The secret life of sperm

New theories on how evolution has reacted to the threat of infidelity. Consider this our post-Valentine's Day gift.

Published February 15, 2008 2:35PM (EST)

Enough about politics -- let's talk for a moment about sperm. (Though, I guess in many cases, those two are related.) Slate just ran a story called "The Merry Band of Wrigglers," which discusses a recent paper in the Journal of Theoretical Biology about possible reasons for sperm mutation. Fascinating stuff. Among the more interesting facts/hypotheses:

-- Men produce millions of sperm in an attempt to "ward off the effects of women's frequent cheating." (See paper for details before writing long comments about your unfaithful exes, please.)

-- In an experiment involving mating nematodes, nematodes whose males mated in the presence of other competitive males soon began producing sperm, on average, that were 20 percent larger than their competitors'. (The bigger the nematode sperm, the better it can crawl. Do with that image what you will.)

-- Men's penises may be designed as semen-removal devices. This one requires a direct quote:

"In a study 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."

-- Chimpanzees, however, do not have coronal ridges. Want to know a potential reason why? Because their semen -- which they produce in copious amounts -- solidifies in the female's vagina like a plug. (Which leads to the obvious question of where the plug goes if another male tries to mate with her. It must just get pushed further in, right? So she ends up with a chunk of dead sperm in her cervix? Monkeys are gross.)

There are some other juicy interesting -- if questionable -- theories mentioned, like the idea that a man is more likely to want to have sex with his partner when they've been separated for a while because he wants to clear out enemy sperm -- not, you know, because absence made the heart grow fonder or anything. But still. If you're looking for some penile distraction that is acceptable to read at work, check it out.

By Catherine Price

Catherine Price is an award-winning journalist and author of Vitamania: How Vitamins Revolutionized the Way We Think About Food. Her written and multimedia work has appeared in publications including The Best American Science Writing, The New York Times, Popular Science, O: The Oprah Magazine, the Los Angeles Times, The San Francisco Chronicle, The Washington Post Magazine, Salon, Slate, Men’s Journal, Mother Jones, PARADE, Health Magazine, and Outside. Price lives in Philadelphia.

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