by Kenneth W. Krause.
Kenneth W. Krause is a contributing editor and “Science Watch” columnist for the Skeptical Inquirer. Formerly a contributing editor and books columnist for the Humanist, Kenneth contributes regularly to Skeptic as well. He may be contacted at firstname.lastname@example.org.
Does anyone ever get the feeling that popular evolutionary science writing is bursting with bull and obsessed with sex? Consider scholar and science writer, Jesse Bering, as one prominent example. In the title cut from his new book, Why Is the Penis Shaped Like That? …And Other Reflections on Being Human (FSG 2012), Bering engages in a neat bit of “reverse engineering” to explain the evolutionary significance behind the mushroom-shaped cap and lip separating the human penis’s shaft from its glans.
Okay, we’re not exploring the origins of the universe here, or even the mystery of human consciousness. But it’s not a bad question either. After all, chimp, gorilla, and orangutan phalluses are essentially all shaft, and cat penises come with about 150 sharp, backward-pointing spines (Youch!). So what on earth was nature thinking, so to speak?
Citing a 2004 study published in the journal Evolutionary Psychology, Bering offers the “semen displacement theory.” Rape, group sex, prostitution, and female promiscuity, the theory goes, have generated intense male competition for access to the female ovum. In response, evolution, an eccentric artist if there ever was one, sculpted a human penis that when thrust into and out of the vagina would “upsuck” and displace the semen of other men.
You might wonder just how this study was conducted. Well, as it turns out, with water, unbleached flour, and prosthetic genitals—both male and female—from a local erotic novelty store, of course! Whatever your appraisal of the study’s methodology, the upshot was generally as follows: Phalluses with coronal ridges displaced 91 percent of the fake semen compared to 35.3 percent for their headless counterparts.
But aren’t chimpanzees at least as promiscuous or violent as we are, and what about those yowling cats? Bering’s answer is identical to the one I once provided to a quarrelsome creationist who wondered why, if evolution was so powerful, it didn’t create perfect solutions or at least very similar solutions among species to the same problems.
Precisely because evolution does not transpire by intelligent design. “It occurs by selection,” Bering observes, “and the raw material for such selection consists of nothing more than random genetic accidents.” Thus, male chimps compete in their own way, by depositing the most or the most potent semen. Male cats, by contrast, with their “scratching posts,” attempt to stimulate ovulation in addition to competing for access.
Now consider Psychologist and Biologist David P. Barash’s account of human menstruation in his new book, Homo Mysterious: Evolutionary Puzzles of Human Nature (Oxford 2012). An extraordinary phenomenon, no other animal comes even close to the human female in terms of both blood loss (about 40 ml) and tissue disruption—forcing her to literally reconstruct her uterine lining by the conclusion of each reproductive cycle.
As such, menstruation is a very costly proposition. Along with the shedding of fresh blood, severe cramps surely rendered ancestral women more susceptible to grisly predation. But the most perplexing fact of all, perhaps, is that menstruation thwarts evolution’s ultimate goal, pregnancy, for three to five days out of every reproductive cycle. All of which, in Barash’s estimation, just screams for a barrage of scientific hypotheses.
First, menstruation could have served early humans as a signal to the woman that she wasn’t pregnant. That would certainly have been a valuable thing to know in a world predating pregnancy tests. Or perhaps it was a signal to men that a girl had finally become a woman. “Logically unlikely,” Barash teases. The costs are still too high. If the evolutionary goal was to signal no pregnancy, why not limit the physical damage to a bit of spotting, as with dogs? And if the objective was to reveal reproductive capacity to men, why not use something more akin to pheromones?
So is all hope lost for the signaling hypothesis? Maybe not. What if nature’s ambition was a bit broader in social terms—to inform the entire tribe or clan, perhaps? Not so fast, Barash warns. Though competition among women is usually less violent than between men, it still exists. Indeed, in many species, dominant females will intimidate, attack, or otherwise inhibit the reproductive opportunities of their subordinates. That humans are among those species, Barash admits, remains an “unproven assumption.” Even so, he says, young and thus vulnerable girls would always be well advised to conceal their reproductive viability.
Second, sex is a very messy business. Intercourse introduces foreign substances deep inside the female body, beyond the influence of her more common defenses. Sperm carry pathogens like Chlamydia, which causes pelvic inflammatory disease, and thrusting penises convey bacteria like Streptococcus and Staphylococcus beyond the vagina, where they pose little if any harm. Thus, some experts contend, the object of menstruation must be primarily hygienic—in other words, to cleanse the woman’s upper reproductive tract.
But the “cleansing hypothesis” has its own set of shortcomings, according to Barash. Menstrual blood, for example, contains nutrients, including iron, that actually facilitate pathogen development. And although one might expect some correlation between menstrual intensity and pathogen load, none seems to exist. Or, if the uterus is unable to sense pathogen invasions, we might alternatively predict a similar relationship between menstruation and sexual frequency. But, again, the evidence just isn’t there.
And if menstruation serves to diminish the uterine pathogen population, why does bacterial diversity actually expand following menstruation? Finally, determined proponents of the cleansing hypothesis should explain why women in traditional, non-technological societies who reproduce more frequently than their Western counterparts and, thus, experience fewer menstrual cycles, aren’t more susceptible to uterine infections.
Yet a third possibility defines menstruation somewhat more cynically as a systematic competency test that essentially pits potential mothers against their weakest embryos. We know that humans invest more time and energy in each offspring, and take more risks in the process, than any other species. We recognize as well that for every successful pregnancy many “spontaneous abortions” are caused by the failure of embryos to implant in their hosts’ uteruses. In addition, as implantation proceeds, embryos begin secreting human chorionic gonadotropin, a hormone that inhibits menstruation.
So perhaps, as Barash suggests, menstruation is “how a woman’s body evaluates her would-be offspring and does so early in the ‘investment’ cycle, thereby minimizing wasted investment in case of a thumbs down.” Evolution, in other words, may have selected against embryos whose burrowing or secretory capacities are found wanting.
But here’s a surprise—the author remains unconvinced. The “evaluation hypothesis,” he cautions, is unsupported by reality because, like the cleansing hypothesis, it should predict a link between menstruation and sexual activity. And what about other species that tend to invest heavily in individual offspring—why, for example, don’t elephants, blue whales, or manatees menstruate? “It’s a mystery,” Barash concludes. “Period.”
So how should we regard these writers’ and researchers’ (let’s say) abundantly creative explanations and methods? Is popular evolutionary science little more than lazy-minded speculation followed by impulsive waffling?
I certainly don’t think so. Imagination and conjecture, after all, are the fertile soils in which truly great scientific ideas are often sowed. That these thinkers reconsider their original hypotheses and frequently conceive new ones is testament to their intellectual integrity. Besides, I’m a big fan of just about anything that makes evolution more fun and accessible for young people.
And what about the obsession—are we being exploited for our fascination with the tawdry and titillating? I don’t buy that either. Indeed, why wouldn’t any evolutionary science writer preoccupy herself with sex to some degree? For humans, heterosexual intercourse is the indispensable mechanism by which our evolutionary machine has functioned for millions of years.
Regardless, show me a person who isn’t fairly obsessed with sex and I’ll lift the cover and show you a toe tag.