An 11-year-old reticulated python named Thelma has given birth to six baby snakes without mating with a male. This is the first time a virgin birth has been documented in this species, Malayopython reticulatus, which holds the world record for “longest snake ever” in captivity.

The 200-pound, 20-foot (6-meter) Southeast Asia native has been housed at the Louisville Zoo in Kentucky for four years without a male partner. Her vivarium-mate is another female, Louise. In the summer of 2012, Thelma laid a clutch of 61 eggs, and she spent two weeks coiled up and brooding them before zoo staff removed the eggs for examination.

“It is not uncommon for a snake to lay infertile eggs, so the staff was surprised when the eggs appeared to be full and healthy instead of shrunken and discolored shells, typical of infertile reptile eggs,” zoo curator Bill McMahan says in a news release. They artificially incubated some of the clutch.

On September 12, 2012, the first of six healthy reticulated python babies hatched. Their average weight was 5.23 ounces (148.3 grams). After the mother and all six of her offspring finished a shedding cycle, the shed skin was collected and analyzed by University of Tulsa’s Warren Booth and colleagues. Genetic tests and molecular analyses revealed that all six baby snakes were produced by their mother alone—without any male genetic contribution. Zoo staff previously thought that Thelma held onto sperm from much earlier. "We had attributed it to stored sperm," McMahan tells National Geographic. "I guess sometimes truth is stranger than fiction."

Asexual reproduction (called parthenogenesis) occurs naturally in many insects and worms, and it’s even the norm for some vertebrates like lizards. Facultative parthenogenesis, on the other hand, is extremely rare in the wild. That’s when females who normally reproduce sexually start having babies without males. These sorts of virgin births have previously been documented in sharks, birds, lizards, and snakes living in zoos and other captive settings, but it was observed in the wild for the very first time just a couple of years ago. Booth’s team described these first cases of wild facultative parthenogenesis in two vipers—the copperhead and the cottonmouth—in 2012.

In Thelma’s case, cells called polar bodies (which contain genetic material leftover from egg cell formation) basically took the place of sperm. Normally they just disappear, but here they fused with the egg, restoring the proper amount of chromosome sets and triggering cell division. This process, called terminal fusion automixis, produces “half-clones” of the mother. And it occurred in all known facultative parthenogenesis snake cases, with one exception: a captive Burmese python once produced full clones. These findings were published in Biological Journal of the Linnean Society earlier this summer.

"It's something we used to consider an evolutionary novelty," Booth tells National Geographic, "that's much more common than we thought." Zookeepers suspect Thelma’s cushy living conditions triggered the virgin birth. Thelma's currently on exhibit in the HerpAquarium, and here’s footage of her and her babies:

Image: Mariluna via Wikimedia CC BY-SA 3.0