Why Masturbation Helps Procreation

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O’Donnell’s mid-’90s crusade against autoeroticism wasn’t based on science. youtube.com

Since Christine “I’m Not a Witch” O’Donnell is campaigning for the U.S. Senate and not the directorship of the Kinsey Institute, maybe we should give her a pass when it comes to her views on sex and, specifically, masturbation. But that would be a mistake: the stakes are simply too high, going all the way up the very survival of our species. For while O’Donnell crusaded against masturbation in the mid-1990s, denouncing it as “toying” with the organs of procreation and generally undermining baby making, the facts are to the contrary. Evidence from elephants to rodents to humans shows that masturbating is—counterintuitively—an excellent way to make healthy babies, and lots of them. No one who believes in the “family” part of family values can let her claims stand.

The science is straightforward. Whenever a behavior is common in the animal kingdom, biologists suspect it has an adaptive function. That is, the behavior enabled individual animals to survive better and leave more offspring than animals that did not engage in the behavior. As a result, genes for the behavior spread throughout that population until it became essentially ubiquitous. And so it is with autoeroticism, which is common—really common. As the Science in Seconds blog noted this week, what with “spanking the monkey,” “charming the snake,” and “freeing willy,” a remarkable number of the slang terms for pleasuring oneself refer to animals. That reflects reality: the practice has been documented in Japanese macaques, gibbons, baboons, chimps, elephants, dogs, cats, horses, lions, donkeys, “and walruses that manage to flog the bishop with their fins.” (Bonus for clicking on the blog link above: excellent photo of an elephant in flagrante dilecto.)

What, then, might be its adaptive function? How can autoeroticism help animals triumph in the war of survival of the fittest? Lucky for us, scientists have been pondering this. There are four basic theories, each with some support in one or another animal species (I’m not counting the sexual-outlet hypothesis, which posits that masturbation is not adaptive but is just a byproduct of sexual arousal, which is definitely adaptive):

1. Masturbation might remove old, worn-out, broken sperm from the reproductive tract. That would increase the fraction of healthy, speedy sperm, improving a male’s chance of becoming a father. “In humans, masturbation increases sperm quality (by promoting younger sperm) without affecting sperm numbers in the female reproductive tract,” notes biologist Jane Waterman of the University of Central Florida in a new paper in the journal PLoS One. As far back as 1993, biologists had observed that masturbating decreased the number of sperm a man delivered the next time he had sex with his partner, but not the number of sperm the woman retained. They concluded that “masturbation is a male strategy to increase sperm fitness.”

Research presented at a science meeting last year offered support for the fitter-sperm idea. Ejaculating daily for seven days improved sperm quality as measured by the amount of DNA damage: levels of damage averaged 34 percent on a standard measurement index after three days’ abstinence, but after a week of ... um, non-abstinence, the level of damage dropped to 26 percent, in the “fair” range for sperm quality. Looking only at men whose sperm damage decreased (in a few, damage got worse for some reason), the average damage level fell to just under 23 percent—putting them in the “good” range. In addition, sperm motility rose significantly. Result: healthier and possibly more babies.

2. Masturbation might be a form of advertising. According to this idea, males that engage in autoeroticism signal to possible mates as well as competitors how much they have to offer. “Males may advertise their high quality,” explains Waterman, “signaling that they have high quantities of sperm and can afford to waste some.” Result: more mating, more babies, more families. We hasten to add that masturbation as advertising does not apply (we really, really hope) to humans.

3. Masturbation might be a sort of victory lap. Some animals masturbate after they mate. Since other members of a group know this, then masturbation signifies that the male engaging in this behavior was the chosen partner of other females. Females who are still shopping for a mate might be inspired by that information to copy their choice, as in, “if he was good enough for her ...” Result: more mating, more babies.

4. Masturbation can serve a hygiene function. According to this idea, males engage in autoeroticism because it cleans the reproductive tract and reduces the chance of acquiring a sexually transmitted disease from a female that he mated with and who had other recent partners. Result: a lower incidence of STDs, better sexual hygiene, more mating, more babies.

The STD function is what Waterman inferred about masturbation after she spent 2,000 (!) hours observing the Cape ground squirrel (Xerus inauris) of Namibia. As she explains in PLoS One, “an oral masturbation was recorded when a male sat with head lowered and an erect penis in his mouth, being stimulated with both mouth (fellatio) and forepaws (masturbation), while the lower torso moved forward and backwards in thrusting motions, finally culminating in an apparent ejaculation.” The behavior was much more frequent on days when females were fertile, and mostly occurred after mating. That would seem odd, since it wastes huge numbers of sperm just when they have the best chance of finding a willing egg. Odd, too, is the observation that males masturbated more when their mate had had a lot of other suitors (female Cape ground squirrels mate with up to 10 males in their three-hour fertile period). But there is one explanation that makes sense of a behavior that occurs after mating, and more often when a male has mated with a particularly promiscuous female: masturbation is a way for males to reduce the chance of infection, since saliva has antibacterial properties. Since STDs can destroy fertility, sexual hygiene through masturbation is a way for male squirrels to keep making babies.

And what about females? There have been far fewer observations of females masturbating in the wild, though bonobos are well known for this form of eroticism (and every other, it seems). One popular theory for why females might indulge, however, is incorrect—namely, the idea that orgasm (during or right after mating) might propel sperm to the egg. To the contrary, scientists reported in a 2002 paper, “vaginal and uterine contractions ... have been misinterpreted as powering rapid sperm transport to facilitate fertilization, but such fast transport would lead to the tubal deposition of noncapacitated, incompetent spermatozoa,” which would not lead to conception. Instead, primatologists conclude, in the case of females the purpose is simply to “produce enjoyable sensations”.

All in all, and across species great and small, autoeroticism (at least among males) is a cornerstone of procreation and thus the formation of families. Were O’Donnell’s unscientific views of the practice to spread, it would be a worrisome threat to family values.

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