Kim Carney / msnbc.com
By msnbc.com contributor
msnbc.com contributor
updated 5/24/2007 2:34:33 PM ET 2007-05-24T18:34:33

Why do women orgasm? Why do they have prominent breasts even when they aren’t nursing a baby? Why can’t people tell when a woman is ovulating? Why would anybody even ask these questions?

Most people don’t, but evolutionary psychologists, biologists and anthropologists make a living doing so, and the possible answers give us some interesting clues to why we have sex the way we do today.

Men just don't seem to have the same number of unanswered questions about our biology. Male primates — male mammals in general — all have penises and testicles and sperm. We use them whenever we can. Unless we are sick or injured, we can make babies. And if anybody wants to know if we're interested, all they have to do is look to see if we're at attention.

Science seems to know why men work the way we do, which is pretty much the way other male primates work. The sexual biology of human females, though, can be puzzling.

David P. Barash, a University of Washington psychology professor who is writing a book about such “evolutionary enigmas,” gave a presentation to the annual meeting of the American College of Obstetricians and Gynecologists in San Diego earlier this month. Afterward, I asked him to follow up with Sexploration and invited other experts to weigh in. Though nobody claims to have the right answers about some of these top female mysteries, they do seem to have fun thinking about them.  

1. Hidden ovulation
Though some studies suggest that men and women do have an unconscious sense of when a woman enters a fertile period and is ripe for mating, there is no obvious outward sign as there is for most mammals. Many female monkeys, for example, get bright red butts when they release an egg. But women are poker butts, even to themselves, which is why they are left to temperature-taking and guessing in order to time ovulation.

Dr. Leonard Shlain, a San Francisco surgeon and author of “Sex, Time and Power: How Women’s Sexuality Shaped Human Evolution,” speculates that ovulation had to be concealed because women wised up and realized sex led to pregnancy, which led to childbirth, which often led to death for the woman. “Once women understood they could die as a result of having sex, why wouldn’t they abstain from sex?” But if women did not know when they ovulated, they wouldn’t know when they had to abstain in order not to risk dying nine months later (a theory that assumes they had a choice about whether to have sex).

On the other hand, since men did not know either, suggests Karen Rosenberg, an evolutionary anthropologist at the University of Delaware, concealed ovulation could have reinforced “pair bonding, the idea that males had to stick around and have sex all throughout the cycle or the female might be pregnant with somebody else’s offspring.”

“I call that the ‘keep ‘em close’ hypothesis,” Barash says. “He not only has to have sex with her, he has to hang around and take out the garbage and mow the lawn.” 

But it is also possible, he argues, that hidden ovulation could have given women the ability to mate with other men during a cycle. Since women do not go into heat like most female mammals, they are sexual all the time and even a devoted male can’t guard his female constantly. While he is hunting a mastodon, she can be flirting with the sensitive cave artist across the way and possibly obtain a shot of his arty genes.

Or maybe, Barash suggests, a bright red butt on one woman would bring out the worst in other women, whose men might be tempted to do a little gene-spreading. Perhaps if nobody knew who was ready for sperm and who wasn’t, harmony could reign.

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His favorite theory for hidden ovulation sounds like Shlain’s. If women did fear childbirth, they may not want as many children as men. If ovulation were not hidden, women might avoid sex during it. “I call this the ‘headache hypothesis’ as in, ‘Not tonight, dear.’” But evolution wins out because women who displayed ovulation may have avoided sex during fertile days, reproduced less and given the upper hand to the offspring of women whose ovulation was less obvious.

“And so, it is those who are ignorant of their own ovulation who have inherited the earth,” Barash says.

2. All those orgasms
It was once thought that women have orgasms so the muscle contractions could shepherd sperm toward the cervix, what Barash calls, alarmingly, “the uterine suck-up theory.” But this has been more or less proven false. 

Shlain believes orgasm is a reward to women, a little something to entice them to have sex rather than focus on the prospect of death in childbirth. “Once she knows death is associated with sex, she needs to have an impetus to keep having her do it,” he says.

“Women have orgasms because men do,” insists Katherine Dettwyler, an anthropologist and colleague of Rosenberg’s. “The clitoris is the homologue of the head of the penis. I think orgasms are a reward to men to [have sex] as much as possible and also the muscles contracting shoot ejaculate out and so it gets as far it can. Women have a clitoris because that’s what’s left of the head of the penis, like men have nipples.”

Rosenberg agrees, calling female orgasm an evolutionary “side effect.”

But Barash says he is “unimpressed with that notion. Male nipples are trivial; they are laughable. Female orgasm is a definite phenomenon, more Technicolor as opposed to male orgasm. It is hard to believe it is a mere byproduct when women are capable of multiple orgasms that would make any man jealous.”

Instead, he prefers what he calls the “grizzly bear hypothesis” (and as if men did not have enough to be anxious about, this one will send us running for the Xanax). When a subordinate male grizzly mates with a female, he constantly swivels his head around looking for the nearby dominant male who will cut in, so to speak. So the subordinate males go for quickies, ejaculating as fast as possible. The dominant males can swagger in and take their time. “With whom would you orgasm?” Barash asks, evoking an image I’d rather not contemplate.

He thinks it’s possible that orgasm is a way for a woman’s body to tell her mind that she is copulating with a powerful, attentive, secure male. That means good genes. “Perhaps orgasm is an evaluation of a male, a way females inform themselves” of the kind of male they are with, he says.

And I thought it was my stock portfolio.  

3. Baffling breasts
Women have breasts, of course, to feed babies. But why do human females cart around prominent breasts even if they are not pregnant or lactating? Most other mammals don’t.

Unless a woman is lactating, or getting ready to, breast size is determined by how much fat a woman stores there. In a statement sure to make everyone feel good, Shlain argues that a “dog belly has no subcutaneous fat. We have the most of any land mammal.”

He says this fat serves the purpose of sexual arousal.

“When we stood up,” he says, referring to our early ancestors, “the anatomy of the pelvis changed. The vagina oriented itself more toward the front.” But this was a problem because most mammals, including primates, have sex doggie style. Hence the big red butts advertising “Sexy girl here!” meant to appeal to our visual sense. (Primates do not smell as well as, say, dogs.) So, since males began facing females for sex, the rough equivalent of big red butts “were transposed to the front of a woman” and became the breasts we know and love.

Not everyone buys it. “I do not find the butt-to-front theory convincing,” Rosenberg says, pointing out, as does Dettwyler, that breasts are not sex objects in many cultures, so it is hardly a universal human trait to obsess about them.

In most non-Western cultures, Dettwyler argues, “for adult men to suck on a woman’s breast is as bizarre as a bull nursing from a cow.” She also believes Western women have relatively large breasts because “we are overfed.”

Barash disagrees, citing several notions, such as the “Goldilocks hypothesis” (too small means she is too young and not fertile, too large and sagging means too old and not suitable for mating) and the “symmetry theory” (breasts are good for telling how symmetric, and therefore healthy, a woman might be).

But his favorite is the “deception hypothesis.” Our forefathers might have been fooled into thinking a woman with large breasts was a nursing superstar, when actually, those bosoms were just loaded with fat. So lured, men would provide food and attention and sex, which could explain the well known “boob-job-equals-5-carats” corollary. 

Breasts as a mating ruse? Bait? An anatomic Potemkin Village? Personally, I don’t care why we like them, but the evolutionary theories are fascinating even if nobody is sure of the answers.

And there are yet more female mysteries, including menstruation and the timing of menopause.

But even though science seems to understand a man's biology better than a woman's, based on what's known about males throughout the animal kingdom, there's another way of looking at all this.

"Females," says Rosenberg, "have departed more from the primitive” — which sounds like a nice way of telling me women are more highly evolved than men.

HARRUMPH!

Brian Alexander is a California-based writer who covers sex, relationships and health. Alexander, also a Glamour contributing editor, recently traveled around the country to find out how Americans get sexual satisfaction for the MSNBC.com special report "America Unzipped" and for an upcoming book for Harmony, an imprint of Crown Publishing.

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