Why do human testicles hang like that?

Earlier this year, I wrote a column about evolutionary psychologist Gordon Gallup’s “semen displacement hypothesis,” a convincing hypothesis presenting a very plausible, empirically supported account of the evolution of the peculiarly shaped human penis. In short, Gallup and his colleagues argued that our species’ distinctive phallus, with its bulbous glans and flared coronal ridge, was sculpted by natural selection as a foreign sperm-removal device. As a companion piece to that work on our phallic origins, Gallup, along with Mary Finn and Becky Sammis, have put forth a related hypothesis in this month’s issue of Evolutionary Psychology. This new hypothesis, which the authors call “the activation hypothesis,” sets out to explain the natural origins of the only human body part arguably less attractive than the penis--the testicles.

In many respects, the activation hypothesis serves to elaborate what many of us already know about descended scrotal testicles: that they serve as a sort of “ cold storage” and production unit for sperm, which keep best at lower body temperatures. But it goes much further than this fact, too. 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 jumpstarts sperm that have been hibernating in the cool, airy scrotal sack. Yet it explains many other things too, including why one testicle is usually slightly lower than the other, why the skin of the scrotum becomes more taut and the testicles retract during sexual arousal, and even why testicular injuries--compared to other types of bodily assault--are so excruciatingly painful to men.

<friendly snip - and I cringe as I write in this context >

Thus, 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, evolutionary speaking. For example, modern elephants’ testicles remain undescended and are deeply embedded in the body cavity (a trait referred to as “testicond”), whereas other mammals, such as seals, have descended testicles but are ascrotal, with the gonads simply being subcutaneous.

Although descended scrotal testicles do satisfy the obvious criterion of being counterintuitively costly, the authors conclude that handicapping is an unlikely explanation. If it were true, we would expect to see scrotal testicles becoming increasingly elaborate and dangly over the course of evolution, not to mention women should display a preference for males toting around the most ostentatious scrotal baggage. “With the possible exception of colored male scrota among a few species of primates,” write Gallup and his colleagues, “there is little evidence that this has been the case.” I’m not aware of any studies on intra-species 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 than serving as an aphrodisiac.

http://www.scientificamerican.com/blog/post.cfm?id=why-do-human-testicles-hang-like-th-2009-11-19

Valk would like all men to send her pictures of their scrotums, so that she may judge and give awards to the biggest and best Roll floor laugh xD

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