Researchers say they've seen the first documented case of virgin birth in the world's longest species of snake, as a 20-foot reticulated python at the Louisville, Ky., zoo gave birth to six offspring -- without the benefit of male company.

The 11-year-old, 200-pound python dubbed Thelma had never had a male anywhere near her during her 4 years at the facility, zoo officials said.

She has lived all that time with another female python named -- what else? -- Louise.

DNA evidence confirmed Thelma was the single parent of the offspring, they said.

"We didn't know what we were seeing," zoo curator Bill McMahan said. "We had attributed it to stored sperm. I guess sometimes truth is stranger than fiction."

The phenomenon of fatherless reproduction in a species that usually requires two parents -- male and female -- is known as parthogenesis.

It happens when cells known as polar bodies, which are produced along with female's egg but which normally die off or disappear, behave like sperm cells and fuse with an egg.

That allows an embryo to develop and grow without fertilization.

A number of snake species have been found to be capable of parthogenesis but it had never before been witnesses in reticulated pythons, says Warren Booth, a University of Tulsa biologist who is lead author of a study reporting the virgin birth.

"Pythons are an old, ancient species," he says. "We've seen this in more advanced species like garter snakes."

The evolutionary origins of parthogenesis remain a mystery, he says, although geographic isolation from males may be a factor.

While Thelma's offspring are healthy so far at the zoo, Booth says he doubts if they would survive living in the wild, since parthogenetic births result in animals that are "highly inbred and often die early."

In addition to reptiles, other species including sharks and birds have been found to experience virgin births, researchers say.

It is also found in many plants and some invertebrate species.

However, naturally occurring parthogenesis in mammals in the wild is unknown, although it has been induced in laboratory animals.

"It's something we used to consider an evolutionary novelty," Booth says, "that's much more common than we thought."

While there are some species that reproduce exclusively through parthenogenesis, much more common are species that can switch between sexual reproduction and parthogenesis.

Such switching can be brought on by seasonal changes, lack of available males, or stresses brought on by changes in population numbers.

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