For women, it's shopping; for men, it's competition

Since starting this column, I've received loads of sex books — soft-cover and hard, by physicians, psychologists, wives, bachelors, and women who want to share their multiple-orgasm secrets.

They all started to look the same until someone sent a sex book by a British zoologist who turned his attention to the human animal.

"Sperm Wars" is aimed at those of us who have ever paused to wonder what, exactly, we're doing. It's a reissue of a 1996 title, yet still holds more surprises than any of the 2006 sex books crowding my shelf.

Robin Baker writes that men's sexual desires evolved to help them detect, eject or otherwise deal with rival sperm. For women, sex is like shopping, with the goal of acquiring just the right sperm, or mix of them.

Baker's ideas fall into the field of evolutionary psychology, but not the sort that speculates on the needs of Paleolithic hunter-gatherers. Some of our most popular bedroom habits arose much further back in evolutionary time. We therefore share them with many of our fellow creatures.

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 (aka 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. Unlike our relatives on the family tree, however, humans do form some pair bonds. So our sexual habits more closely resemble those of birds and some small mammals — "animals that don't look anything like humans," says Baker, who recently retired from the University of Manchester and is now writing in Spain, where I interviewed him by phone.

Baker says he's been criticized by "male and female chauvinists," although his book extols the wonders of both male and female bodies. For example, it does away with the idea that producing 300 million or so sperm for every ejaculation is an example of the inefficiency of men.

To the contrary: 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.

In Baker's scientific view, a lot of nonprocreative 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. 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.

When it comes to the old question of whether men and women evolved to be monogamous, Baker's ideas parallel those of other evolutionary psychologists such as David Buss at the University of Texas. There is variety among humans: Some instinctively want one lifetime partner while others desire 1,000. Some are fine with swapping; others abhor it. Many feel the desire to cheat, and some of those people are instinctively driven to keep their partner from cheating.

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.

Carnal Knowledge appears Sundays in The Seattle Times