For most guys, it's just another fact of life -- but for scientists, the existence of the scrotum is something of a mystery.
Now, thanks to a new genetic study released Wednesday in the journal Genome Research, the duck-billed platypus is shedding some light where, well, the sun doesn't usually shine.
Suffice it to say, the scrotum is a crucial structure. In humans, sperm do best at temperatures a bit below 98.6 degrees. By holding the testes away from the body, the scrotal sac allows optimal development of sperm.
But the scrotum isn't exactly practical in other ways. Ask any man who's ever been kicked there. So, why didn't we just evolve lower body temperatures? Or, rather, sperm that like it hot?
Evolution is a funny thing. It builds on twists and turns, due to chance, keeping what's useful and discarding what's not. If a mutation occurs that promotes survival, it tends to be passed along. In the case of the platypus, that's led to a face like a duck and a tail like a beaver.
In humans, it's given us a high body temperature, which lets us stay active, even when it's cold outside. And it's given us the male scrotum.
So, how does a funny-looking animal from Australia help us explain human male anatomy?
The platypus shares characteristics with a number of other kinds of animals. It has milk glands, which means it's a mammal -- same as humans, dogs, cats, elephants, and so on. It has fur like most mammals, too.
But it walks like a reptile and lays eggs like a bird. And like chickens and frogs (and unlike most other mammals), it keeps its testes safely inside its abdomen. The platypus can get away with this because its body temperature is cooler than ours.
All this means is that, in many ways, the platypus is sort of a link between those other kinds of animals and us. And, in fact, scientists have been studying the platypus genome to try to understand more about how mammals evolved.
One group of researchers at Stanford University, led by assistant professor of obstetrics and gynecology Sheau Yu Teddy Hsu, has looked at what the platypus can teach us about the male anatomy.
Fish, frogs, platypuses and humans all share a common ancestor at some point in the distant past, which means we have a lot of genes in common. Mutations in those genes are why we all look so different now. But a gene can't just suddenly change its function.
"If you have only one gene, and it needs to do something, it can't change because it would change the whole cellular machinery," Hsu said.
The key turns out to be two genes that have a common origin. Sometime before frogs, platypuses and humans all evolved, that original gene was accidentally duplicated. In frogs, both copies make a similar protein. In humans, the two copies have very different roles. One of those roles is to make the testes descend into the scrotum.
But again, it isn't likely that these changes happened all at once. The genes had to change one at a time, so that their original function wasn't lost all at once.
That's where the platypus comes in. It has one gene that resembles a human one, and one that resembles the version in frogs. Basically, Hsu said, the platypus reveals the evolutionary "bridge" between frogs, birds and reptiles, which keep their testicles inside their bodies, and humans, which have them swinging outside.
And in fact, it didn't take much to get to the gene that causes testicular descent: If just one more small mutation had occurred, platypus males might have developed scrotums, just like us.
Then again, it wouldn't have done them much good: With their lower body temperature, they don't need to keep their testes cool.
But for humans, that one small change led to a whole new process -- and a way to keep the advantage of a high body temperature without losing the ability to reproduce efficiently.