No-dad dragons: Komodos reproduce without males.

Two female Komodo dragons in zoos have startled their keepers by laying viable eggs without any contribution from males.

The world's largest lizard species had previously been observed to reproduce only in the usual morn-and-pop way, explains Kevin Buley of the Chester Zoo in England. So, he and the staffat the London Zoo were surprised when, at each institution, a female with no access to males managed to have offspring. Genetic tests have verified that each female was the sole parent of her clutch, Buley and his colleagues report in the Dec. 21 Nature.

Solo moms have turned up in only 70 vertebrate species. Mammals never reproduce this way, according to the scientific literature. A few reptiles, amphibians, fish, and birds manage some variation on single-parent reproduction. The Komodo dragon "is certainly the largest," says Buley.

Female Komodo dragons without males around have been known to lay infertile eggs. However, in May, one of the Chester Zoo's Komodo dragons laid a clutch that 'looked really good" says Buley, "On a whim, we put them in an incubator." Three eggs collapsed, and when staff members opened them, "to our amazement, we found blood vessels and small embryos," he says.

Flora, the Chester Zoo's new morn, had never been housed with a male. Buley sent tissue samples from Flora's embryos to the University of Liverpool. There, coauthor Phillip Watts and his colleagues found that although the embryos weren't exact replicas of the mother, only her genetic material had contributed to them. The doubling of sex chromosomes that occurs in this kind of asexual reproduction creates males among reptiles.

The remaining eight of Flora's eggs lie in the Chester Zoo's incubator. Buley has calculated that they could hatch sometime from late December to next February.

London Zoo keepers also had a surprise when their Komodo dragon Sungai laid eggs in August 2005. She hadn't been near a male for 2 1/2 years. Four of the eggs hatched this year into healthy little Komodos. Many reptiles can store sperm for several years, but tests in Liverpool confirmed that Sungai was indeed the sole parent.

After Sungai had produced her solo clutch, keepers introduced her to a male, and she produced a clutch the usual way.

"There's a lot to think about here," says Charles Cole of the American Museum of Natural History in New York City. He has studied other lizards that don't need males to reproduce, but he says that he's never encountered a vertebrate species in which the females switch to asexual reproduction in a pinch.

The power to change reproductive modes according to the company one keeps might enable a lone female to colonize new territory, he notes, although she'd eventually have to mate with her sons.

Cole agrees with concerns about conservation expressed by the report's authors. Only 4,000 Komodo dragons remain, and zoo breeders don't want to lose genetic diversity through single-parent reproduction.

The Komodos' one-parent eggs are "fascinating" says Robert Vrijenhoek of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. He has studied various nonstandard-reproduction modes in fish.

The example closest to the Komodos', he says, is the occasional fatherless bird among turkeys. Poultry researchers tried to harness this trait to create economically efficient, female-only superstrains. The project ended when the researchers realized that their superfemales produced only sons.