance, and submission—is one of the most pervasive metaphors for the family of humans and dogs. It originates where dogs originated: dogs emerged from wolflike ancestors, and wolves form packs. Thus, it is claimed, dogs form packs. The seeming naturalness of this move is belied by some of the attributes we don’t transfer from wolves to dogs: wolves are hunters, but we don’t let our dogs hunt for their own food.13 And though we may feel secure with a dog at the threshold of a nursery, we would never let a wolf alone in a room with our sleeping newborn baby, seven pounds of vulnerable meat.

What domestic dogs do seem to have inherited from wolves is the sociality of a pack: an interest in being around others. Indeed, dogs are social opportunists. They are attuned to the actions of others, and humans turned out to be very good animals to attune to.

Undesired behavior gets no attention, no food: nothing that the dog wants from you. Good behavior gets it all. That’s an integral part of how a young child learns how to be a person. And that’s how the dog-human gang coheres into a family.

Despite their sometime wildness, dogs never revert to wolves. Stray dogs—those who lived with humans but have wandered away or been abandoned—and free-ranging dogs—provisioned with food but living apart from humans—do not take on more wolflike qualities. Strays seem to live a life familiar to city dwellers: parallel to and cooperative with others, but often solitary. They do not self-organize socially into packs with a single breeding pair. They don’t build dens for the pups or provide food for them as wolves do. Free-ranging dogs may form a social ordering like other wild canids—but one organized by age more than by fights and strife. Neither hunts cooperatively: they scavenge or hunt small prey by themselves. Domestication changed them.

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.

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.1

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.

Humans stink. The human armpit is one of the most profound sources of odor produced by any animal; our breath is a confusing melody of smells; our genitals reek. The organ that covers our body—our skin—is itself covered in sweat and sebaceous glands, which are regularly churning out fluid and oils holding our particular brand of scent. When we touch objects, we leave a bit of ourselves on them: a slough of skin, with its clutch of bacteria steadily munching and excreting away. This is our smell, our signature odor. If the object is porous—a soft slipper, say—and we spend a lot of time touching it—putting a foot in it, clutching it, carrying it under an arm—it becomes an extension of ourselves for a creature of the nose.

When we return home at a day’s end, dogs typically greet our cocktail of stink promptly and lovingly. Should we come home after having bathed in unfamiliar perfume or wearing someone else’s clothes, we might expect a moment of puzzlement—it is no longer “us”—but our natural effusion will soon give us away.

Not only can dogs identify individuals by odor, they can also identify characteristics of the individual. A dog knows if you’ve had sex, smoked a cigarette (done both these things in succession), just had a snack, or just run a mile. This may seem benign: except, perhaps, for the snack, these facts about you might not be of particular interest to a dog. But they can also smell your emotions.

How does that strange, menacing-looking dog smell our apprehension or fear as he approaches us? We spontaneously sweat under stress, and our perspiration carries a note of our odor on it: that’s the first clue to the dog. Adrenaline, used by the body to gear up for a good sprint away from something dangerous, is unscented to us, but not to the sensitive sniffer of the dog: another hint. Even the simple act of increased blood flow brings chemicals more quickly to the surface of the body, where they can be diffused through the skin. Given that we emit odors that reflect these physiological changes accompanying fear, and given the budding evidence of pheromones in humans, chances are that if we’ve got the heebie-jeebies, a dog can tell. And as we’ll see later, dogs are skilled readers of our behavior. We can sometimes see fear in other people in their facial expressions; there is sufficient information in our posture and gait for a dog to see it, too.

All canids—wild and domestic dogs and their relations—leave urine conspicuously splashed on all manner of object. Urine marking, as this method of communication is called, conveys a message—but it is more like note-leaving than a conversation. The message is left by one dog’s rear end for retrieval by another’s front end. Every dog owner is familiar with the raised-leg marking of fire hydrants, lampposts, trees, bushes, and sometimes the unlucky dog or bystander’s pant leg. Most marked spots are high or prominent: better to be seen, and better for the odor in the urine (the pheromones and affiliated chemical stew) to be smelled. Dogs’ bladders—sacs that serve no known purpose except as a holding pen for urine—allow for release of just a little urine at a time, allowing them to mark repeatedly and often.

From observations of the behavior of the sniffing dogs, it appears that the chemicals in the urine give information about, for females, sexual readiness, and for males, their social confidence. The prevailing myth is that the message is “this is mine”: that dogs urinate to “mark territory.”

Marking changed by seasons, and happened more often when courting or when scavenging. The “territory” notion is also belied by the simple fact that few dogs urinate around the interior corners of the house or apartment where they live. Instead, marking seems to leave information about who the urinator is, how often he walks by this spot in the neighborhood, his recent victories, and his interest in mating.

dogs have one other trick in their marking book: they scratch the ground after defecation or urination. Researchers think that this adds new odors to the mix—from the glands on the pads of the feet—but it may also serve as a complementary visual cue leading a dog to the source of the odor for closer examination. On a windy day, dogs may seem friskier, more likely to scratch the ground; they may in fact be leading others to a message that otherwise would waft away.

we deprive our dogs of something by bathing them so much—to say nothing of our culture’s overenthusiastic cleaning of our own homes, including our dogs’ bedding. What smells clean to us is the smell of artificial chemical clean, something expressly non-biological. The mildest fragrance that cleansers come in is still an olfactory insult to a dog. And although we might like a visually clean space, a place rid entirely of organic smells would be an impoverished one for dogs.

planning shifted in the eighteenth and nineteenth centuries toward elaborate “deodorization” of cities: paving streets and replacing dirt paths with concrete to trap odors. In Manhattan it even prompted a grid-based street system that, it was thought, would encourage odors to race out of the city to the rivers, instead of settling into pleasant nooks and alleys.

To understand the dog umwelt, then, we must think of objects, people, emotions—even times of day—as having distinctive odors.

Dogs, like so many non-human animals, have evolved innumerable, non-language-driven methods to communicate with one another. Human facility at communication is unquestionable. We converse with an elaborate, symbol-driven language, quite unlike anything seen in other animals. But we sometimes forget that even non-language-using creatures might be talking up a storm. What animals have are whole systems of behavior that get information from a sender (speaker) to a recipient (listener). That is all that is needed to call something a communication. It needn’t be important, relevant, or even interesting information, but between animals it often is. Communication is only sometimes within our range of hearing, or even vocal: it is often made through body language—using limbs, head, eyes, tails, or the entire body—or even through such surprising forms as changing color, urinating and defecating, or making oneself larger or smaller.

Our auditory range is from 20 hertz to 20 kilohertz: from the lowest pitch on the longest organ pipe to an impossibly squeaky squeak.1 We spend most of our time straining to understand sounds between 100 hertz and 1 kilohertz, the range of any interesting speech going on in the vicinity. Dogs hear most of what we hear and then some. They can detect sounds up to 45 kilohertz, much higher than the hair cells of our ears bother to bend to.

The crystal resonator used in digital alarm clocks emits a never-ending alarm of high-frequency pulses audible to canine ears. Dogs can hear the navigational chirping of rats behind your walls and the bodily vibrations of termites within your walls. That compact fluorescent light you installed to save energy? You may not hear the hum, but your dog probably can.

Dogs hear all sounds of speech, and are nearly as good as we are at detecting a change of pitch—relevant, say, for understanding statements, which end in a low pitch, versus questions, which in English end in a raised pitch: “Do you want to go for a walk(?)” With the question mark, this sentence is exciting to a dog with experience going on walks with humans. Without it, it is simply noise.

The research with dogs suggests that they do understand language—to a limited degree.

One component in understanding a word is the ability to discriminate it from other words. Given their sensitivity to the prosody of speech, dogs do not always excel at this. Try asking your dog on one morning to go for a walk; on the next, ask if your dog wants to snow forty locks in the same voice. If everything else remains the same, you’ll probably get the same, affirmative reaction.

High-pitched sounds are naturally interesting to dogs: they might indicate the excitement of a tussle or the shrieking of nearby injured prey. If a dog fails to respond to your reasonable suggestion that he come right now, resist the urge to lower and sharpen your tone. It indicates your frame of mind—and the punishment that might ensue for his prior uncooperativeness. Correspondingly, it is easier to get a dog to sit on command to a longer, descending tone rather than repeated, rising notes. Such a tone might be more likely to induce relaxation, or preparation for the next command from their talky human.

Low moans or grunts are also very common in puppies, and seem not to be signs of pain but rather a kind of dog purr. There are snuffling moans and sighing moans—what some call “contentment grunts,” and they all seem to mean about the same thing. Pups moan when they are in close contact with littermates, their mother, or a well-known human caretaker. The sound might be simply a result of heavy, slow breathing, which indicates it might not be intentionally produced: there is no evidence that dogs moan on purpose (neither is there evidence that they do not; neither has been proven). But whether they do or not, moans probably function to affirm the bond between family members, whether heard as a low vibration or felt through skin-to-skin contact.

The play slap, an audible landing on the two forefeet at once, is an inevitable part of play. It conveys sufficient exuberance that it can be used by itself to ask a dog to play with you.

The dog laugh is a breathy exhalation that sounds like an excited burst of panting. We could call it social panting: it is a pant only heard when dogs are playing or trying to get someone to play with them.

Dogs don’t seem to laugh to themselves, off sitting in the corner of the room, recollecting how that tawny dog in the park outsmarted her human this morning. Instead, dogs laugh when interacting socially. If you have played with your dog, you have probably heard it. In fact, doing your own social panting toward a dog is one of the most effective ways to elicit play.

Just as our laughs are often inadvertent, reflexive responses, so may dog laughs be: simply the kind of panting that results when you’re throwing your body around in play. Though it might not be under the control of the dog, social panting does seem to be a sign of enjoym...

when experimenters analyzed the spectrograms of thousands of dog barks during one of three contexts—a stranger ringing the doorbell, being locked outside, or in play—they found three distinct types of barks. Stranger barks were the lowest in pitch and the harshest: they are nearly spat out. Less variable than the other types, stranger barks are well designed to send a message over a distance, something necessary if caught in a threatening situation alone. They can also be combined into “superbarks,” concatenations of barks that together last much longer than the duration of barking in other contexts. The end result is a bark that most human listeners find to be aggressive. The isolation barks tended to be higher-frequency and more variable: some ranged from loud to soft and back again, some went from high to low. These barks are lobbed into the air one by one, sometimes with great intervals between them. They sound “fearful,” people tend to say. Play barks, too, are high-frequency, but they happen more often one after the other than the isolation barks. They’re directed at someone else, unlike isolation barks: at a dog or human playmate. There is considerable individual variation, of course: not every dog barks alike. The stranger bark of a small dog may come out as rar, rar or raoaw, raoaw, while a larger dog emits a capital-r Rumph.

These differences between bark types make evolutionary sense: the lowest sounds are used in threatening situations (again to appear bigger); higher sounds are entreaties—to friends, for companionship—and as such are submissive requests, not warnings.

For dogs, posture can announce aggressive intent or shrinking modesty. To simply stand erect, at full height, with head and ears up, is to announce readiness to engage, and perhaps to be the prime mover in the engagement. Even the hair between the shoulders or at the rump—the hackles—may be standing at attention, serving not just as a visual signal of arousal but also releasing the odor of the skin glands at the base of the hairs. To exaggerate the whole effect, a dog might stand not just up but over another dog, head or paws on his back. That’s about as declarative a statement as you can make that you are feeling dominant. The opposite body posture, crouching with head down, ears down, and tail tucked away, is submissive. To lie all the way down and expose the belly is even more so.4

A dog’s “grin,” with jaw closed, is submissive; as the mouth is opened, the arousal increases; and if the teeth are exposed, the look gets aggressive. Coming full circle, a wide-open mouth with teeth mostly covered—a yawn—is not a sign of boredom, as often assumed by analogy with our own yawning; instead, it may indicate anxiety, timidity, or stress, and is used by dogs to calm themselves or others. The ears can also go through these gymnastics: they can be pricked, relaxed and down, or folded tightly along the head. Eyeballing another dog directly can be threatening or aggressive; by contrast, looking away is submissive—an attempt to quell one’s own anxiety and the other dog’s arousal.

tails held high indicate confidence, self-assertion, or excitement from interest or aggression, while low-hanging tails indicate depression, stress, or anxiety. An erect tail also exposes the anal region, allowing a bold dog to air his odor signature. By contrast, a long tail held so low as to curl back between the legs, closing off the rump, is actively submissive and fearful. When a dog is simply waiting around, his tail is relaxed, hanging low, dropped down but not rigid. A tail gently lifted is a sign of mild interest or alertness.

the dog wags asymmetrically. On average, dogs’ wags tend more strongly to the right when they suddenly see their owners—or even anything else of some interest: another person, or a cat. When presented with an unfamiliar dog, dogs still wag—more that tentative wag than the happy wag—but tending to the left.

Urine marking is peeing with intention. The morning’s blissful release relieves the strain on the bladder, but most of the time some is held back for later use in marking. Presumably the urine is the same urine: there’s no evidence of an independent channel or means by which to modify the odor they are emitting. But marking behavior differs in a few key ways. First, in most adult males, and some gender-bending females, marking is characterized by a prominently raised leg. There are individual and contextual variations on the so-called “raised-leg display,” from a modest retraction of the rear leg up toward the body, to hoisting the leg up above the hip points, above vertical—certainly also a visual display for any other dogs in the vicinity. Both allow for a directional flow of urine, aimed to land at a conspicuous site. (One can squat and mark, too, although it is a quieter affair, perhaps for messages better whispered than shouted.) Second, the bladder is not emptied when marking; urine is doled out a little at a time, allowing for greater distribution of scent over the course of the dog’s travels. If you’ve left your dog indoors long enough that he races outside to squat, this urgency might preempt his ability to cache some urine for later marking. Thus the fruitless raised-leg displays you may witness, waving dryly at bushes, lampposts, and trash cans. Finally, dogs usually urine mark only after spending some time sniffing the area. This is what elevates the odor excha...

Countermarking—covering old urine with new—is a common behavior of male dogs, when the old urine is that of less dominant male dogs. Everyone’s marking increases when there is a new dog around.

With age, the higher-frequency sounds, above 11–14 kilohertz, go undetected by the human ear. This knowledge prompted the inspired design of a product with the teenager’s umwelt in mind. The device emits a 17 kilohertz tone—out of the range of most adults’ hearing, but unpleasantly audible to youngsters. Shop owners have used it as a teenager repellent, to discourage loitering around their businesses.

The order of operations is turned upside-down for dogs. Snout beats eyes and mouth beats ears. Given the olfactory acuity of dogs, it makes sense that vision plays an accessory role. When a dog turns his head toward you, it is not so much to look at you with his eyes; rather, it is to get his nose to look at you. The eyes just come along for the ride.

small differences between the eyes of humans and dogs immediately become apparent. First, our eyes are smack in the front of our face. We look forward, and images in the periphery fall away to darkness around our ears. While there is variation among dogs, most dogs’ eyes are situated more laterally on their heads in the manner of other quadrupeds, allowing a panoramic view of the environment: 250–270 degrees, as contrasted to humans’ 180 degrees.

While dog and human eyes are similarly sized, our pupil size—the black center of the eye that lets light in—varies considerably when we are in a darkened room or are aroused or fearful (expanding to up to 9 millimeters wide) or in the bright sun or are highly relaxed (contracting to 1 millimeter). Dogs’ pupils, by contrast, are relatively fixed at about 3 to 4 millimeters, regardless of the light or the dog’s level of excitement. Our irises, the muscles that control pupil size, tend to be colored to contrast with the pupil, blue or brown or green. Not so in most dogs, whose eyes are often so monochromatically dark that they remind me of bottomless lakes, repository of all attributions of purity or desolation we might ascribe to them. And the human iris sits amidst the sclera—the white—of the eye, while many (but not all) dogs have very little sclera. The anatomical sum effect is that we can always see where another person is looking: the pupil and iris point the way, and the amount of sclera revealed underlines it. Without a prominent sclera or a distinct pupil, the eyes of a dog don’t indicate the direction of their attention nearly as much.

Dogs manage to gather more light than we do. Once light enters the eye of a dog, it travels through the gel-like mass that holds nerve cells to the retina (we’ll get to that in a moment), then through the retina to a triangle of tissue, which reflects it back. This tapetum lucidum, in Latin “carpet of light,” accounts for all the photographs you have of your dog with brilliant light shining out where their eyes should be. Light entering the dog’s eye hits the retina at least twice, resulting not in the redoubling of the image but in a redoubling of the light that makes images visible. This is part of the system enabling dogs to have such improved night and low-light vision.

Our eyes face forward, and our retinae have foveae: central areas with an extra abundance of photoreceptors. Having so many cells in the center of the retina means that we are very good at seeing things right in front of us in high detail, great focus, and strong color. Perfect for identifying that blob of color and form coming at you as your boyfriend or your mortal enemy. Only primates have foveae. By contrast, dogs have what is called an area centralis: a broad central region with fewer receptors than a fovea, but more than in peripheral parts of the eye. Things directly in front of a dog’s face are visible to him, but they are not quite as sharply in focus as they would be for us. The lens of the eye, which adjusts its curvature to focus light onto the retina, doesn’t accommodate to nearby sources of light. In fact, dogs might overlook small things right in front of their nose (within ten to fifteen inches), because they have fewer retinal cells committed to receiving light from that part of the visual world.

The shorter the nose, the less visual streak; the longer the nose, the more visual streak. Dogs with the visual streaks have better panoramic, high-quality vision, and much more peripheral vision than humans.