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

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.
    In these ways, the fleeing criminal being tracked by dogs is doubly doomed. Dogs can be trained to track based not just on pursuit of a specific person's odor, but also based on a certain kind of odor: the most recent odor of a human in the vicinity (good for finding someone's hiding place), or a human in emotional distress—fearful (as one running from the cops might be), angry, even annoyed.
    The smell of disease

    If dogs can detect trace amounts of chemicals we leave behind on a doorknob, or in a footprint, might they be able to detect chemicals indicating disease? If you're lucky, when you come down with a disease difficult to diagnose, you'll have a doctor who recognizes, as some have, that a distinctive smell of freshly baked bread about you is due to typhoid fever, or that a stale, sour scent is due to tuberculosis being exhaled from your lungs. According to many doctors, they have come to notice a distinctive smell to various infections, or even to diabetes, cancer, or schizophrenia. These experts come unequipped with the dog's nose—but more equipped to identify disease. Still, a few small-scale experiments indicate that you might get an even more refined diagnosis if you make an appointment with a well-trained dog.
    Researchers have begun training dogs to recognize the chemical smells produced by cancerous, unhealthy tissues. The training is simple: the dogs were rewarded when they sat or lay down next to the smells; they weren't rewarded when they didn't. Then the scientists collected the smells of cancer patients and patients without cancer, in small urine samples or by having them breathe into tubes able to catch exhaled molecules. Although the numbers of trained dogs are small, the results were big: the dogs could detect which of the patients had cancer. In one study, they only missed on 14 out of 1,272 attempts. In another small study with two dogs, they sniffed out a melanoma nearly every time. The latest studies show trained dogs can detect cancers of the skin, breast, bladder, and lungs at high rates.
    Does this mean your dog will let you know when a small tumor develops in you? Probably not. What it indicates is that dogs are able to do so. You might smell different to them, but your changing smell might be gradual. Both you and your dog would need training: the dog to pay attention to the smell, you to pay attention to behaviors indicating your dog has found something.*

    The smell of a dog

    Since odor is so conspicuous to a dog, it gets great use socially. While we humans leave our scents behind inadvertently, dogs are not only advertent, they are profligate with their scents. It is as though dogs, realizing how well the odor of our bodies comes to stand for ourselves (even in our absence), determined to use this to their advantage. 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,