Inside of a Dog_ What Dogs See, Smell, and Know - Alexandra Horowitz [33]
Generations of schoolchildren have been admonished to "never show fear" to a strange dog.* It is likely that dogs do smell fear, as well as anxiety and sadness. Mystical abilities need not be invoked to account for this: fear smells. Researchers have identified many social animals, from bees to deer, who can detect pheromones emitted when one animal is alarmed, and who react by taking action to get to safety. Pheromones are produced involuntarily and unconsciously, and through different means: damaged skin may provoke release of them, and there are specialized glands that release chemicals of alarm. In addition, the very feeling of alarm, fear, and every other emotion correlates with physiological changes, from changes in heart rate and breathing rate, to sweating and metabolic changes. Polygraph machines work (to the extent that they work at all) by measuring changes in these autonomic bodily responses; one might say that animals' noses "work" by being sensitive to them as well. Laboratory experiments using rats confirm this: when one rat is given a shock in a cage, and learns to be fearful of the cage, other rats nearby pick up on the shocked rat's fear—even without seeing the rat being shocked—and themselves avoid the cage, which was otherwise not distinguishable from nearby cages.
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.
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