From fire hydrants to rescue work, dogs perceive the world through smell

Specially trained dogs have been known to sniff out explosives, drugs, missing persons and certain cancer cells, but author Alexandra Horowitz tells Fresh Air's Terry Gross that extraordinary olfactory abilities aren't just the domain of working dogs.

Horowitz says that all dogs have the ability to create "a picture of the world through smell," thanks, in part, to the design of their snouts. A canine's nose is "stereoscopic," she explains, which means that each nostril is controlled separately, allowing the dog not only to detect a particular smell, but also to locate it in space.

In her new book, Being a Dog, Horowitz discusses the mechanics of canine smell and explains how dogs can use their noses to understand what time of day it is or whether a storm is coming.

Horowitz warns that pulling dogs away from smell-rich environments, such as fire hydrants and tree trunks, can cause them to lose their predisposition to smell. When dogs are living in "our visual world," she says, "they start attending to our pointing and our gestures and our facial expressions more, and less to smells."

Interview Highlights

On the detection work dogs can do

The types of work that dogs now do as detection dogs is really stunning. ... Dogs are, as we know, able to find explosives and drugs with some high acuity. They also are really good at finding us — finding missing people, whether dead or alive, but they're also used to detect goods that are brought into the country illegally. ...

They're also used to find animals whose population we're trying to count through detecting their scat. They can smell out illicit cellphones or computers, if trained on electronic components.

On dogs' ability to detect tumors

One of the most amazing things is that accidentally researchers discovered that dogs could detect melanoma. The accident was that there were individual dogs who were persistently smelling something on their owners. When their owners finally went to a doctor and had it checked out it turned out to be a melanoma.

So since that time there's been a budding research program in training dogs to detect various cancers on the breath, in urine, in blood and on the skin. Most of these programs report very high levels of success. Dogs are definitely able to detect whatever it is in the cell that makes it cancerous.

On a dog nose compared to a human nose

Starting at the nostril — their nostrils are doing a little bit better work than ours are. They have all this musculature — we do as well, with our nose, but theirs really allows them to get a different odor sample with each nostril, especially up close, which might be why they bring their noses close to things, one of the reasons.

Then they have this amazing long snout, many dogs, which humidifies and filters the air and kind of rushes the air up to the back of the nose. We both have that same apparatus, but ours is less complex.

At the end of the nose, right between the eyes, we both have a little patch of tissue called the olfactory epithelium which has the receptor cells — the ones that really grab the odors and are going to send the signal to the brain. The dog's has just hundreds of millions more receptor cells than ours does, and that's probably partially responsible for their increased acuity.

On how the dog's exhale enhances its sense of smell

I love the fact that not only is the dog's sniff different than ours, but their exhale is different. I mean, it goes so deep, the differences between us.

In this case, researchers looking at the fluid dynamics of air flow found that dogs exhale through the side slits of their nose. They inhale through the nostrils but exhale through the side, and what that does is it allows the odors that they've inhaled to stay in there a little bit longer, in the back of the nose. When you want to get a smell out of your nose you can exhale it out, you can kind of push it out with an exhale, but dogs don't push all the smell out with a single exhale.

It's like a circular breathing of smelling. It also creates a little puff on the ground, a puff of air that might actually allow more odor molecules to come up toward their nose to be sniffed.

On dogs detecting time and weather

Smells tell time, in other words, strong odor is probably newer odor, laid down more recently. A weaker odor is something that was left in the past. So being able to detect the concentration of a smell, they're really seeing not only what it is, but how long ago it was left.

So the past, for instance, when you walk outside your door, is underfoot — who's walked by, what skin have they sloughed, leaving some evidence of their voyage, what animals have passed by. And the future, in a way, is smelled on a breeze from up ahead or around a corner.

So I feel like time is rubber banded for dogs through smell. It also allows them to detect things which we don't think are really visible yet, like dogs often are said to be able to detect an upcoming storm. One of the reasons this might be is that when a low pressure system moves in, the air above the ground kind of feels extra roomy and the earth loosens its grip on odors within it. They become volatile, they evaporate and go up in the air, and the dog can detect that and you might see a change in their behavior when they notice this new smell.

On how a dog can tell the time of day by smell

Smells in a room change as the day goes on. Hot air rises, and it usually rises in currents along the walls and will rise to the ceiling and go kind of to the center of the room and drop. If we were able to visualize the movement of air through the day, what we're really visualizing is the movement of odor through the day. In the mid-afternoon you might feel tangibly on your skin, or see through the light in the window, that it's afternoon and the sun is half-way in the middle of the sky. The dog, I think, can smell that through the movement of that air through the room.

On why dogs smell human crotches

It's a really smelly place. Dogs are extremely good at honing in on the parts of us that happen to smell and we secrete a lot of smells from a couple parts of our body — the crotch, the armpits, the mouth. One of the things you could do when you don't like that, which most people don't, is give them something else to smell. They might be preoccupied with the smell of your ear, for instance. We have lots of glands that give off odors around our face, and that might suffice to be information about you. That's all, the dog is trying to get information.

On "smell walks"
I really am trying to counter what I and lots of owners have done our whole lives, which is discourage smelling. In fact, instead I'm trying to embrace it. So on a "smell walk," I just let the dog choose what we're going to do, where we're going to go, and how long we're going to stay there. ... I just let the dog take charge. Sometimes our walks are pretty much standing around, actually, but I think the dog is enjoying himself. Copyright 2019 Fresh Air. To see more, visit Fresh Air.