Platypus Genes Hint at Human Scrotum Origins

May 8, 2008— -- 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.