"Virgin birth" among animals may not be a rare, last-resort, save-the-species stopgap after all.

For the first time, animal mothers, specifically pit vipers, have been discovered spawning fatherless offspring in the wild. More to the point, the snakes did so even when perfectly good males were around.

Among vertebrate animals that normally reproduce sexually, virlgin birth, or parthenogenesis, had been observed in only captive female snakes, Komodo dragons, birds, and sharks.

Until now it's been considered an evolutionary novelty, albeit one that made a sort of sense—a way for a bloodline to continue in the absence of suitable fathers.

For the study behind the find, published in this week's issue of the journal Biology Letters, a team led by biologist Warren Booth of the University of Tulsa in Oklahoma captured pregnant copperhead and cottonmouth (see picture) females from fields where males were present. When the snakes gave birth, the researchers documented the physical and genetic characteristics of the litters.

Tests showed that 1 of the 22 copperhead mothers had given birth parthenogenetically, as had 1 of the 37 cottonmouth snakes collected—a ratio Booth finds surprisingly high.

"The fact that we find it in such small sample sizes is quite remarkable," Booth said. "What we're going to do now is go back to these populations and do sampling year-to-year to see if we can find instances of parthenogenesis again."

Virgin-Birth Mysteries

Why would female snakes undergo parthenogenesis when males are available?

One possibility: It might have been the only way they could reproduce. Booth noted that the copperhead that underwent parthenogenesis was smaller than usual—and perhaps passed over by males in favor of fitter females.

Other scientists have suggested that parthenogenesis is a kind of random reproductive mistake. Booth himself is investigating the possibility that a bacteria or virus is the trigger.

Another mystery: Each of the two virgin-birth litters consisted of a single snake. A normal copperhead litter might number between 6 and 9, while cottonmouths can spawn up to 20 offspring.

Also, the two virgin-birth baby snakes were both male. That might just seem like the luck of the draw, except that every known parthenogenetic snake offspring has been male—a certifiable scientific mystery.

"It would be interesting to see if we can find females," Booth said. "There's no reason realistically why we shouldn't find females, but in all of the [snake] species that we've looked at ... they've all been males."

It's also unknown whether animals born by parthenogenesis can reproduce normally or have virgin births themselves.

Parthenogenetic offspring often exhibit abnormalities or die early. That shouldn't surprise anyone, Booth said, since it's essentially "a severe form of inbreeding."

You're Safe for Now, Men

A virgin birth occurs when a polar body—a cell produced along with the egg—essentially functions like a sperm and "fertilizes" the egg.

As a result, the DNA of a virgin-birth offspring, or "parthenogen," doesn't perfectly match that of its parent—the offspring is a sort of half clone.

So far, parthenogenesis has only been observed among sharks, reptiles, and birds (which are closely related to reptiles). Mammals aren't thought to be capable of parthenogenesis, because their reproduction requires copies of genes from both parents.

"So no human parthenogenesis anytime soon," said Stony Brook University marine biologist Demian Chapman, who discovered virgin birth among blacktip sharks.