Love might be in the air on Valentine's Day, metaphorically speaking. But scientists have long debated whether love—or, at least, sexual attraction—is literally in the air, in the form of chemicals called pheromones.
Creatures from mice to moths send out these chemical signals to entice mates. And if advertisements about pheromone-laden fragrances are to be believed, one might conclude that humans also exchange molecular come-hithers.
Still, after decades of research, the story in humans is not quite so clear. Rather than positing that single, pheromone-esque compounds strike us like Cupid's arrow, investigators now suggest that a suite of chemicals emitted from our bodies subliminally sways potential partnerings. Smell, it seems, plays an underappreciated role in romance and other human affairs.
"We've just started to understand that there is communication below the level of consciousness," says Bettina Pause, a psychologist at Heinrich Heine University of Düsseldorf (H.H.U.), who has been studying pheromones and human social olfaction for 15 years. "My guess is that a lot of our communication is influenced by chemosignals."
Animals, plants and even bacteria produce pheromones. These precise cocktails of compounds trigger various reactions in fellow members of a species—not all of which are sexual. Pheromonal messages can range from the competitive, such as the "stink fights" of male lemurs, to the collaborative, such as ants laying down chemical trails to food sources.
The term "pheromone" itself came about in 1959 with the identification of bombykol, a powerful aphrodisiac secreted by female silk moths that can work over kilometers of distance. "The males are enormously sensitive to it," says Tristram Wyatt, a zoologist at the University of Oxford. "Just a very few molecules are enough to get the male to fly to the female."
Nothing quite so obvious is happening with people. But the scientific search for human pheromones is still in the early stages. The first steps have focused on areas of the body that already omit noticeable odors—in particular our gland-filled armpits. "Early on it was discovered that there's some action there," says Charles Wysocki, an olfactory neuroscientist at the Monell Chemical Senses Center.
Some of the first evidence for subtle smell cueing came from reports that women who lived in close quarters, such as those in college dorms, ended up with synchronized menstrual cycles. Subsequent research has shown that armpit sweat—from females or males—when placed on women's upper lip, can modify cycle time. But a putative pheromone behind this time-of-the-month alignment has not been isolated, Wysocki says, and subsequent work has poked statistical holes in the initial findings.
In nature, pheromones that induce gradual physiological changes of this kind are dubbed "primers." Those that cause a behavioral response—such as with the smitten male silk moths—are called "releasers." In humans, the most salient example for a releaser pheromone does not involve sex but rather its product: newborn babies, who seem to be guided to a mother's breast by scent. "Newborns will move in the direction of the odor source," Wysocki says. Research published last year pointed to secretions from the areolar gland "bumps" on mother's nipples as the source of the behavior-modifying, odorous molecules that cue a baby to find its food source.
Other results over the years have hinted at pheromones altering adults' moods. Odors given off by the breasts of breast-feeding women, for example, can render childless females downright randy—although a particular chemical messenger remains unidentified. H.H.U.'s Pause, meanwhile, has demonstrated that humans can sense alarm scents in anxious or fearful people's perspiration. Yet more studies with sweat have explored the strongest isolated candidate so far for a human pheromone, known as androstadienone, which derives from the male hormone testosterone. The presence of this compound has been reported to make women feel more relaxed. Wysocki and his colleagues are currently seeking National Institutes of Health grants to find out just what the "magic bullet—or bullets—are in male body odor" that elicit female responses, he says. They also hope to study whether female odors can similarly influence male mood and hormonal activity.
The nose knows
Although the nitty-gritty of their dispersal remains obscure, pheromonal detection mechanisms are becoming clearer. Scientists have long thought that a specialized structure in animals' noses, called a vomeronasal organ (VNO), detects pheromones. The problem with that theory when applied to humans, however, is that the tiny VNO duct behind each of our nostrils is not always present, plus the genes for its receptors seem to be inoperative. But as it turns out, regular mammalian nasal tissue seems to be able to pick up pheromones just fine—at least in some animals. For example, sows, upon smelling a pheromone in boars' saliva, assume a mating stance, even if researchers plug the pigs' VNOs. In humans, a 2011 study showed that when volunteers were exposed to androstadienone, all their brains showed a reaction, even if they lacked VNOs or had their VNOs blocked. "The VNO need not be the pheromone-sensing organ," Wysocki says. "The olfactory system can be the input."
Other work suggests that less familiar inputs might exist for a human pheromonal network. Investigations continue into a possible pheromone nerve, known as cranial nerve 0, or the terminal nerve. [For more about the terminal nerve, read "Sex and the Secret Nerve," by Douglas Fields, in Scientific American MIND, February/March 2007.] Initially discovered in sharks in 1878 and humans in 1913, this pair of nerves runs from the nose directly into the brain in front of cranial nerve 1, the olfactory nerve (the traditional first of a dozen recognized cranial nerves). Animal research points to important sexual, pheromonal roles for the terminal nerve. Hamsters with severed terminal nerves fail to mate, and when male zebra fish get an electrical zap to theirs, the fish ejaculate. In humans, just what part the terminal nerve might have for adults remains sketchy, Wysocki says. It does have one clear purpose, however: During fetal development, the terminal nerve works as a pathway for certain sex hormones to migrate into the brain crucial for later development during puberty.
Whether or not pheromones initially affect sexual attraction, other research has indicated that humans might be using a different set of subtle smell cues to help select our mates. Variation in the major histocompatibility complex (MHC), an important set of immune system genes, imbues each of us with a unique "odorprint," like a fingerprint. "With the exception of identical twins, no two individuals are likely to have the same odorprint," Wysocki says. In nature, the sexual union of unlike MHCs yields offspring with more diverse and thus more robust immune systems. Instinct may also guide us in this manner: Previous research has revealed that human females preferred the musk of sweaty T-shirts worn by men with suitably different MHC genes.
Because scores, if not hundreds, of unidentified odorants comprise an odorprint, Oxford's Wyatt has argued that it cannot be considered a pheromone in the classic sense. Evidently, the complex cloud of aromas we emit needs a lot more parsing before science closes the book on pheromones. The olfactory cues of many insects remain better understood than our possible covert realm of social and sexual chemistry. "The real problem," Wyatt says, "is simply a lack of knowledge so far as humans are concerned." Wysocki agrees: "There's no good literature in the biomedical field to support that sexual-attractant pheromones exist," he says. "But that is not to say they aren't out there. I think we have to go in with an open mind.
Until more is known, then, those on Valentine's Day dates should probably just follow their noses.
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