What the Nose Knows: The Science of Scent in Everyday Life (9 page)

The physicist Richard Feynman had a great party trick in which he would identify by smell objects briefly handled by other guests when he wasn’t looking. He said it was easy to do because peoples’ hands have surprisingly different scents. (A 1977 study confirmed that hand odor is individually distinctive and discriminable.) There are other stupid human tricks besides Feynman’s. For example, in a lineup of dirty laundry we can pick out the T-shirt worn by our spouse or partner. A mother can identify the smell of her own baby, and a baby can pick out the scent of its mother’s breast.

How do humans measure up at the quintessential doggy task—scent tracking? Researchers at UC Berkeley had people get on their hands and knees and follow a 10-meter-long chocolate-scented trail using only their noses. The test subjects wore goggles, gloves, and kneepads, which restricted nonolfactory input. Two-thirds of the people tested successfully followed the trail under these conditions. (When subjects wore nose plugs, none of them could follow the trail.) With a few days of training, tracking speed was doubled and people strayed less from the scent trail. Dog lovers (of which I am one) may also be surprised to learn that drug dogs and humans have almost identical sensitivity to methyl benzoate, the smell used to track cocaine. Dogs have great noses, but it’s time to stop the trash talk and give ourselves more credit.

Many people take it for granted that the human nose is inferior, and scientists often make the same assumption. Charles Darwin thought our evolutionary ancestors made good use of smell, but felt it was “of extremely slight service, if any” to modern man. The sex psychologist Havelock Ellis agreed: “Among the apes it has greatly lost importance and in man it has become almost rudimentary, giving place to the supremacy of vision.” The attitude persists. As recently as 2000, some French researchers asserted “The sense of smell in primates is greatly reduced (microsmatic) with respect to other mammals such as dogs or rodents.”

Scientists are taking a fresh look at the conventional wisdom regarding the sense of smell in animals. The anatomists Timothy Smith and Kunwar Bhatnagar, for example, are questioning the textbook distinction between macrosmatic and microsmatic animals, i.e., those with good and poor olfactory abilities. The long-standing assumption is that what separates macrosmatic and microsmatic species is the amount of surface area inside the nose. This turns out to be a bad assumption; internal surface area is more about air conditioning—warming and filtering incoming air—than about odor perception. Of more relevance is the amount of sensory tissue in the nose. But Smith and Bhatnagar find that the amount of sensory tissue varies from species to species independently of total surface area. Further muddying the waters, the number of olfactory nerve cells per square inch varies among species. All in all, surface area is a dubious proxy for smell ability. Smith and Bhatnagar suggest that the traditional macrosmatic/microsmatic distinction has outlived its usefulness. Size isn’t everything.

The Yale University neurobiologist Gordon Shepherd agrees that counting nerve cells is a poor way to estimate sensory talent. In his view, the number of cells available for odor detection is less important that what the brain does with the information those cells provide. He makes the analogy to hearing: humans have about the same number of auditory nerve fibers as cats and rats, yet we have far superior speech abilities. It’s the brain areas that analyze and interpret speech sounds that provide the advantage, not the number of cells in the ear.

The German sensory physiologist Mathias Laska cuts right to the chase by measuring odor perception in different animal species. He has used reward-based conditioning techniques to find odor detection thresholds in spider monkeys, squirrel monkeys, and pigtail macaques. According to conventional wisdom, these primates are less sensitive than dogs and rabbits, but Laska finds they perform quite well—monkey thresholds are comparable to those of dogs and rabbits across a variety of odors. And contrary to Darwin’s gloomy belief, Laska finds that humans have odor sensitivity similar to that of apes and monkeys.

New evidence suggests that humans and animals may be more similar in odor perception than we thought. In 1991, Linda Buck and Richard Axel discovered a large family of mammalian olfactory receptor genes, work for which they eventually received the Nobel Prize. Each gene produces a different receptor. In general, more receptors means more detectable odors, and therefore greater smell ability. Rats have about 1,500 functional receptors, followed by dogs with about 1,000, mice with about 900, and chimpanzees with about 350. Humans have somewhere between 340 and 380. Dolphins have zero.

Does this mean rats are five times better smellers than we are? Not really. We can use DNA sequence similarity to arrange odor receptors into families and subfamilies. In theory, similar receptors detect similar odor molecules, so a receptor subfamily detects a class of related odors. When we compare odor receptor subfamilies, the human-animal gap doesn’t look too large. Humans and dogs have about 300 subfamilies, rats have 282, and the mouse 241. The overlap between species is substantial. About 87 percent of human receptor subfamilies have counterparts in the mouse genome, while 65 percent of mouse subfamilies are shared by humans. This suggests to Linda Buck and her colleagues that “the majority of odorant features [i.e., smells] detectable by one species may also be recognized by the other.” Perhaps a mouse can smell more of our world than we can smell of his. (Unlike us, he may have a whole subfamily of receptors devoted to cat urine.) For man and mouse the differences are not as big as the similarities. For man and chimp this is even more the case—there is a corresponding human gene for 85 percent of chimp odor receptor genes. Chimp, dog, man, or mouse, we perceive the general features of the smellscape in much the same way.

Physical equipment—size of brain areas, number of nerve cells or receptor types—may be less important than what the brain does with the information once it arrives. For many animals, a smell is a call to action, a trigger for a biologically hard-wired survival response: “scent of lion means flee.” In contrast, human cognitive abilities turn smells into symbols and let us make flexible use of their signal value. When it comes to comparative smell ability, it’s the brain, stupid.

Better Than the Rest

One morning when I walked on my monk’s alms-rounds to collect food, my nose became like that of the most sensitive dog. As I walked down the street of a small village, every two feet there was a different smell: something being washed, fertilizer in the garden, new paint on a building, the lighting of a charcoal fire in a Chinese store, the cooking in the next window. It was an extraordinary experience of moving through the world attuned to all the possibilities of smell.

—J
ACK
K
ORNFIELD,
A Path with Heart

My friend Larry Clark is an ornithologist. I have hiked trails with him as he identified bird after bird by song alone. His skill leaves me awestruck. It’s the same feeling I get when a perfumer talks about a fragrance—he seems to be smelling more than I do, finding notes that my blundering nose doesn’t detect until he points them out. How do olfactory experts accomplish these feats? Are their noses that much better than yours or mine? What does it take to become an expert smeller?

Pure nose-sensitivity is not the answer. The average person probably detects odors at about the same concentration as the professional wine taster. What the expert has are cognitive skills that make better use of the same sensory information. The practiced wine expert can name varietals and tell one vintage from another, just as the trained perfumer classifies a new cologne with ease and zeroes in on its unique notes. The expert’s advantage, in other words, is brain power rather than nose power, and in the regular exercise of these specialized mental skills. Wine experts, for example, routinely make notes as they taste. Experts outperform novices in matching their own descriptions to wines on subsequent tastings. Their mental discipline helps experts avoid a trap called the “verbal overshadowing effect” that can snare novices when the effort to generate a verbal label interferes with the perception of the aroma itself.

The perfumers Robert Calkin and Stephan Jellinek believe their job can be done with only an adequate nose. What makes for professional success is specific mental skills and thought processes. My own research confirms that fragrance professionals think differently. Perfumers, fragrance evaluators, chemists, and sales executives have better olfactory imagery ability than nonexperts from outside the industry. The ability to bring to mind the scent of specific perfumes, and to imagine how ingredients will smell when blended, is central to the job description.

Constant honing of perceptual skills may actually change how an expert’s brain responds to scent. The brain-wave patterns of professional perfume researchers have been compared to those of less specialized workers. When smelling an odor, the pros show distinctive frontal lobe activity in an area known as the orbitofrontal cortex—one that is involved in cognitive judgments. This pattern of brain response in the pros may reflect their more analytic way of perceiving odor. Another study examined brain activity in wine sommeliers and nonexperts, as each group sipped and savored wine samples. The sommeliers had activity in areas associated with cognitive processing (the orbitofrontal cortex again) and in an area where taste and smell information are integrated. In contrast, the nonexperts showed activity in the primary sensory areas and zones associated with emotional response. Practice in making deliberate judgments about what one smells leads to changes in brain function and makes a person into a better smeller.

Superpowers

Is there such a thing as an olfactory prodigy? What talents would a Mozart of the nose possess? He would ace tests of odor identification, notice smells at trace concentrations, and quickly zero in on the difference between highly similar scents. He would effortlessly arrange samples according to concentration, name odors without hesitation, and pick the individual components out of a complex mixture. He would have an enormous store of remembered odors, and the ability to memorize new ones in a single sniff. He couldn’t be fooled into false recognition with distracters and decoys. Finally, he would have a profound ability to imagine odors and to anticipate how they would smell when mixed together.

If such a person exists, science hasn’t found him. This doesn’t stop novelists from imagining characters endowed with superhuman ability. Take Grenouille, the hero of Patrick Süskind’s novel
Perfume: The Story of a Murderer.
Many people have recommended this book to me, thinking I would enjoy the depiction of Grenouille’s incredible olfactory powers, but I am not impressed. Where the Laing Limit keeps normal people from smelling more than four odors in a complex mixture, Grenouille is born with the ability to recognize dozens of them. Even if we buy this fantasy, how does it instantly make Grenouille the best perfumer in Paris? Analyzing a perfume isn’t the same thing as creating one. (I can hear every note in a Mozart symphony, but that doesn’t make me a composer.) We know that perfumers work from the top down; they first recognize a perfume’s type, and then the nuances that make it unique. Grenouille starts by cracking a perfume into its raw materials, the very opposite of how real perfumers work. As a fan of slasher films, I don’t mind that Grenouille is a repellent freak with no body odor of his own who murders female virgins to extract their body scent. Neither do fans of
Perfume
—they are so enthralled by the romance of essential oils and blending that they ignore Grenouille’s nasal necrophilia and the novel’s soul-deadening grimness.
Perfume
is about perfume-making the way
The Texas Chainsaw Massacre
is about sausage-making.

The novelist Salman Rushdie created a hero with supernormal olfactory power named Saleem Sinai, who is born with an enormous nose. The smelly passages in
Midnight’s Children
are fun to read even as they verge on the phantasmagorical. Here Rushdie conjures the smellscapes of Karachi, Pakistan:

…the fragrances poured into me: the mournful decaying fumes of animal faeces in the gardens of the Frere Road museum, the pustular body odours of young men in loose pajamas holding hands in Sadar evenings, the knife-sharpness of expectorated betel and opium: “rocket paans” were sniffed out in the hawker-crowded alleys between Elphinstone Street and Victoria Road. Camel-smells, car-smells, the gnat-like irritation of motor-rickshaw fumes, the aroma of contraband cigarettes and “blackmoney,” the competitive effluvia of the city’s bus-drivers and the simple sweat of their sardine-crowded passengers.

Like Grenouille, Saleem Sinai comes from the land of make-believe. His olfactory ability goes far beyond normal experience: he uses it to detect emotions in other people, read their character, and peer directly into their souls. Similarly bizarre characters appear in Chitra Banerjee Divakaruni’s
The Mistress of Spices
and Tom Robbins’s goofy burlesque
Jitterbug Perfume.
Why are the authors of magic realist fiction so fond of supersmellers? Transforming a “primitive” animal sense into an all-knowing form of perception is apparently an irresistible literary conceit. Whatever their entertainment value, fictional supersmellers don’t shed much light on real people.

Busted

I suggest that if the police really wish to know where stills and “speakeasies” are located, they take me with them. It would not be a bad idea for the United States Government to establish a bureau of aromatic specialists.

—H
ELEN
K
ELLER

In April 2005, an Indiana man arrived at the Decatur County jail to bail out his brother-in-law. As he handed over $400 in cash, the dispatch clerk noticed that the bills reeked of marijuana. Police officers got the man’s permission to search his car. They found a pipe and some pot and charged him with possession. The episode has a certain Cheech-and-Chong quality to it, but the use of odor as evidence raises serious questions about the Fourth Amendment’s guarantee against unreasonable search and seizure. In February 1999, an Ohio State Highway Patrol officer stopped a motorist for running a red light. When the driver rolled down his window, the officer smelled marijuana smoke. A search turned up rolling papers and joints in the driver’s pocket, and a stubbed-out doobie in the ashtray. The driver was arrested. At trial, he succeeded in having the charges dismissed on the grounds that a search based only on odor—without other visible, tangible evidence—was improper. The case made its way to the Ohio Supreme Court, which ruled that the “plain smell” of burning pot was, by itself, sufficient probable cause for a warrantless search. The supreme courts of Michigan, Colorado, Wisconsin, Arkansas, and at least fifteen other states have reached similar conclusions.