Sunday, November 29, 2009

Attraction & Olfaction

The fact human females would want sex at all outside of their monthly 6-day window of receptivity is an evolutionary problem. What practical purpose does it serve? Don't females have better things to do?

Well, of course we do - but even so, those females who prescribe to a "dual sexuality" (conceptive and not conceptive) mating strategy are actually more likely to reap both the genetic and material benefits in their mate(s). There have been studies that suggest when a female is not receptive she prefers males who exhibit strong paternal and supportive attributes. These are the "nice" guys who are often pitied because they don't "get the girl." This isn't entirely true - except perhaps from a male's perspective. It really depends upon how "getting the girl" is interpreted. All but six days of the month studies have shown females to show preference towards the devout paternal male, not the raucous bad boy who is well endowed (genetically).

I suppose I should pause here to explain an important concept in evolutionary biology that's relevant to this entry. I'm inferring that he who is considered of high genetic quality is the aggressive, eyebrow-pierced, pink polo-adorned male who is incapable of articulating anything that has two or more syllables. This is true to at least one extent explained by the sexy son hypothesis. The idea is that if a female mates with a male who is considered "attractive," she will have attractive male offspring who will be perceived as having the same appeal to the next generation of women. A vibrant example of this is the male peafowl (below).

image source

One can easily see why the dual sexuality strategy would be adaptive to our female ancestors. A female prefers the male with whom she can share parental duties and, when she's ovulating, the male who can maximize her reproductive success in the long run. In human and nonhuman monogamous animals, females engage in extra-pair copulations during ovulation if the benefits outweigh the costs (ie. if the extra-pair male is much more genetically compatible and of course, if they don't get caught).

Follow your nose...
There is mounting evidence that suggests female mate choices are made through cues that reflect genetic quality ("sexy son" indicators) and material benefits ("good dad" indicators). Sexy son indicators include vocal and facial masculinity, muscles, symmetry, and body scent intensity. Good dad indicators are passiveness and feminine facial features (eg. round face and eyes).

So... what does the pill have to do with how sexy a woman perceives a man's stench?

The reason the pill has such a strong effect on female mate preference is because it keeps a high level of progestin (a substitute for progesterone) and estrogen in the female's system. Progesterone is a mellowing hormone and the sister to estrogen, the controlling, all-powerful hormone that is responsible for feelings of aggression and seduction. The pill tricks the body into thinking it is in a pregnant state, during which estrogen and progesterone are peaking. When a female is pregnant she does not continue the uterine cycle because a fertile egg is already gestating.

So what do the two key components to hormonally-based birth control pills actually do?Progestin and estrogen prevent gonatropin-releasing hormone (GnRH) from being secreted by the hypothalamus. GnRH is the signal to the pituitary gland to produce follicle-stimulating hormone (FSH) and leutinizing hormone (LH). Once GnRH is blocked, the body does not secrete FSH, which prompts the body to grow a follicle, and LH, which triggers the release of the egg. The pill essentially freezes the natural cycle so it remains at a single stage for the entire month.

The data suggests that females who are on the pill (or pregnant) do not have the same sexual palette as the non-pill user. In a study that manipulated the facial features in 20 pictures of male faces to masculinize or feminize their attributes found normally cycling women prefer less masculine faces in a long-term context compared to more masculine faces in a short-term context. Pill-users had an inverse preference and a weaker preference.

Love at first smell and contraceptives

The Major Histocompatibility Complex (MHC) is a gene family that serves as an important component to a person's immune system. It creates molecules capable of identifying pathogens, or foreign invaders, so that the soldiers of the immune system can be called upon for combat. The MHC is so essential that the olfactory cues reflecting the genetic compatibility among two individuals are thought to be based off of it alone.

Mate choice is influenced by MHC diversity through olfactory cues. MHC–peptide complexes present at the cell surface carry information about our cellular genetic make-up. When these complexes are shed from the cell surface they dissolve in our bodily fluids. This releases the peptide ligand component of the complex, which is then free to interact with other receptors (such as olfactory sensory neurons). The relevance of this has to do with the patterns of sensory neuron activation, which are unique to the structure of distinct peptide ligands. A person's distinct body odor reflects their MHC genotype, (drum roll) thus allowing us to assess the genetic compatibility of prospective mates!

This is where contraceptive pills come into play: natural selection has played a large role in making sure individuals with similar genetic material - siblings, for instance - do not breed because the resulting offspring will have weaker immune systems and be more likely subject to a whole suite of genetic faults. The cost of mating with someone who has a similar set of MHC alleles is large for the same reason. The idea is that with lots of diversity in our MHC genes, the body is ready to take on a lot of different invaders. This diversity can only be maintained by sexual outbreeding which leads to different combinations of MHC alleles representing a richer defense strategy.

Then the brilliant stinky t-shirt experiment was conceived:

In summary, (naturally cycling) women found the odors of MHC-dissimilar males more pleasant. These results yielded a negative correlation between number of shared alleles and pleasantness ratings.

Note, however, this test examined the preference of exclusively normally cycling women. When the same sweaty t-shirt test was performed on women who are taking contraceptives they found surprising results. These women didn't show a preference for the scent of males with dissimilar MHC genes. In fact, these women showed a higher preference for males with MHC genes similar to their own. If your body believes you to be in a pregnant state, this odor preference could be advantageous. Our primitive brains yearn to be surrounded by genetic similarity (ie. relatives) during this stressful and dangerous part of our life cycle.

Past studies suggest there are consequences to females preferring genetically similar males as long-term partners. Couples with dissimilar MHC genes are more satisfied and more likely to be faithful to their mate. Those who have similar MHC genes report less satisfaction and more wandering eyes. There are also fertility problems encountered by MHC-similar couples, who have a much higher probability of recurrent spontaneous abortions (RSA). There have also been studies that highlight the importance of odor perception as it plays a significant role in maintaining attraction within relationships.

So if a female acquires her male counterpart after beginning the pill, what happens when the woman goes off of it? Something I personally will never know. I'll take my sweaty man stench without the extra hormones, please.

Thornhill, R., Gangestad, S. W., Miller, R., Scheyd, G., McCollough, J. K., and M. Franklin. Compatibility complex genes, symmetry and body scent attractiveness in men and women. The Research & Education Division of The Fragrance Foundation.

Havlicek, J. and S. Roberts. 2009. MHC-correlated mate choice in humans: A review. Psychoneuroendocrinology 34: 497 - 512.