do your balls hang low? here's why...

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.

The opening line of Gallup's new article helps readers to appreciate the oddity of the scrotum:

It is almost unthinkable to ask why ovaries do not descend during embryological development and emerge outside the female’s body cavity in a thin, unprotected sack…

After you’ve finished exorcising that unsettling image from your mind, consider that the dangling gonads of many male animals....: "

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