The old adage says that a wife can't change her husband, but the truth is that women for thousands of years have been shaping one crucial male attribute: sperm. Men tend to produce as many sperm as possible as quickly as possible, a manufacturing decision that sacrifices quality control: Their sperm are frequently mutated or deformed as a result. Why, then, do men make millions of sperm at once? Because they're adapting to ward off the effects of women's frequent cheating, according to a paper published in December in the Journal of Theoretical Biology.
Humans aren't especially good at monogamy. Evidence gathered from surveys and paternity tests suggests that 25 percent of women and 30 percent of men cheat on their spouses at least once during marriage. The evolutionary reason that men cheat is pretty simple: to father as many children as they can. It's more complicated for women, who can only give birth so many times. The quality of the child, then, wins over quantity. Because men with the best genes aren't always the most stable and resourceful partners (they don't have to be), women might marry the latter but cheat with the former. Then they can become pregnant with a genetically superior child who will, if her mother can pull it off, grow up with the help of her unwitting spouse.
Women aren't consciously playing out these underlying reasons for their urge to cheat, of course. But if this cheating "pays off" in children who are more likely to survive and reproduce, the predilection to cheat will become an evolutionally advantageous trait—and the net result, over many generations, will be women who cheat.
Clearly, though, this is no good for their husbands, who are duped into raising kids who aren't theirs. Some scientists believe men have adapted clever ways to prevent this from happening. Not only are there studies to indicate that husbands do things to keep their wives from cheating in the first place—such as tracking their whereabouts—there is research that suggests men have evolved so as to correctfor women's cheating after the fact, by maximizing the chances that their sperm will conceive over someone else's.
The idea of "sperm competition," as it is called, is a familiar one in the animal world—there's lots of literature about it. For example, male flour beetles have spiny penises designed to remove rival sperm from a female's reproductive tract. And in a study published in 2002, scientists compared two groups of nematode worms: those that mated with females in the presence of competing males and those that mated without any male competition. After 60 generations had passed under these conditions, the offspring of the worms with the competition ejaculated sperm that were on average 20 percent larger than the sperm of other males. In nematodes, larger sperm crawl faster and can reach the eggs sooner.
The study of sperm competition in humans, though, is fairly new. In a study (with photos!) published in 2003, researchers at the State University of New York-Albany used a variety of dildos, artificial vaginas, and a homemade semen recipe to test whether the penis might be elegantly designed not only to deposit semen in the vagina, but also to remove it. The researchers speculated that when a man has sex with a woman who has recently slept with another man, the first man's semen is pulled out with the second man's penis (because it gets caught behind the second man's coronal ridge, which separates the head of the penis from the shaft). This lends meaning to the term "sloppy seconds": Sex the second time around is sloppy, because the semen that is removed ends up, well, making a mess.
The researchers found evidence to support their hypothesis: Dildos featuring a coronal ridge, like a real penis, displaced 91 percent of semen that got there first. Dildos without ridges displaced only 35 percent. Given that chimpanzees, our closest primate relatives, do not have ridges on their penises, this is "pretty strong evidence for the fact that the human penis evolved to compete with rival male semen in the female reproductive tract," says SUNY psychologist Gordon G. Gallup, who led the study. (Chimpanzees are very promiscuous, but they appear to solve the sperm competition problem differently: They produce extremely large volumes of semen that solidify in the vagina. Like a plug.)
That the human penis might have evolved as a semen displacement device is "not an outrageously ridiculous idea," says Todd Shackelford, an evolutionary psychologist at Florida Atlantic University. Size, too, may matter here—the longer a man's penis is, the more likely it is to deposit semen out of reach of other men.
Of course, if a man has sex with a woman twice in quick succession, then there is the risk that he will displace his own semen. But researchers believe that men have evolved ways of preventing this from happening—for instance, men usually need to "recover" for a few hours between orgasms, and they rarely continue to thrust after they ejaculate. Both of these behaviors help keep men from accidentally removing their own goods.
Men also act differently when they have reason to believe that their partner may have cheated. Several survey-based studies suggest that the more time that has passed since a man has last seen his spouse, the more he will want to have sex with her. When men sleep with their partners in these circumstances, they also thrust deeper and harder than usual—consistent with the idea that they are attempting to "scoop out" rival semen—and release more sperm when they ejaculate. Not that men are aware that they're doing this.
If men have evolved adaptations to thwart women from having other men's children, what is to stop women from evolving counter-adaptations? Nothing. "You have what biologists call an evolutionary arms race of sorts," Gallup explains. Just as a man is more likely to initiate sex with his partner if he suspects she has been unfaithful, a woman who has cheated does the opposite, according to other research. She tries to wait as long as possible before having sex again, perhaps to maximize the chances that her egg will be fertilized by the superior male she dallied with. In addition, women are most likely to cheat when they are at the most fertile point in their menstrual cycle, and they're more likely to orgasm, too; some biologists argue that the female orgasm, which is accompanied by vaginal and intrauterine contracts, helps to pull semen into the reproductive tract.
Of course, there are other more obvious explanations for the waiting and the orgasms: guilt in the first instance and pleasure in the second. Perhaps, too, women wait because they don't want their husbands to smell the scent of another man. Cheating for evolution's sake is one thing; getting caught on an individual basis is another. A husband isn't likely to be more sympathetic when his wife tells him that evolution made her do it.