Decoding Love - Andrew Trees [23]
BATTLE OF THE SEXUAL GAMETES?
I know this sounds far-fetched. Women don’t cheat on their husbands with the express thought of getting their hands on better genetic code. Remember, though, that much of what we are talking about in this chapter occurs below the level of conscious thought. The skeptical among you might use this as a chance to dismiss what I’m saying, but you don’t have to take my word for it. You simply have to look at the human body itself. And what does it show? A constantly evolving arms race between men and women—only in this case the battle is waged not by us but by our proxies, our sperm and our eggs. What we have is a classic Red Queen contest where each side wants to gain an advantage. The man would like to impregnate every woman he sleeps with, while the woman would like to be able to choose who fathers her child. She might find a man who is good enough to marry but still want top-notch sperm from someone on the side. If what I’m saying is true, you would expect both men and women to develop measures to increase their chances. We’ve already seen one very successful strategy of women to thwart a man’s ability to police her fertility: concealed ovulation. But it doesn’t stop with concealed ovulation. Both men and women have developed a host of subtle and shocking measures to increase their chances of genetic success. These measures are both a sign of how deeply we have been shaped by sexual selection and also a disturbing indication of how we are unwitting pawns in a largely invisible genetic struggle. In short, we are a long way from the romantic story line now.
The clearest evidence of this battle comes from an ingenious study of the fruit fly, published by William Rice in 1996. What Rice did was allow male fruit flies—but not the females they were breeding with—to evolve for forty-one generations. This was a boon for the males. They gradually developed much more effective sperm so that when they mated with a female who had already mated with another of Rice’s evolved males, they were much more likely to impregnate her. The reason for their success? Much more toxic sperm, which basically killed off the rival sperm. That trait came with a cost, though, and the cost reveals quite clearly that sexual evolution is not a matter of both sexes cooperating—it’s usually a case of one sex developing a quality that is actually antagonistic to the other sex. Just how antagonistic? The sperm had grown so toxic that if a female fruit fly had sex with enough of these evolved males, the sperm had a good chance of killing her—talk about a toxic bachelor! Luckily, human sperm remain far from lethal, but an equally fierce competition is being waged every day between our sperm and our eggs.
So, let’s wade right in and start with one of the more controversial aspects of human sexuality—female orgasm. There is still a fairly vigorous debate about whether or not there is any point to a female orgasm. Although feminists might feel that this is some sort of chauvinistic joke masquerading as science, the subject of female orgasm is a serious issue. The reason for a male orgasm is obvious—the release of sperm. And that also helps explain the almost clocklike regularity of male orgasm (as well as the tendency of some men to ejaculate prematurely—there are many positive benefits to the man coming as quickly as possible, and the downside is borne by women). But women ovulate once a month and can be impregnated without any orgasm, leaving scientists