For flatworms, "Who's your daddy?" is a loaded question. In a bizarre bout lasting up to an hour, the first flatworm to stab and inseminate its mate becomes the father.

Flatworms have both male and female sex organs.

But when it comes time to make little flatworms, they have to decide who plays which role...and that they fight over.

The two flatworms rear up, exposing their midsections.

Those jutting white nibs aren't weapons-at least, not intentional ones.

They are actually the worm's penises-a double-barreled inseminator-if they can shoot first.

Flatworm sex consists of the two attempting to stab their lover with their pointed pair and inseminate each other...an act delicately referred to as "penis-fencing."

But there's nothing delicate about it.

Flatworms have been known to gouge holes in each other in battles lasting up to an hour.

Why do they cross swords? Producing offspring requires a lot of energy.

The winning father can go out and about in the world without any further responsibility.

The losing mother absorbs the winner's sperm.

It must now work longer and harder to find food and stay alive.

The father floats merrily along. The mother crawls along looking for a meal, already the responsible one.