Hermaphrodites duel for manhood.

Talk about a battle of the sexes. Researchers have found hermaphroditic flatworms that rear up, expose their stubby penises, and literally duel.

In bouts that can last 20 minutes to an hour, marine worms of the suborder Cotylea feint and writhe for position. Each attempts to stab its penis into an exposed area of its sex partner's body while avoiding getting jabbed itself.

A worm that scores a hit injects sperm into whatever region of flesh it penetrates, report Nicolaas K. Michiels of the Max Planck Institute for Behavioral Physiology in Starnberg, Germany, and Leslie J. Newman of the Smithsonian Institution in Washington, D.C. After the strike, sperm stream through the partner's body tissue, creating pale streaks like lightning jags on their way to fertilizing eggs in the ovaries.

Researchers knew that these flatworms inject sperm into random areas of flesh, but "penis fencing is new," Newman says. No one had described the dueling until she and Michiels spent some 20 hours continuously watching pairs of captured worms mating in old ice cream containers. The details for one moderately aggressive species appear in the Feb. 12 Nature.

"It's better to stab than to be stabbed," says Newman. The stabber fathers offspring without the energy drain of healing torn flesh or producing eggs.

"The interesting thing," says Michiels, "is that hermaphroditic partners run into conflicts because they usually have identical but incompatible interests." People may not realize how simple many human sexual conflicts are in the grand scheme of nature. "Once a male and a female decide to mate, there is no discussion about who will give and who will receive sperm," Michiels says.

The dueling worms illustrate one extreme of hermaphroditic difficulties--both partners vying for the male role--but other flatworms have the opposite problem. "Individuals have to `beg' to receive sperm," Michiels says. In these species, hermaphrodites project what looks like a penis but reaches out to a partner's male organs and withdraws sperm.

Hermaphrodites, which share the same sexual interests and strategy, may be more likely to evolve physically damaging sex, Michiels speculates. For the marine flatworms, particularly aggressive duelers may produce more offspring than so-so stabbers. "It results in a kind of escalation," he says.

In two-sex species, the females have a strong interest in not getting their flesh ripped to pieces during mating and so may avoid injurious males. Natural selection will favor males that are "well-behaved," Michiels says.

Jabbing sperm directly into flesh may have developed to circumvent female devices to control fertilization or wrest some unintended benefit from it, Michiels speculates. A number of species with set routes for sperm have special female adaptations so that "most of the sperm go straight into the gut."

Michael Siva-Jothy of the University of Sheffield in the United Kingdom says that among species studied so far, "I think the dueling and the overtness of the dueling are quite unusual." He adds that he would not be surprised if the worm report inspired closer observations of other hermaphrodites, whose romances may turn out to be just as weird.

Another specialist in mating conflicts, William G. Eberhard of the University of Costa Rica in San Jose and the Smithsonian Tropical Research Institute in Panama City, Panama, urges researchers to follow the sperm. He'd like to know whether the stabbed worm digests or otherwise manipulates the sperm it receives. He warns against a common bias: "Females are generally taken as relatively passive."

Eberhard also points out the difficulty of untangling the interests of the sexual combatants. Is a dodging partner just filtering out lousy duelers? Then, he says, the traditional battle of the sexes becomes "selective surrender."