CARNAL NATION

Scientific American columnist Jesse Bering focuses today on a new hypothesis about the design of male testicles.  Evolutionary psychologists Gordon Gallup, Mary Finn and Becky Sammis have their "activation hypothesis" published in the most recent issue of the journal Evolutionary Psychology.

It has been assumed for a while now that the scrotum act as a sort of cold storage unit for sperm, which thrive at a temperature about 3 degrees lower than human body temperature.  (Thin-skinned scrotum, which dangle away from the body, are slightly cooler than the rest of the human male.) 

But Gallup and his colleagues notice that sperm become highly active for a short time when warmed slightly.  When they get near a vagina, say.  The vagina, Bering says, "radically jumpstarts sperm that have been hibernating in the cool, airy scrotal sack." 

The trouble is that the increase in activity doesn’t last for more than four hours.  But the vagina, which generally stays the same temperature, is warm enough to four hours is enough time for a sperm to reach an egg, and in terms of evolution that’s really all that matters.

The study also addresses things like why one testicle usually hangs lower than the other, why the scrotum retracts during intercourse, why men shouldn’t wear tight jeans, and why men are so squeamish about testicular injuries.   It also dismisses the idea that male testicles are flashy and ornamental, like peacock feathers, because by this point there would be a lot of flashy variation in color and size.