"We didn't know what we were seeing. We had attributed it to stored sperm," the zoo's snake curator, Bill McMahan, told National Geographic. "I guess sometimes truth is stranger than fiction."
This is the first case of such a birth recorded in Thelma's species, but it's not the first seen in the animal kingdom — or even in the snake world.
In a phenomenon called parthenogenesis, females who would typically use two-parent sexual reproduction are sometimes able to accomplish the deed on their own.
Researchers say that Thelma's six female offspring were "half-clones," National Geographic reports. That means that her egg cell probably interacted with something called a polar body — a cell made up of genetic material that's left over when the egg cell forms. These polar bodies usually just die, but sometimes they can meet up with egg cells and (if they form the right number of chromosomes once they're put together) an embryo can begin to form.
Thelma's reproductive system may have triggered the rare phenomenon because of her unusually luxurious surroundings: She lived in a large habitat, surrounded by heating pads, and was fed massive quantities of food. With conditions so optimal for reproduction, what should have been a cellular fluke ended with six healthy babies.
Of course, those healthy babies are still pretty inbred — and while they're doing just fine in their zoo enclosure, their keepers don't think they'd have survived in the wild.