How Are They Hanging? This Is Why They Are   A few years ago, the evolutionary psychologist Gordon Gallup, whom we'll 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 woman's 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 elephant's 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 you've wiped that image from your mind's 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 doesn't 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." I'm not aware of any studies on intraspecies individual variation in scrotal design, but I'm 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 it's adaptive for sperm to be hyperactive, sperm are stored and produced most efficiently in the cool, breezy surroundings of the relaxed scrotal sac.

One doesn't want his scrotum to be too cold, however, since nature has calibrated these temperature points at precisely defined optimal levels. Fortunately, human scrota don't 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. It's also why it's 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 it's no longer 1988. Now, I know what you're 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 that's 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 Gallup's 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 you're especially close to is that in contrast to the slackened scrotal skin accompanying flaccid, nonaroused states, penile erections are usually accom.