The book's title refers to a phenomenon called sperm competition, which is what happens when females mate with multiple males in the same fertility window (a.k.a. estrous cycle). Of course, no one you know would ever do such a thing. But DNA testing shows that it happens.

Sperm competition is more intense among chimpanzees: Females often mate with every male in the band within minutes, filling their reproductive tracts with a diverse assortment. To compete in this game, male chimps evolved bigger testicles capable of producing higher sperm counts.

Gorillas evolved on a path to more direct competition: Males have tiny testicles and huge bodies, the better to beat each other up for exclusive access to females.

Humans fall in between when it comes to testicle size, suggesting that our ancestors weren't all monogamous.

In case you were wondering, baseballs are to softballs are to basketballs as gorilla testicles are to human testicles are to chimpanzee testicles.

Not only must men ejaculate in high numbers in order to compete, he says, they also can (unconsciously) change the number of sperm released depending on what is known of the intended destination. If they are headed into a vagina, a man can assess the likely competition and adjust accordingly. That means sending more for a one-night stand than for a steady partner - unless the partner has been away.

Wow. Our subconscious is pretty stupid.

In Baker's scientific view, a lot of non-procreative sex actually has a higher purpose. The urge for oral sex evolved in a number of animals as a form of sleuthing - its olfactory clues can be a tip to a partner's cheating. And masturbation can increase a man's fertility by ejecting old, feeble sperm, which are then replaced by freshly minted seeds. Think of it as a form of house cleaning. Baker says masturbation holds an advantage for women, too, but that's more complicated.

All of this is subconscious, of course. "No man is going to say, 'My sperm were getting old and I wanted a slightly more competitive ejaculate,' " Baker says.

He'll just feel the urge.

That sounds like an awesome ad for one of those late-night TV ads for a sperm potency tablet or something else that doesn't work. "Take Potent Pills™, and get a slightly more competitive ejaculate!" I bet it'd be a big hit.

Biology, says Baker, is not aimed at helping people have better sex. But Sperm Wars might help you understand and perhaps even predict your partner's behavior. Like Christopher Columbus with his almanac, you never know when some basic knowledge could come in handy.

His sex almanac, that is.

Carnal Knowledge | Finally, a sex book with surprises [Inky]
1993 Philadelphia Phillies [Baseball Reference]
Earlier today: So Be Sure To Wear Boxers