Sex changes things. The act of intercourse between two people inevitably changes some aspect of a pair’s interaction. This may be a dual effect, or it may simply manifest from modifications of the behavior of the individuals.
As human beings, we tend to focus more on the psychological or sociological implications of coupling, but there are also biological ramifications that can be independent of or intertwined with cognitive facets of sexual congress. It is thought that many of the behavioral effects have evolved to increase the likelihood that a union yields offspring, or simply to increase the reproductive success of one of the putative parents.
A recent article appearing in Nature highlights one striking example of such a convolved effect, the so-called sex peptide and its cognate receptor in the fruit fly Drisophila Melanogaster. The work, from the lab of Barry J. Dickson at the Research Institute of Molecular Pathology in Vienna, both identifies the receptor and the sites at which it can be found in the fly’s body1.
It had been known for some time that sex peptide (found in the male’s seminal fluid), acts on the female in such a way that she is more likely to lay eggs, and less likely to copulate again. This has the effect of increasing the male’s chance of producing heirs; keeping his sperm from being diluted by competing sires, and increases the number of potential progeny. Of course, this has a cost for the female, as she is now less likely to find a more-fit partner.
There are also many other examples worth mentioning (these were brought to my attention by an excellent comment on the paper, also in Nature2):
In mammals, intercourse alters the environment of the reproductive tract at an immunological level, increasing the probability of fertilization and implantation, perhaps explaining the increased likelihood that a female can contract a urinary tract infection.
The males of many species (avian, reptile, rodent) will simply guard a female post-coitally to prevent her from finding a (potentially) more desirable mate.
In snakes and some insects, sex can actually lead to changes in the pheromones that a female produces, rendering her less attractive to other possible suitors.
If a female rodent is exposed to the pheromones of a dominant male, she develops a sort of taste for power, making her more likely to reject a subordinate gentleman caller.
Female jewel wasps have an even simpler version: their response to male sex-pheromones changes from attraction to aversion after copulation. Similarly, and again in the fruit fly, females will actively turn-down new beaus, running away from newly encountered males and even kicking them.
While there is no known human homologue of the genes identified by the study I mentioned above, all of these examples beg the question of what forms of this sex-mind-control might be at work in our species. It’s also interesting to consider that all the thus-far identified effects are of males on females, could there also be some method that women have of controlling the men as well? In any case, knowing that our very biology might change as the result of certainly make me wonder what might at work in my brain & body after a roll in the hay.
1. Yapici, N. et al (2008) A receptor that mediates the post-mating switch in Drosophila reproductive behaviour. Nature 451, 33–37
2. Griffith L.C. (2008) Neuroscience: Love hangover. Nature 451, 24-25