Media playback is unsupported on your device Media caption Dr David Jacoby: "Numbers of this species are extremely sparse now"

Seven sawfish in Florida have become the first virgin-born animals ever found in the wild from a sexually reproducing species.

The discovery suggests that such births may be a natural response to dwindling numbers, rather than a freak occurrence largely seen in captivity.

It was made by ecologists studying genetic diversity in a critically endangered species of ray.

They say that births of this kind may be more common than previously thought.

The findings appear in the journal Current Biology.

Surprisingly common?

There are many species, particularly invertebrates, that naturally reproduce alone; some types of whiptail lizard, meanwhile, are bizarrely all-female.

But for an animal that normally reproduces by mating, a virgin birth is an oddity.

And yet a number of captive animals have produced virgin births. This roster of surprise arrivals includes sharks, snakes, Komodo dragons and turkeys - all species that normally use sexual reproduction.

And in 2012 a US research group reported two pregnant pit vipers, caught in the wild, each gestating baby snakes (inside eggs) that appeared to be fatherless.

But the smalltooth sawfish, a strange-looking beast that grows up to four metres long, is the first sexually reproducing species whose virgin-born babies have been found roaming free and healthy in their native habitat.

Andrew Fields, a PhD student at Stony Brook University in New York and the study's first author, said the find was entirely unexpected. It came during a survey of the sawfish population in the estuaries of southwest Florida.

"We were conducting routine DNA fingerprinting of the sawfish found in this area in order to see if relatives were often reproducing with relatives due to their small population size," Mr Fields said.

"What the DNA fingerprints told us was altogether more surprising: female sawfish are sometimes reproducing without even mating."

Image copyright Getty Images Image caption The smalltooth sawfish, seen here in an aquarium, is critically endangered

Of the 190 individual sawfish that Mr Fields and his colleagues surveyed, seven had DNA that indicated they only had one parent. Specifically, these seven historic fish had identical copies of at least 14 of the 16 genes that the scientists looked at; if they had arisen from normal sexual reproduction, the team calculated that the chance of the animals being "homozygous" for all those genes was less than one-in-100 billion.

So they concluded that the seven sawfish - all of them female, five of them sisters - were produced by "parthenogenesis": a process by which an unfertilised egg develops into an embryo. Researchers believe this takes place in vertebrates when the egg absorbs an identical sister cell.

Because the resulting offspring have much less genetic diversity than normal sexual offspring, their chances of survival are usually thought to be very low. But the seven fish in Mr Fields' study were up to one year old, normal in size and apparently getting on fine.

No miraculous recovery

"Occasional parthenogenesis may be much more routine in wild animal populations than we ever thought," said Dr Kevin Feldheim of the Field Museum of Chicago, a co-author of the study.

The researchers suggest it might be a last-ditch evolutionary strategy that takes hold when a population goes through an extremely lean patch - such as that presently faced by the smalltooth sawfish, whose numbers have plummeted to less than 5% of what they were a century ago.

But for this to make sense, the "parthenogens" themselves would have to be fertile, so that they could help the species to bounce back. It is too early to know whether that is the case for the seven sawfish.

Image copyright Florida Fish and Wildlife Conservation Commission Image caption The researchers tagged and released 190 sawfish in the study

Dr David Jacoby is a behavioural ecologist and marine biologist at the ZSL Institute of Zoology. He told the BBC that the Florida findings were "interesting and groundbreaking", particularly with regard to the question of whether virgin births are a natural adaptation.

"We kind of associate parthenogenesis with invertebrates: corals or crustaceans or things things like that. There are instances of this happening in vertebrates - birds and reptiles, and some shark species - but it's all been in captivity.

"The fact that this has not only been inferred in the wild, but also in a species that is seeing critically low levels of population - it definitely raises the question as to whether this is a strategy which has evolved."

But Dr Jacoby cautioned, as did the researchers themselves, that although this remarkable ability might slow the demise of the sawfish, it is unlikely to halt it altogether.

"It doesn't seem as though this type of adaptation is going to be a way of restoring population levels," he said. "It's not a get-out clause."

They might have produced a natural miracle, but the sawfish of the Florida estuaries still need human help to avoid extinction.

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