Not all dogs have the same genius for sniffing. Dogs with longer snouts generally have more sensitive noses than dogs with stubby noses, like bulldogs. This is one of the reasons why you will probably never see a pug as a military working dog, unless the military decides it needs something rather amusing-looking to distract the enemy.
Let’s see why size matters. If you have more odor analyzers in your nose, you are going to be more sensitive to smell. We humans have about five million odor receptors in our noses. The area these take up if unfurled would be about the size of a postage stamp. Dogs with long noses have far more of these scent receptors. Dachshunds have 125 million. But German shepherds have 225 million of them. So do beagles, which is pretty amazing considering they’re half the size of shepherds. Bloodhounds have the most, with three hundred million. A bloodhound’s olfactory receptor area is about the size of a handkerchief. (You will not see bloodhounds in the military, though. Doc Hilliard explains that while they have great noses, and can be excellent trackers, most do not retrieve or play with Kongs or balls the way they need to for the training. I wonder if another reason could be that their droopy, drooly countenances don’t seem very “military.”)
Smell is the dominant sense in dogs—even in those with less prominent snouts. From the outer nose (known as a dog’s “leather”) to the brain, a dog’s olfactory system makes ours look like it needs to go back to the manufacturer. (But our eyes have all the grandeur of their noses, so it all works out.)
Here’s a quick look at what happens when a military working dog we’ll call Sam, a German shepherd, sniffs an odor of interest; let’s say ammonium nitrate. Sam is close to a scent, but not sure quite where it is yet. He sniffs more rapidly, so the air coming into his nose is more turbulent and more of it can be distributed onto his olfactory membranes. He can sniff up to twenty times for every exhalation if he’s really interested in a scent. He can even pull a neat trick of inhaling at the same time he exhales. And he can move his nostrils independently, which helps him figure out just where a scent is coming from. When Sam thinks that he may be quite near the source, he will sniff more deeply, actively drawing air over the source to confirm its location. At this point he may even be able to compare the concentration of odor between left and right nostrils, which will both confirm that the source is nearby and further help to pinpoint its location.
As he sniffs, scent molecules stick to the moisture on Sam’s nose. (The moisture is actually mucus, which helps snortle the molecules all the way through the olfaction process.) Scent molecules dissolve in the mucus, and the sniffing carries them into the nose, to two bony plates called turbinates. This is the home of those millions of scent-detecting cells discussed earlier.
Adding to Sam’s nasal prowess is a body part that doesn’t seem to exist in any functional form in humans: an extra olfactory chamber known as the vomeronasal organ, aka Jacobson’s organ. It’s located above the roof of a dog’s mouth, just behind the upper incisors. It has ducts that open to the nose and mouth so scent molecules can be processed. Most mammals and reptiles have a vomeronasal organ. It’s used primarily to detect pheromones (not terribly helpful for Sam on the job), but some scientists think it may have other functions we’re not yet aware of.
The brains of dogs and people and most vertebrates contain two structures called olfactory bulbs, which help us decode smells. The olfactory bulbs of dogs are about four times as big as those of people, despite the fact that dogs’ brains are far smaller than ours. Between this and his 225 million scent receptors, Sam is able to find the ammonium nitrate quickly. He sits, stares, and is called back by his handler, who gives him his cherished Kong and praises him up like mad.
All in a day’s work for a good nose.
Oh, and lest you think it’s all about sniffing and letting nature take it from there, a dog also has to do a great deal of legwork to locate an odor. Bradshaw explains that a dog’s first strategy is to run cross-wind to figure out whether or not he is directly downwind of the source. But wind spreads scent in unpredictable ways, so this may not be very informative. If the scent is continuous, the source must be very close by, so the wind direction is an unreliable clue, and visual cues may provide the best indication of the source. If the scent is discontinuous, its source is probably some distance away, so the dog will briefly switch to proceeding upwind; if the scent is quickly lost, he will switch back to running cross-wind to try to position himself more precisely in the odor “corridor.” Then, switching between upwind and cross-wind running will bring the dog, somewhat crabwise, to within close range of the source.
But what if the scent itself is moving?
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A CLOUD OF SCURF
In the silence of the vast desert, you can hear the
The air remains thick with dust when out of the helicopter, whose rotors are still churning, run a dog and handler, followed by another dog team. The handlers hunch forward slightly as they run, to better protect themselves from the churning sand and air. They race off into the distance, then stop. The dogs sniff the ground with great interest, but just then the helicopter lifts off, and we lose sight of everything again.
Gunny and I catch up with these teams, and a few others, while they’re resting up after having tracked a “bad guy” varying distances at the Yuma Proving Ground. The dogs are combat tracker dogs. While explosives dogs find bombs, these dogs find the people who plant them. (In friendly operations, they can track down lost people.) The dogs here today are training for real-life combat missions, which often involve these rapid helicopter drops. The dogs need to get used to these so they’re ready when they deploy.
One of the dogs was so scared of the helicopter this day that he put the brakes on as he approached it and had to be nearly carried aboard. He spent the ride with his head tucked firmly into the crotch of his handler, who had to move her rifle to make room for him. (Once off the helicopter, though, he tracked his man beautifully, I’m told.) Most dogs stayed very close to their handlers, and hunkered down and strapped in, for the ride. One veteran dog took in the view from the helicopter’s edge while firmly grasped by his handler.
These dogs are trained to track human odor over distances. They pick up a scent where a handler “suggests” (near an IED, for instance, or the last place an insurgent stood) and follow it. Tracks can be miles long and hours—or even days—old. Recall James Earl Ray, who broke out of a Tennessee prison in 1977 and was pursued by sister bloodhounds named Sandy and Little Red. They started their manhunt days after his escape, but within a few hours they had found him a mere three miles away. (Now
Tracking dogs keep their heads down and follow the scent on the ground. The track is a combination of human scent and crushed vegetation or stirred-up dirt or sand. Disturbed environments, like crushed grass (the grass “bleeds,” in a sense), give off unique smells. But nothing as unique as the smell of the person being pursued.
You may think that if you shower well and wear deodorant, you smell like just about everyone else out there. But our individual scent fingerprints are unique. As Horowitz says, “To our dogs, we
Anyone who is thinking of outwitting a tracking dog one day should read what she has to say about our scents:
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.
