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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