Carnal Knowledge | Hermaphroditic worms fight the battle of 2 sexes in 1

As many a frustrated spouse will admit, males and females can differ in their desire for sex. So, what happens in animals that function as hermaphrodites, built with equipment to have sex as either males or females?

It's sad but true that when boy/girl meets girl/boy, they often engage in a brutal fight over who gets to be the boy. In their desperation to get the male position, many resort to stabbing, drugging, or even chemical weapons.

Animals in this fix are surprisingly common, says biologist Nico Michiels of the University of Tübingen, Germany. Flatworms go both ways, as do earthworms, slugs and many snails. They're called simultaneous hermaphrodites because they can play male and female roles at the same time, as opposed to animals that change sex.

Take Pseudoceros bifurcus, a little sea worm that lives on the Great Barrier Reef. When two of these meet, they start trying to stab each other with their penises. Whichever one gets stabbed first is fertilized and thereby becomes the de facto female.

"Both are trying to stab the other without being stabbed," Michiels says. Everybody wants to be the male.

That happens through a concept known as sexual conflict. Whenever it's easier to give up sperm than to deposit eggs (because sperm are smaller, for example), a hermaphrodite will succeed in the evolution game better as a he than a she. The sperm become more abundant, and the more crowded the species' environment, the cheaper sperm get and the harder the males try to unload them.

The bottom can really drop out of the sperm market when a creature's female side can store someone else's sperm. Then nobody wants to buy sperm and everybody wants to sell. And that can make the sellers aggressive.

In the polyclad flatworm, another sea-dwelling invertebrate, males squirt corrosive semen that burns holes in the flesh of other flatworms. When two of them engage in battle, the loser ends up not only pregnant but covered in big burns, Michiels says. "It can be quite severe."

In his most recent paper, Michiels describes an even dirtier tactic played by the tiny yellow sea slugs that live on the Great Barrier Reef. They try to stab each other with penises that inject an anesthetic. Once a stab is achieved, the winner's male side has sex with the loser's female side while he/she's under the influence of this built-in date-rape drug.

"It's a very interesting system because it's never clear beforehand which one will win," Michiels says. In this animal, "the penis is heavily armed with hooks and spines and anchors itself in the female genital tract." So maybe it's better to be anesthetized.

In a two-sex system, he says, the female could just run away if she didn't want to be stabbed or burned or drugged, but her male half won't let her leave, so intent is he on doing the same deed to the other one's female side. Like the single human body shared by Steve Martin and Lily Tomlin in the movie All of Me, the male and female sides don't want the same thing.

But sometimes the tables can turn. In some species, females have the power to reject a male after sex by rerouting the sperm to her stomach. That can make the male less eager to give them up.

So as a countermeasure, in one worm species, the female wields a penislike organ that stabs the male, pokes a hole in him, and uses the appendage like a straw to "suck the sperm out," Michiels says.

Sexual conflicts also surface in ordinary two-sex species, including us. It's most intense when males don't invest much in either making sperm or nurturing babies. That can turn sex into an evolutionary free lunch - you pass on your genes at no cost to you.

Could sexual conflict explain why author Joan Sewell is admitting she doesn't want sex more than twice a month with her frustrated and horny husband in I'd Rather Eat Chocolate? Possibly.

On the other hand, the rules of the game are changing fast. Contraceptives give women control over our fertility, thus making sex more attractive to us, while DNA paternity testing makes it harder for males to get that free lunch. So it's only a matter of time before some guy comes out with I'd Rather Drink Beer.