Dec. 20 (Bloomberg) -- Komodo dragons, the world's largest lizards, are capable of virgin births, scientists said after two female specimens laid fertile eggs without mating.

The dragons, the only two sexually mature females in European zoos, laid fertile eggs within a year of each other, U.K. scientists say in tomorrow's edition of the journal Nature. One of them had never been in contact with a male, and the other hadn't mated for at least 2 1/2 years, they said.

The production of offspring without fertilization by a male, known as parthenogenesis, has been observed before in about 70 vertebrates -- or 0.1 percent of known species -- though never in one as large as a komodo dragon, which can grow up to 3 meters (10 feet) in length and weigh 100 kilograms (220 pounds).

``It's a complete surprise; no one had suspected it before,'' Richard Gibson, one of the paper's co-authors, said in a telephone interview from London Zoo, where one of the females laid her eggs. ``This has ramifications for the natural history of the species in the wild, and its capacity to perpetuate.''

Virgin births have been recorded previously in species of fish, reptiles and amphibians, as well as a turkey, though recorded instances are usually one-offs, according to Gibson. The discovery of parthenogenesis in two Komodo dragons suggests the phenomenon is more common, he said.

``Not only is it happening in a rare and new species that's also the world's largest lizard, it's happening in two separate, unrelated individuals in different zoos,'' Gibson, curator of amphibians and reptiles at the Zoological Society of London, said. ``That suggests parthenogenesis isn't so rare or unusual as we all thought.''

Vulnerable Species

The Komodo dragon, whose Latin name is Varanus komodoensis, is listed as ``vulnerable'' in the Red List of endangered species published by the IUCN, or International Union for Conservation of Nature and Natural Resources. That's the third-highest degree of threat for a species still found in the wild.

Sungai, an adult female dragon who was loaned to London Zoo by France's Thoiry Zoo, laid a clutch of 22 eggs in August 2005, 2 1/2 years after last living with a male, called Kimaan, according to the scientists. Four of the eggs produced offspring.

Sperm storage, whereby female animals preserve sperm after mating, is ``really common'' in reptiles such as lizards and turtles, so samples were sent to Liverpool University for genetic testing, to make sure Kimaan wasn't the father, Gibson said. The tests showed the offspring were a result of asexual reproduction.

`Immaculate Conception'

A komodo called Flora in May laid 25 eggs at Chester Zoo, northern England, according to the paper. She had been bred in captivity, and never kept with a male. Eleven of the eggs were viable, and three of those collapsed during incubation, providing material for genetic testing, which also confirmed they were the product of a sole parent, according to the paper. The remaining eggs are due to hatch next month.

``Essentially what we have here is an immaculate conception,'' Kevin Buley, a curator at Chester Zoo and co-author of the paper, said in a statement on Nature's Web site.

Found on just three islands in Indonesia, Komodo, Flores and Rinca, fewer than 4,000 dragons are thought to remain, Chester Zoo said. Their saliva contains a host of ``deadly'' bacteria, and after biting their prey, they will track it for up to two days until it dies from blood poisoning, according to the zoo.

A final discovery made by the scientists was that once females have reproduced asexually, they are able to switch back to normal sexual reproduction with males. After her parthenogenetic eggs, Sungai mated with London Zoo's male, Raja, producing a batch of normal eggs before she died in March.

Risks and Advantages

The phenomenon could have both positive and negative impacts on komodo dragons, according to the paper. While asexual reproduction reduces the genetic diversity of a species, it also means that a lone female can reproduce when males are absent, perpetuating its survival, they said.

``Inbreeding carries an associated risk of reduced fitness and an increased probability of extinction,'' the paper said. Still, ``parthenogenesis in wild komodo dragons could be adaptive, given that viable offspring are always male and that sexual reproduction can resume, albeit between related individuals, in a colony founded by a single unfertilized female.''

To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net.

Last Updated: December 20, 2006 12:56 EST