Why are testicles kept in a vulnerable dangling sac? It’s not why you think.

Soccer fans call it brave goalkeeping, the act of springing into a star shape in front of an attacker who is about to kick the ball as hard as possible toward the goal. As I shuffled from the field, bent forward, eyes watering, waiting for the excruciating whack of pain in my crotch to metamorphose into a gut-wrenching ache, I thought only stupid goalkeeping. But after the fourth customary slap on the back from a teammate chortling, “Hope you never wanted kids, pal,” I thought only stupid, stupid testicles.

A vervet monkey's neon scrotum  

Courtesy of Gijs Joost Brouwer

Natural selection has sculpted the mammalian forelimb into horses’ front legs, dolphins’ fins, bats’ wings, and my soccer ball-catching hands. Why, on the path from the primordial soup to us curious hairless apes, did evolution house the essential male reproductive organs in an exposed sac? It's like a bank deciding against a vault and keeping its money in a tent on the sidewalk.

Some of you may be thinking that there is a simple answer: temperature. This arrangement evolved to keep them cool. I thought so, too, and assumed that a quick glimpse at the scientific literature would reveal the biological reasons and I’d move on. But what I found was that the small band of scientists who have dedicated their professional time to pondering the scrotum’s existence are starkly divided over this so-called cooling hypothesis.

Reams of data show that scrotal sperm factories, including our own, work best a few degrees below core body temperature. The problem is, this doesn’t prove cooling was the reason that testicles originally descended. It’s a straight-up chicken-and-egg situation—did testicles leave the kitchen because they couldn't stand the heat, or do they work best in the cold because they had to leave the body?

Vital organs that work optimally at 98.5 degrees Fahrenheit get bony protection: My brain and liver are shielded by skull and ribs, and my girlfriend’s ovaries are defended by her pelvis. Forgoing skeletal protection is dangerous. Each year, thousands of men go to the hospital with ruptured testes or torsions caused by having this essential organ suspended chandelierlike on a flexible twine of tubes and cords. But having exposed testicles as an adult is not even the most dangerous aspect of our reproductive organs’ arrangement.

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