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Why Is the Penis Shaped Like That?: And Other Reflections on Being Human


Why Is the Penis Shaped Like That?: And Other Reflections on Being Human Cover

ISBN13: 9780374532925
ISBN10: 0374532923
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How Are They Hanging? This Is Why They Are


A few years ago, the evolutionary psychologist Gordon Gallup, whom well meet again later in this section, along with his colleagues Mary Finn and Becky Sammis, set out to explain the natural origins of the only human male body part arguably less attractive than the penis—the testicles. In many respects, their so-called activation hypothesis elaborates on what many of us already know about descended scrotal testicles: they serve as a sort of cold storage and production unit for sperm, which keep best at a temperature slightly lower than the norm for the rest of our bodies. But the activation hypothesis goes much further than this fun fact.

It turns out that human testicles display some rather elaborate yet subtle temperature-regulating features that have gone largely unnoticed by doctors, researchers, and laymen alike. The main tenet of the activation hypothesis is that the heat of a womans vagina radically jump-starts sperm that have been hibernating in the cool, airy scrotal sac. This heat aids conception. Yet it explains many other things too, including why one testicle is usually slightly lower than the other, why the skin of the scrotum sometimes becomes rugose (prune-like and as wrinkled as an elephants elbow), why the testicles retract during sexual arousal, and even why testicular injuries—compared with other types of bodily assaults—are so excruciatingly painful.

To help us all get on the same page, consider an alternate reality, one in which ovaries, like testicles, descend during embryological development and emerge outside the female body cavity in a thin, unprotected sac. After youve wiped that image from your minds eye, note that the dangling gonads of many male animals (including humans) are no less puzzling. After all, why in all of evolution would nature have designed a body part with such obviously enormous reproductive importance to hang outside the body, so defenseless and vulnerable? We tend to become accustomed to our body parts, and it often fails to occur to us to even ask why they are the way they are. Some of the biggest evolutionary mysteries are also the most mundane aspects of our lives.

So the first big question is why so many mammalian species evolved hanging scrotal testicles to begin with. The male gonads in some phylogenetic lineages went in completely different directions, evolutionarily speaking. For example, modern elephant testicles are deeply embedded in the body cavity (a trait referred to as testicond), whereas other mammals, such as seals, have descended testicles but are without scrota, with the gonads simply being subcutaneous.

Gallup and his colleagues jog through several possible theories of our species testicular evolution by descent. One of the more fanciful accounts—and one ultimately discarded by the researchers—is that scrotal testicles evolved in the same spirit as peacock feathers. That is to say, given the enormous disadvantage of having your entire genetic potential contained in a thin satchel of unprotected, delicate flesh and swinging several millimeters away from the rest of your body, perhaps scrotal testicles evolved as a sort of ornamental display communicating the genetic quality of the male. In evolutionary biology, this type of adaptationist account appeals to the handicapping principle. The theoretical gist of the handicapping principle is that if the organism can thrive and survive while still being hobbled by a costly, maladaptive trait such as elaborate, cumbersome plumage or (in this case) vulnerably drooping gonads, then it must have some high-quality genes and be a valuable mate.

But the handicapping hypothesis doesnt quite fit the case of descended scrotal testicles, explain the authors, because if it were true, then we would expect to see these body parts becoming increasingly elaborate and dangly over the course of evolution, not to mention that women should display a preference for males toting around the most ostentatious scrotal baggage. “With the possible exception of colored scrota among a few species of primates,” writes Gallup, “there is little evidence that this has been the case.” Im not aware of any studies on intraspecies individual variation in scrotal design, but Im nonetheless willing to speculate that most human males have rather bland, run-of-the-mill scrota. Anything deviating from this—particularly a set of unusually pendulous testicles suspended in knee-length scrota—is probably more likely to have a woman dry heaving, screaming, or staring in confusion rather than serving as an aphrodisiac.

Again, a more likely explanation for scrotal descent, and one that has been around for some time, is that sperm production and storage are maximized at cooler temperatures. “Not only is the skin of the scrotal sac thin to promote heat dissipation,” the authors write, “the arteries that supply blood to the scrotum are positioned adjacent to the veins taking blood away from the scrotum and function as an additional cooling/heating exchange mechanism. As a consequence of these adaptations average scrotal temperatures in humans are typically 2.5 to 3 degrees Celsius lower than body temperature (37 degrees Celsius), and spermatogenesis is most efficient at 34 degrees Celsius.”

Sperm are extraordinarily sensitive to even minor fluctuations in climate. When the ambient temperature rises to body levels, there is a momentary increase in sperm motility (they become more lively), but only for a period of time before fizzing out. To be more exact, sperm thrive at body temperature for fifty minutes to four hours, the length of time it takes for them to journey through the female reproductive tract and fertilize the egg. But once the spermatic atmosphere rises much above 37 degrees Celsius, the chances for a successful insemination consequently plummet—any viable sperm become the equivalent of burned toast. So in other words, except during sex, when its adaptive for sperm to be hyperactive, sperm are stored and produced most efficiently in the cool, breezy surroundings of the relaxed scrotal sac. One doesnt want his scrotum to be too cold, however, since nature has calibrated these temperature points at precisely defined optimal levels.

Fortunately, human scrota dont just hang there holding our testicles and brewing our sperm; they also “actively” employ some interesting thermoregulatory tactics to protect and promote males genetic interests. I place “actively” in scare quotes, of course, because, although it would be rather odd to ascribe consciousness to human scrota, testicles do respond unintentionally to the reflexive actions of the cremasteric muscle. This muscle serves to retract the testicles so they are drawn up closer to the body when it gets too cold—just think cold shower—and also to relax them when it gets too hot. This up-and-down action happens on a moment-to-moment basis; thus male bodies continually optimize the gonadal climate for spermatogenesis and sperm storage. Its also why its generally inadvisable for men to wear tight-fitting jeans or especially snug “tighty whities”; under these restrictive conditions the testicles are shoved up against the body and artificially warmed so that the cremasteric muscle cannot do its job properly. Another reason not to wear these things is that its no longer 1988.

Now, I know what youre thinking. “But, Dr. Bering, how do you account for the fact that testicles are rarely perfectly symmetrical in their positioning within the same scrotum?” In fact, the temperature-regulating function governed by the cremasteric muscle can account even for the most lopsided, one-testicle-above-the-other, waffling asymmetries in testes positioning. According to a 2009 report in Medical Hypotheses by the anatomist Stany Lobo and his colleagues, each testicle continuously migrates in its own orbit as a way of maximizing the available scrotal surface area that is subjected to heat dissipation and cooling. Like ambient heat generated by individual solar panels, when it comes to spermatic temperatures, the whole is greater than the sum of its parts. With a keen enough eye, presumably one could master the art of “reading” testicle alignment, using the scrotum as a makeshift room thermometer. But thats just me speculating.

From an evolutionary perspective, the design of male genitalia makes sense only to the extent that it adaptively complements the female anatomy, which, I realize, I should really go into more (but there are only so many hours in a day). By contrast to males, unless a woman is engaging in strenuous exercise, the female reproductive tract is maintained continuously at standard body temperature. This is the crux of Gallups activation hypothesis: the rise in temperature surrounding sperm as occasioned by ejaculation into the vagina “activates” sperm, temporarily making them frenetic and therefore enabling them to acquire the necessary oomph to penetrate the cervix and reach the fallopian tubes. “In our view,” write the authors, “descended scrotal testicles evolved to both capitalize on this copulation/insemination contingent temperature enhancement and function to prevent premature activation of sperm by keeping testicular temperatures below the critical value set by body temperatures.”

One of the things you may have noticed in your own genitalia or those of someone youre especially close to is that in contrast to the slackened scrotal skin accompanying flaccid, nonaroused states, penile erections are usually accompanied by a telltale retraction of the testicles closer to the body. (This is the sort of thing easiest to demonstrate using visual illustrations, and a quick Google image search should provide ample examples. Just choose your own search terms and disable “safe search”—though if youre out in public right now, you may want to save this as homework for later.) According to Gallup and his coauthors, this is another smart scrotal adaptation. Not only does the cremasteric reflex serve to raise testicular temperature, thus mobilizing sperm for pending ejaculation into the vagina, but (added bonus) it also offers protection against possible damage to too-loose testicles resulting from vigorous thrusting during intercourse.

There are many other ancillary hypotheses connected to the activation hypothesis as well. For example, the authors ponder whether humans well-documented preference—and one rather unique in the animal kingdom—for nighttime sex can be at least partially explained by temperature-sensitive testicles. Although the authors note the many additional benefits of nocturnal copulation (such as accommodating clandestine sex or minimizing the threat of predation), this preference may also reflect a circadian adaptation related to descended scrota. Given that our species evolved originally in equatorial regions where daytime temperatures often soared above body temperature, optimal testicular adjustments would be difficult to maintain in such excessive heat. In contrast, ambient temperatures during the evening and at night fall below body temperature, returning to ideal thermoregulatory conditions for the testes. Additionally, after nighttime sex the female partner is likely to sleep, thus remaining in a stationary, often supine position that also maximizes the odds of fertilization.

Although the activation hypothesis helps us to better understand the functional, if quirky, architecture of the human male gonads, it may still seem odd to you that nature would have invested so heavily in such a precipitously placed genetic bank. After all, were still left with the curious fact that these precious gametes are literally hanging in the balance in a completely unprotected vessel. Gallup and his coauthors weigh in on this, too:

Any account of descended scrotal testicles must also address the enormous potential costs of having the testicles located outside the body cavity where they are left virtually unprotected and especially vulnerable to insult and damage. To be consistent with evolutionary theory the potential costs of scrotal testicles would have to be offset not only by compensating benefits (e.g., sperm activation upon insemination), but one would also expect to find corresponding adaptations that function to minimize or negate these costs.

Enter pain. Not just any pain, but the unusually acute, excruciating pain accompanying testicular injury. Most males have some horrific stories to tell on this score—whether it be a soccer ball to the groin or the flailing foot of a sibling—but all of us men have something in common: weve all become extraordinarily hypervigilant against threats to the welfare of our scrotal testicles. According to the authors, the fact that males are so squeamish and sensitive regarding this particular body part can again be understood in the context of evolutionary biology. If youre male, the reason you probably wince more when you hear the word “squash” or “rupture” paired with “testicle” than you do with, say, “arm” or “nose” is that testicles are disproportionately more vital to your reproductive success than these other body parts. I, for one, had to pause to cover myself even typing those words together.

Its not that those other body parts arent adaptively important or that it doesnt hurt when theyre injured. Rather, its a question of the degree of pain. Variation in pain sensitivity across different bodily regions, according to this view, reflects the vulnerability and importance that different adaptations play in your reproductive success. Many children have been born of broken-nosed men, but not a single one has ever been sired by a man with two irreparably damaged testicles. The point is that male ancestors who learned to protect their gonads would have left more descendants, and pain is a pretty good motivator for promoting preemptive defensive action. Or to think about it another way: any male in the ancestral past who was oblivious to or freakishly enjoyed testicular insult would have been quickly weeded out of the gene pool.

The wonders of the cremasteric muscle dont end here. It also flexes in response to threatening stimuli, in effect pulling the testicles up closer to the body and out of harms way. In fact, the authors point out, Japanese physicians have been known to apply a pinprick to the inner thigh of male patients as a surgical prep: if the patient displays no cremasteric reflex, the spinal anesthesia has kicked in, and hes ready to go under the knife. Other evidence suggests that fear and the threat of danger trigger the cremasteric reflex. There are a number of ways to test this at home, if youre so inclined. Just make sure the owner of the fearfully reflexive testicles knows what youre up to before frightening him.

So, there you have it—an evolutionarily informed account of descended scrotal testicles in humans. Is the whole thing nuts? Dont leave me hanging, folks. Balls in your court.


Copyright © 2012 by Jesse Bering

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abeck01, August 5, 2012 (view all comments by abeck01)
What a delicious surprise! Jesse Bering, Ph.D., a frequent contributor to Scientific American and an evolutionary psychologist, delivers a cheeky but science-based look at a variety of quirky subjects that many of us have been curious about but have, ahem, cough, um, er, been reluctant to share outside of our consciousness. Among the subjects he tackles with good humor from an evolutionary perspective include premature ejaculation, why humans are the only species to masturbate, why public hair has different characteristics than other body hair, the shocking contents of human semen and their evolutionary purposes, why only humans get acne, the roots of teenage behavior and the deep brain origins of fetishes. The book is funny, affirming and very easily understood, and should appeal to a lay reader interested in the evolutionary patterns of human sexual response and gender roles. He also somewhat somberly explores the notion of suicide and questions whether or not it could be a genetic adaptation. Yes, this is another book that explores and furthers recent brain research that questions the role of free will and consciousness.
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Bering, Jesse
Scientific American
Anthropology - General
Life Sciences - Evolution
Health and Medicine-Medical Specialties
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Product details 320 pages Scientific American / Farrar, Straus and Giroux - English 9780374532925 Reviews:
"Publishers Weekly Review" by , "Bering, a research psychologist, columnist for, and contributor to Slate and NPR, gathers 30 eclectic essays from Scientific American and Slate that address numerous human activities, such as human sexuality, with an evolutionary spin. He explains, among others, why men masturbate (to expel older sperm that might have less power to impregnate than newer sperm); the prevalence of sexual fantasies ('everyone tends to imagine someone — or something — else when they are having sex with their partner'); the evolutionary advantages of premature ejaculation; and the relationship between a woman's orgasms and her partner's status and looks. Bering is often personal, and always takes a wry approach to his topics, but despite the inherent prurient interest that invariably attaches to human sexuality, he is a rationalist and his insights and opinions are always supported by science. And he isn't all about sex: his essays tackle the 'adaptive' aspect of suicide, the penchant for believing Christians to contribute more to charity on Sundays than on other days, and even free will. These entertaining essays offer a cornucopia of ideas that will reward readers with hours of conversational gambits. Agent: Peter Tallack, the Science Factory. (July)" Publishers Weekly Copyright PWxyz, LLC. All rights reserved.
"Review" by , "If David Sedaris were an experimental psychologist, he'd be writing essays very much like these. Bering's unique blend of scientific knowledge, sense of humor, intellectual courage, and pure literary skill is immediately recognizable; no one writes quite the way Bering does. Read this book. You'll learn, laugh, and then learn some more."
"Review" by , "Jesse Bering is the Hunter S. Thompson of science writing, and he is a delight to read — funny, smart, and madly provocative.
"Review" by , "Jesse Bering is the intellectual spawn of Helen Fisher and Oliver Sacks, and Why Is the Penis Shaped Like That? is brainy, informative, compassionate — and hilariously naughty."
"Review" by , "Anyone interested in reading about the latest developments in sex research told with a generous dose of self-deprecating humor will enjoy this essay collection."
"Review" by , "Nothing sacred is spared in Jesse Bering's deft, rivetingly informative, and relentlessly hilarious new book. Bering's addictive curiosity and wry, dexterous humor make this a collection that's as funny as it is impossible to put down."
"Review" by , "Bering has an uncanny way with words, an incisive capacity for logical thinking, and a stunning talent for breathing new life and enthusiasm into science."
"Synopsis" by ,
Why do testicles hang the way they do? Is there an adaptive function to the female orgasm? What does it feel like to want to kill yourself? Does “free will” really exist? And why is the penis shaped like that anyway?

     In Why Is the Penis Shaped Like That?, the research psychologist and award-winning columnist Jesse Bering features more than thirty of his most popular essays from Scientific American and Slate, as well as two new pieces, that take readers on a bold and captivating journey through some of the most taboo issues related to evolution and human behavior. Exploring the history of cannibalism, the neurology of people who are sexually attracted to animals, the evolution of human body fluids, the science of homosexuality, and serious questions about life and death, Bering astutely covers a generous expanse of our kaleidoscope of quirks and origins. 

     With his characteristic irreverence and trademark cheekiness, Bering leaves no topic unturned or curiosity unexamined, and he does it all with an audaciously original voice. Whether youre interested in the psychological history behind the many facets of sexual desire or the evolutionary patterns that have dictated our current mystique and phallic physique, Why Is the Penis Shaped Like That? is bound to create lively discussion and debate for years to come.

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