November 16, 2010

Premature Ejaculation — Something Evolution Selects For?

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does evolution select from premature ejaculation?

No matter what we do, it always seems to come back to semen.

We’re blaming this one on Jesse Bering over at Scientific American, who recently decided to examine the evolutionary merits of premature ejaculation.

Ejaculation, biologically speaking, has one function: to shoot semen into the female body, in the hopes that one of the sperm survives the hostile reception long enough to penetrate an egg. “Given these basic biological facts, and assuming that ejaculation is not so premature that it occurs prior to intromission and sperm cells find themselves awkwardly outside of a woman’s reproductive tract flopping about like fish out of water,” Bering reflects, “what, exactly, is so “premature” about premature ejaculation?”

He goes on to hypothesize that in the very distant human past, ejaculating as quickly as possible may have even been preferable for the human species. Sexual reproduction in the wild is, after all, a dangerous thing. Survival requires attention.

Bering isn’t alone in this. A 1984 study by California State University sociologist Lawrence Hong, published in the Journal of Sex Research, offered the same line of thought. Bering notes:

The author compares the mating habits of human beings to other rapid–and not-so-rapid–ejaculators in the primate family, noting that the faster a primate species is in the coital realm, the less aggressive it is when it comes to mating-related behaviors. He calls this the “slow speed-high aggressiveness hypothesis.” For example, male rhesus macaque monkeys often engage in marathon mounting sessions, where sex with a female can be drawn out for over an hour at a time (including many breaks and therefore non-continuous thrusting). That may sound great, but libidinous anthropomorphizers beware: macaque sex is a chaotic and violent affair, largely because the duration of the act often draws hostile attention from other competitive males. By contrast, primate species whose males evolved to ejaculate rapidly would have largely avoided such internecine violence, or at least minimized it to a considerable degree.

Key to Hong’s analysis therefore is the idea that intravaginal ejaculation latencies in males is heritable — there was initially greater within-population level variation in the male ancestral population, he surmises, but over time, “the ancestry of Homo sapiens became overpopulated with rapid ejaculators.” This is because, according to Hong, young reproductive-aged males who ejaculated faster (i.e., had more sensitive penises) avoided injury, lived longer and therefore had a greater chance of attaining high status and acquiring the most desirable females.

The notion that premature ejaculation is an inherited trait, one possibly naturally selected for, is supported by research by Finnish psychologists, who published an article on the topic last year in the International Journal of Impotence Research.

Where is female orgasm in all this? It’s not. Female orgasm, you see, is not necessary for conception. Bering notes several critiques of this aspect of Hong’s research, among them the sexual aggressiveness of females in species that follow this “survival of the fastest” model and the problems resulting from lack of arousal and thus, lack of lubrication in females.

These critiques don’t seem to take into account research on female response during sexual assault, which report genital arousal, leading one to wonder whether sexual “readiness” has much to do with actual desire. A fast vaginal response to sex greatly reduces the likelihood of injury, which, because said injury could lead to death or impair fertility, would also result in being naturally selected.

Compelling. Of course, the game has changed. Sex is no longer something we have for the purpose of reproduction alone. Our immediate surroundings are no longer hostile. Some 50 years after the introduction of the birth-control pill, sex is something we enjoy for pleasure and the emphasis on conception has shifted dramatically to mutual pleasure. So much so, in fact, that we medicate hypothesized evolutionary advantages like premature ejaculation and seek out partners who display maladaptive traits, such as the ability to go and go and go for hours.

Via Scientific American.

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