The smell of love, part four

Why do some people smell better to you? A look at how human body odor influences sexual attraction. To read part one of this article click here.

Anti-pheromones?
Despite the poor showing of androsterone in smell ratings, Karl Grammer of Austria's Institute for Human Biology thought it might be the sought-after human pheromone and studied women's reactions to it. He expected to find that women have a strong, favorable reaction to the smell of androsterone around ovulation, when their sense of smell becomes more acute and when they are most likely to conceive. Changes in their bodies' estrogen levels around ovulation, Grammer suspected, may change how women react to androsterone's smell.

He found that women's reactions to androsterone indeed change around ovulation -- but not in the manner he expected. Instead of attraction, Grammer's ovulating volunteers shrugged their shoulders and reported Androsterone, it seems, offers little hope to men looking for a $19.95 solution to their dating slumps.

Of mice and men
The empirical proof of odor's effect on human sexual attraction came out of left field. Medical geneticists studying inheritance rules for the immune system, not smell physiologists, made a series of crucial discoveries that nobody believed were relevant to human mate preferences -- at first.

Research on tissue rejection in organ transplant surgery patients led to the discovery that the body recognizes an alien presence (whether a virus or a surgically implanted kidney) because the body's own cells are coated with proteins that our immune system recognizes as "self." But the immune system gets a lot more subtle about recognizing "nonself" intruders. It can recognize specific types of disease organisms, attach protein identifiers to them, and muster antibodies designed specifically for destroying that particular disease. And it can "remember" that particular invader years later, sending out specific antibodies to it.

A segment of our DNA called the major histocompatibility complex (MHC) codes for some of these disease-detecting structures, which function as the immune system's eyes. When a disease is recognized, the immune system's teeth -- the killer T cells -- are alerted, and they swarm the intruders, smothering them with destructive enzymes.

Unlike many genes, which have one or two alternative versions (like the genes that code for attached or unattached ear lobes), MHC genes have dozens of alternatives. And unlike earlobe genes, in which the version inherited from one parent dominates so that the version inherited from the other parent is not expressed, MHC genes are "co-dominant." This means that if a lab mouse inherits a version of an MHC gene for resistance to Disease A from its mother and a version lending resistance to Disease B from its father, that mouse will be able to resist both diseases.

When a female mouse is offered two suitors in mate choice trials, she inevitably chooses to mate with the one whose MHC genes least overlap with her own. It turns out that female mice evaluate males' MHC profile by sniffing their urine. The immune system creates scented proteins that are unique to every version of each MHC gene. These immune by-products are excreted from the body with other used-up chemicals, allowing a discerning female to sniff out exactly how closely related to her that other mouse is.

By choosing MHC-dissimilar mates, a female mouse makes sure that she doesn't inbreed. She also secures a survival advantage for her offspring by assuring that they will have a wider range of disease resistance than they would had she mated with her brother.

It's not that she seeks out diverse MHC genes for her young on purpose, of course. Ancestral females who preferred the smell of closely related males were simply outrun through evolutionary time by females who preferred the scent of unrelated sires.