Given all the energy required to get there, it’s surprising that the female reproductive tract is not a particularly welcoming place for sperm cells. Researchers Robin Baker and Mark Bellis found that approximately 35 percent of the sperm are ejected within half an hour of intercourse and those that remain are anything but home free. The female’s body perceives sperm as antigens (foreign bodies) that are promptly attacked by anti-sperm leucocytes, which outnumber sperm 100:1. Only one in 14 million ejaculated human sperm even reach the oviduct. In addition to the obstacles imposed by the female’s body, even those lucky few sperm are going to run into competition from other males (at least, if our model of prehistoric human sexuality has any validity).
But even while presenting obstacles to most sperm, the woman’s body can assist others. There is striking evidence that the female reproductive system is capable of making subtle judgments based upon the chemical signature of different men’s sperm cells. These assessments may go well beyond general health to the subtleties of immunological compatibility. The genetic compatibility of different men with a given woman means that sperm quality is a relative characteristic. Thus, as Anne Pusey explains, “Females may benefit from sampling many males, and different females will not necessarily benefit from mating with the same ‘high quality’ male.”
This is a crucially important point. Not every “high quality” male would be a good match for any specific woman—even on a purely biological level. Because of the complexities of how the two sets of parental DNA interact in fertilization, a man who appears to be of superior mate value (square jaw, symmetrical body, good job, firm handshake, Platinum AMEX card) may in fact be a poor genetic match for a particular woman. So, a woman (and ultimately, her child) may benefit by “sampling many males” and letting her body decide whose sperm fertilizes her. Her body, in other words, might be better informed than her conscious mind.
So, in terms of reproduction, the “fitness” of our prehistoric male ancestors was probably not decided in the external social world, where conventional theories tell us men competed for mates in Darwinian struggles for status and material wealth. Rather, paternity was determined in the inner world of the female reproductive tract where every woman is equipped with mechanisms for choosing among potential fathers at a cellular level. Remember this next time you read something like, “The predisposition for influence, substance and prestige are all merely expressions of a male positioning himself to acquire women with whom to mate,” or, “Mate competition will involve contests over resources [men’s] wives will need to raise children.” This may well be the situation for most people today, but our bodies suggest our ancestors faced an entirely different scenario.
For Woody Allen's take on what really lies behind ejaculation, take a look at this clip.
Given all the energy required to get there, it’s surprising that the female reproductive tract is not a particularly welcoming place for sperm cells. Researchers Robin Baker and Mark Bellis found that approximately 35 percent of the sperm are ejected within half an hour of intercourse and those that remain are anything but home free. The female’s body perceives sperm as antigens (foreign bodies) that are promptly attacked by anti-sperm leucocytes, which outnumber sperm 100:1. Only one in 14 million ejaculated human sperm even reach the oviduct. In addition to the obstacles imposed by the female’s body, even those lucky few sperm are going to run into competition from other males (at least, if our model of prehistoric human sexuality has any validity).
