You only think self-mating sounds trouble-free

Science may have found an answer to that question made famous by Maureen Dowd: Are men necessary?

If other creatures are anything to go by, men are essential to keeping our species healthy, and would still be necessary even if women started making their own sperm. That's more or less what happened to the male clam shrimp. One day millions of years ago, a mutant female started producing both sperm and eggs; that mutation spread until all the females were replaced by hermaphrodites. You'd think that would spell doom for the males.

Hermaphrodites also dominate one of our fellow vertebrates, a swamp dweller called a killifish. And yet male killifish crop up here and there.

Are these males in any way contributing to the gene pool? Are they necessary?

An even more serious question is why all animals don't become hermaphrodites, with its obvious perk of increased sexual opportunity. If you want to colonize new ponds or tide pools, say, it's advantageous to be able to mate with anyone of your species, including, in a pinch, yourself.

It works for some corals, shellfish, worms and plants. But there are drawbacks.

"In some ways, it is a matter of 'jack of all trades is a master of none,' " says Stephen Weeks, an evolutionary ecologist from the University of Akron, Ohio. The hermaphroditic shrimp he studies, for example, can't mate with each other. Only the males come equipped with a clawlike clasper for positioning the hermaphrodite so its eggs meet his sperm at the right time.

Without the males, the shrimp are stuck mating with themselves, says Weeks, and that's a problem. "That's often associated with something called inbreeding depression, which is, basically ... have you seen 'Deliverance'?"

(In the classic '70s film, it's implied that inbreeding led to defects suffered by a mute banjo-playing savant.)

You can't get more incestuous than sex with yourself. Unlike asexual reproduction, which creates genetically identical clones, sex with yourself can double up copies of deleterious genes.

Say you're a hermaphrodite and you carry a gene for banjo-savant syndrome, or BSS. You need two copies of the mutant BSS gene to have the disease, and your cells carry just one, plus a normal copy.

But you divide your genes in half to make sperm, so half your sperm gets a bad copy of the BSS genes. The same thing happens in your eggs. That means one of every four of your offspring will get two bad BSS genes and come out banjo savants. And if they're fertile, so will their offspring.

Preventing this kind of thing is the job of the male clam shrimp, and apparently has been for some time. All of the more than 30 known species of this creature mix hermaphrodites and males, suggesting they inherited this sexual strategy from a common ancestor before they diverged.

Genetic diversity also keeps the male killifish in business despite the challenge of having to mate with hermaphrodites — and mean ones, at that. When you put killifish together in a tank, the hermaphrodites tend to eat the males, says biologist John Elder of Valdosta State University in Georgia.

But in the wild, some obviously mate and escape, at least in Belize, where DNA tests show that males inject much-needed genetic diversity into the pool. For years, the scientists weren't finding males among Florida's killifish, but recent genetic tests reveal the Floridian fish are too genetically diverse to be having sex just with themselves.

The hermaphrodite fish might be mating with one another, Elder said. And it might be that males not yet spotted are mixing up the genes.

In which case those male killifish, like male humans, are indeed necessary.

Faye Flam's Carnal Knowledge column appears Sundays in The Seattle Times