Reproductive Warfare: Sperm Invasion on Battlefield Vagina

Cristen Conger

Vaginas don't take kindly to all those sperm. (Duncan Smith/Getty Images)

Earlier this week, I wrote about the possible antidepressant effects of semen in women. A 2002 study by psychologist Gordon Gallup hypothesizes that the vagina absorbs a host of mood-boosting hormones contained in semen, which makes women feel better. This would make it seem like vaginas and semen are BFFs, right?

Well, according to Dr. Gallup himself, that isn't the case. In fact, vaginas don't take too kindly sperm, and semen contains chemicals to neutralize sperm-attacking pathogens in the vagina. Speaking to Popular Science, Gallup said:

"The vagina is a very hostile environment for sperm. When a female is inseminated, the presence of the semen triggers an immune reaction, so semen-and particularly the sperm-are treated as pathogens. Male seminal plasma contains all kinds of chemicals that are designed to take this into account. Seminal plasma is alkaline, and a couple seconds after ejaculation the pH of the vagina approaches neutrality, which makes it a friendly environment for sperm. Sperm also contains a lot of immunosuppressants that suppress the female's immune system and counteract this immune reaction to semen."

PopSci explains this antagonistic sperm-vagina relationship as an example of antagonistic coevolution. "In nature's sex wars, males campaign for more offspring-the proverbial seed-spreading-while females play hard-to-get because they bear most of the burden of reproduction and parenthood. "

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