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.)