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.
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?