Researchers in the US caught pregnant females from two snake species and genetically³ analysed the litters.

 

That proved the North American pit vipers⁴ reproduced without a male, a phenomenon called facultative parthenogenesis（兼单性生殖） that has previously⁵ been found only in captive species. 

 

Scientists say the findings could change our understanding of animal reproduction and vertebrate evolution.

 

It was thought to be extremely rare for a normally sexual species to reproduce asexually.

 

First identified in domestic chickens, such "virgin births" have been reported in recent years in a few snake, shark, lizard⁶ and bird species.

 

Crucially though, all such virgin births have occurred in captivity⁷, to females kept away from males.

 

Virgin births in vertebrates in general have been viewed as "evolutionary⁸ novelties", said Warren Booth, from the University of Tulsa, Oklahoma, US.

 

Professor Booth is lead author of a paper published in the Royal Society's Biological Letters that challenges this label.

 

He and his collaborators investigated virgin births in wild populations of two geographically⁹ separated and long-studied species of snake.

 

They captured pregnant copperhead and cottonmouth female pit-vipers from the field, where males were present.

 

The snakes gave birth, allowing the scientists to study the physical and genetic² characteristics of the litters.

 

Of the 22 copperheads（铜斑蛇）, the scientists found one female that must have had a virgin birth. 

 

Another single virgin birth occurred within the 37 cottonmouth litters.

 

"I think the frequency is what really shocked us," said Prof Booth.

 

"That's between 2.5 and 5% of litters produced in these populations may be resulting from parthenogenesis.

 

"That's quite remarkable¹⁰ for something that has been considered an evolutionary novelty," he said.

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