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The Merry Band of WrigglersMen, women, passion, and sperm.

By Melinda WennerPosted Wednesday, Feb. 13, 2008, at 11:57 AM ET

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.

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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 correct for 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.)

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Melinda Wenner is a science writer living in Brooklyn, N.Y.

Illustration by Robert Neubecker.

COMMENTS

Remarks from the Fray:

Surveys taken of today's sexual attitudes and behaviors must be taken as absolutely meaningless as regards human evolution. Our attitudes and behaviors are determined far more by cultural factors than by biology. The proof is that you will get very different results in very different cultures and different times.

Just because in our time 25 percent of women and 30 percent of men 'cheat' doesn't mean we have any idea of the actual behavior of the humanoids who evolved into us. One hundred years ago the same study might have gotten very different numbers. What makes our numbers more 'valid' than other numbers? What even makes them valid at all?

Furthermore, it's not at all clear that 'cheating' is something that has any meaning outside of our culture and time. 'Cheating' implies individuals who can choose who they would like to mate with. And favoring some mates over others also implies a degree of autonomy. But if we look at human history, or even at other countries, we see very few people who have any choice in who they mate with. Very few of us, for instance, are forced into marriages that our parents have arranged for us. And very few of us are ever likely to be sold into slavery as captives. Yet these were extremely common experiences throughout history. It's our lives that are out of the norm.

--nerdnam

(To reply, click here.)

(2/13)

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