Alpha Dominance

Pheromones: we’ve long known their power in the animal world but much less was known about how they influenced us. Since we had difficulty in identifying a mechanism for them in humans, some believed they were vestigial and that they no longer had an impact in humans. A body of research is growing however that documents a variety of pheromonal effects in humans. Nonetheless, our knowledge of this field is in its infancy and much remains to be learned, making this an exciting field to monitor for future developments.

For some time, the debate has centered around the lack of a functional VNO (vomeronasal organ) in humans since it is now vestigial, however research has found that pheromonal detection occurs via the conventional olfactory system in humans.  A brief history of the debate is as follows:

In the following century, a rich array of animal pheromones were documented for seals, boars, rodents, and all manner of other critters. But not for human beings.

Some of Darwin’s contemporaries embraced human uniqueness in this regard as evidence of our inevitable ascendance, as if Nature’s Plan somehow called for the evolution of a nearly naked two-legged primate with a poor sense of smell to conquer the Earth. The French physician Paul Broca—noting that primates’ social olfactory abilities are diminished compared to those of other mammals—asserted that monkeys, apes, and humans represent ascending steps from four-legged sniffing beasts to sight-oriented bipeds.

Monkeys, he argued, have smaller “smell brains” than other mammals, and apes’ brains are even smaller than that. Among humans, only the tribal “primitives,” Broca wrote, could still attach erotic import to the bodily smells of man.

More enlightened researchers dismissed such views as racist tripe. But they still noted that humans engage in very little scent-driven socializing—compared to, say, the urine-washing displays of monkeys (during which urine is rubbed on the feet to attract mates).

To make matters worse, humans seemed to lack the hardware for communicating by scent. Pheromone reception in other species is the business of two little pits (one in each nostril) known collectively as the vomeronasal organ (VNO). Few scientists of the time claimed to have been able to locate a human VNO. Those who did complained that the VNO is so small that they could detect it only rarely.

But most scientists, without bothering to look, simply dismissed the idea of a VNO in humans. It’s been scientific dogma for most of this century that humans do not rely on scent to any appreciable degree, and that any VNOs found are vestigial throwbacks. Then, in the 1930s, physiologists declared that humans lack the brain part to process VNO signals, firmly closing the book on any role for body odor in human sexual attraction. Even if we had a VNO, the thinking was, our brains wouldn’t be able to interpret its signals.

Recent discoveries suggest, however, that the reports of our olfactory devolution have been greatly exaggerated.

Further research has identified a new class of chemoreceptors present in the mammalian olfactory system and the genes TAARS which encode them.  It has been speculated that these may encode pheromone receptors.

The mammalian olfactory system detects chemicals sensed as odours as well as social cues that stimulate innate responses. Odorants are detected in the nasal olfactory epithelium by the odorant receptor family, whose 1,000 members allow the discrimination of a myriad of odorants. Here we report the discovery of a second family of receptors in the mouse olfactory epithelium. Genes encoding these receptors, called ‘trace amine-associated receptors’ (TAARs), are present in human, mouse and fish. Like odorant receptors, individual mouse TAARs are expressed in unique subsets of neurons dispersed in the epithelium. Notably, at least three mouse TAARs recognize volatile amines found in urine: one detects a compound linked to stress, whereas the other two detect compounds enriched in male versus female urine—one of which is reportedly a pheromone. The evolutionary conservation of the TAAR family suggests a chemosensory function distinct from odorant receptors. Ligands identified for TAARs thus far suggest a function associated with the detection of social cues.

So it seems the detection apparatus is in place, but that the VNO is non-functional in humans.  Detection occurs via the normal olfactory system.

Rodents and other mammals secrete chemical signals called pheromones that carry information about their gender or reproductive state, and influence the behaviour of others. Pheromones are detected by a specialised sensory system, the vomeronasal organ (VNO), which consists of a pair of structures that nestle in the nasal lining or the roof of the mouth. Although most adult humans have something resembling a VNO in their nose, neuroscientist Michael Meredith of Florida State University in Tallahassee has no hesitation in dismissing it as a remnant.

“If you look at the anatomy of the structure, you don’t see any cells that look like the sensory cells in other mammalian VNOs,” he says. “You don’t see any nerve fibres connecting the organ to the brain.” He also points to genetic evidence that the human VNO is non-functional. Virtually all the genes that encode its cell-surface receptors – the molecules that bind incoming chemical signals, triggering an electrical response in the cell – are pseudogenes, and inactive.

So what about the puzzling evidence that humans respond to some pheromones? Larry Katz and a team at Duke University, North Carolina, have found that as well as the VNO, the main olfactory system in mice also responds to pheromones. If that is the case in humans too then it is possible that we may still secrete pheromones to influence the behaviour of others without using a VNO to detect them.

So what signals are we transmitting to each other under the conscious radar?  Do we really produce that many pheromones?  Why are they so elusive?  We certainly have the hardware to produce them in quantity:

For an animal whose nose supposedly plays no role in sexual attraction or social life, human emotions are strongly moved by smells. And we appear to be profoundly overequipped with smell-producing hardware for what little sniffing we have been thought to be up to. Human sweat, urine, breath, saliva, breast milk, skin oils, and sexual secretions all contain scent-communicating chemical compounds. Zoologist Michael Stoddart, author of The Scented Ape (Cambridge University Press, 1991), points out that humans possess denser skin concentrations of scent glands than almost any other mammal. This makes little sense until one abandons the myth that humans pay little attention to the fragrant or the rancid in their day-to-day lives.

Part of the confusion may be due to the fact that not all smells register in our conscious minds. When those telltale scents were introduced to the VNO of human subjects, they didn’t report smelling anything—but nevertheless demonstrated subtle changes in mood.

What could be a source of what might be our very own pheromone?

Humans possess three major types of skin glands—sebaceous glands, eccrine (or sweat) glands, and apocrine glands. Sebaceous glands are most common on the face and forehead but occur around all of the body’s openings, including eyelids, ears, nostrils, lips, and nipples. This placement is particularly handy, as the secretions of these glands kill potentially dangerous microorganisms. They also contain fats that keep skin supple and waterproof and, on the downside, cause acne. Little is known, however, about how sebaceous glands contribute to human body odor.

The sweat glands exude water and salt and are non-odorous in healthy people. That leaves the third potential source of a human pheromone—the apocrine gland. Apocrine glands hold special promise as the source of smells that might affect interpersonal interactions. They do not serve any temperature-managing functions in people, as they do in other animals. They occur in dense concentrations on hands, cheeks, scalp, breast areolas, and wherever we possess body hair—and are only functional after puberty, when we begin searching for mates.

Men’s apocrine glands are larger than women’s, and they secrete most actively during times of nervousness or excitement. Waiting colonies of bacteria turn apocrine secretions into the noxious fumes that keep deodorant makers in business. Hair provides surface area from which apocrine smells can diffuse—part of the reason why hairier men smell particularly pungent. (Is it any coincidence that hair at the arm pit and the genitals sprouts at puberty, when apocrine glands start producing food for our skin bacteria?)

Most promising of all, apocrine glands exude odorous steroids known to illicit sexual behavior in other mammals. Androsterone—a steroid related to the one that nearly doomed the hapless musk deer—is one such substance. Men secrete more androsterone than women do, and most men become unable to detect the stuff right around the time they start producing it themselves—at puberty.

We have seen that pheromones play a role in sexual attraction for humans, but how does it work?  How do we judge other’s desirability using these scents?  It turns out that much of the information being communicated has to do with variation and robustness of our immune systems.  An important component in the immune system has to do with MHCs (major histocompatibility complexes).  We have a biological imperative to reproduce with individuals whose MHCs are most different from our own as the resulting offspring will have immune systems prepared to deal with a wider variety of microorganisms and parasites.  This process is discussed here:

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.

Continued from the above article, research on human subjects found the same results:

It was found, by Wedekind and his team, that how women rate a man’s body odor pleasantness and sexiness depends upon how much of their MHC profile is shared. Overall, women prefer those scents exuded by men whose MHC profiles varied the most from their own. Hence, any given man’s odor could be pleasingly alluring to one woman, yet an offensive turnoff to another.

Raters said that the smells they preferred reminded them of current or ex-lovers about twice as often as did the smells of men who have MHC profiles similar to their own, suggesting that smell had played a role in past decisions about who to date. MHC-similar men’s smells were more often described as being like a brother’s or father’s body odor… as would be expected if the components of smell being rated are MHC determined.

Somewhat more surprising is that women’s evaluations of body odor intensities did not differ between MHC-similar and MHC-dissimilar men. Body scent for MHC-dissimilar men was rated as less sexy and less pleasant the stronger it was, but intensity did not affect the women’s already low ratings for MHC-similar men’s smells.

That strong odor turned raters off even with MHC-dissimilar men may be due to the fact odor is a useful indicator of disease. From diabetes to viral infection to schizophrenia, unusually sweet or strong body odors are a warning cue that ancestral females in search of good genes for their offspring may have been designed to heed. (In the case of schizophrenia, the issue is confounded—while some schizophrenics do actually have an unusually sweet smell, many suffer from delusions of foul smells emanating from their bodies.)

Nobody yet knows what roles MHC may play in male evaluations of female attractiveness. Females’ superior sense of smell, however, may well be due to their need to more carefully evaluate a potential mates merits—a poor mate choice for male ancestors may have meant as little as a few minutes wasted, whereas a human female’s mistake could result in a nine-month-long “morning after” and a child unlikely to survive.

Interestingly, this process becomes inverted in women who take the pill.  This is postulated to be in part responsible for the rise in infertility witnessed in western society in recent years.

The Swiss researchers found that women taking oral contraceptives (which block conception by tricking the body into thinking it’s pregnant) reported reversed preferences, liking more the smells that reminded them of home and kin. Since the Pill reverses natural preferences, a woman may feel attracted to men she wouldn’t normally notice if she were not on birth control—men who have similar MHC profiles.

The effects of such evolutionary novel mate choices can go well beyond the bewilderment of a wife who stops taking her contraceptive pills and notices her husband’s “newly” foul body odor. Couples experiencing difficulty conceiving a child—even after several attempts at tubal embryo transfer—share significantly more of their MHC than do couples who conceive more easily. These couples’ grief is not caused by either partner’s infertility, but to an unfortunate combination of otherwise viable genes.

Doctors have known since the mid-1980s that couples suffering repeated spontaneous abortions tend to share more of their MHC than couples for whom pregnancies are carried to term. And even when MHC-similar couples do successfully bring a pregnancy to term, their babies are often underweight.

The Swiss team believes that MHC-related pregnancy problems in humans are too widespread to be due to inbreeding alone. They argue that in-couple infertility problems are due to strategic, unconscious “decisions” made by women bodies to curtail investment in offspring with inferior immune systems—offspring unlikely to have survived to adulthood in the environments of our evolutionary past.

When Broca and other social Darwinists pointed out that “uncivilized races” were more sensitive to body odor, they may have been correct—insofar as Europeans tend to go to greater lengths to perfume and wash away their natural scents. But this is hardly evidence of European superiority over “less evolved” peoples, as Broca insisted. Paying careful attention to the health of others and their suitability as sires to one’s offspring in the disease-rich tropics, whose cultures Broca derided, actually makes exceedingly good sense.

Perfume; daily, soapy showers; convenient contraceptive pills—all have their charms. But they also may be short-circuiting our own built-in means of mate choice, adaptations shaped to our unique needs by millions of years of ancestral adversities. The existence of couples who long for children they cannot have indicates that the Western dismissal of body scent is scarcely benign.

Those who find offensive the notion that animal senses play a role in their attraction to a partner need not worry. As the role of smell in human affairs yields to understanding, we see not that we are less human but that our tastes and emotions are far more complex and sophisticated than anyone ever imagined.

This too could explain part of the increase in dissolution of relationships in the years since the pill has come into wide usage.  Perhaps we have partially derailed our natural chemical sex appeal systems with excessive use of artificial scents and pharmaceutical contraception.  We may be creating unintended consequences in our attempt to eliminate all natural odors from our lives, a generation of offspring with less robust immune systems and truncated marriages.  Depending on the degree of impact that this has on the health of our youth, this may turn out to parallel the rise of superbugs resulting from overuse of antibiotics.

Personally I dislike strong chemical laden shampoos, perfumes and other body products.  Often their scent is more offensive than that of the natural healthy and hygienic human body.  For those desiring a little something extra, essential oils and flower waters accomplish this naturally and less acridly.  The chemical scents that are added to everything from body products to laundry detergent to candles and cleansers are more noticeable when you eliminate them from your daily life.  Once having done so, you may find yourself straining to hold your breath while on the elevator with someone doused in perfume.

Together these findings present a cogent picture of the importance of pheromones to our sex appeal and mate selecti0n.  We know that they are at work and the mechanism of their action.  In the next features we will explore sex-specific pheromones and the effects they have on the opposite sex.  Stay tuned for the next installment: male sex pheromones.