Sex at Dawn

persuader, conqueror.” Recent research on human fertilization

suggests something of a role reversal. Small suggests the

ovum “reaches out and envelops reluctant sperm.” “Female

biology,” she concludes, “even at the level of egg and sperm

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interaction, doesn’t necessarily dictate a docile stance.”

In addition to enveloping eggs, a cervix that filters or favors sperm, and vaginal contractions that may expel the sperm of one man while boosting that of another, women’s orgasms provoke changes in vaginal acidity. These changes appear to assist the sperm cells of the lucky guy who provoked the orgasm. The environment at the cervical opening tends to be highly acidic and thus hostile to sperm cells. The alkaline pH of semen protects the spermatozoa in this environment for a while, but the protection is short-lived; most of the sperm cells are viable within the vagina for only a few hours, so these changes in acidity alter the vaginal environment in ways that can favor sperm that arrive with the female’s orgasm.

The benefits may run both ways. Recent research suggests women who do not use condoms are less likely to suffer from depression than either women who do use condoms or who are not sexually active. Psychologist Gordon Gallup’s initial survey of 293 women (data congruent with those from another survey still to be published that included 700 women) found that women can develop a “chemical dependency” on the boost they get from the testosterone, estrogen, prostaglandins, and other hormones contained in semen.

These chemicals enter the woman’s bloodstream through the

vaginal wall.³⁰

If it’s true that multiple mating was common in human evolution, the apparent mismatch between the relatively quick male orgasmic response and the so-called “delayed” female response makes sense (note how the female response is “delayed” only if the male’s is assumed to be “right on time”). The male’s quick orgasm lessens the chances of being interrupted by predators or other males (survival of the quickest!), while the female and her child would benefit by exercising some preconscious control over which spermatozoa would be most likely to fertilize her ovum.