The birth took place back in 2012, but was just announced recently as the zoo did some analysis behind-the-scenes. Like many snakes, female reticulated pythons can store sperm for lengthy periods of time, so the folks at Louisville wanted to confirm that the babies weren't the result of some long-ago coupling. In the end, DNA analysis verified that the little pythons were pure Thelma. For the first time ever, parthenogenesis had been confirmed in the world's longest snake.
Parthenogensis ("virgin birth") is the process by which a female gives birth to offspring that are not fathered by a male. Many of the cases of it are reported in zoos, aquariums and laboratories - not that it is more likely to happen there then in the wild, but in those controlled conditions, it is easier to verify that no male was present and to do DNA tests on the offspring. The phenomena has been observed in many species, including many that normally reproduce sexually. Examples include Komodo dragons, zebra finches, and numerous sharks. It has never naturally been observed to occur in mammals. In some populations of lizard, parthenogensis is the only way in which reproduction has been observed to occur - the populations are all female!
It's uncertain how or why exactly these virgin births occur to some animals but not others, even within the same species. Hopefully, data collected at the Louisville Zoo and other institutions that have documented these incredible births will be able to shed a little more light on these occurrences. Until then, congratulations to Louisville on their fascinating discovery!
A reticulated python (but not Thelma... or one of her miraculous offspring)