Earlier this year, staff at the New England Aquarium in Boston noticed that one of their anacondas was in the process of giving birth. However, something didn’t quite add up – the Aquarium only has female anacondas.

DNA testing has now shown that this was not an administrative mess-up, divine intervention, or any sneaky behavior from a lothario snake. It was actually the result of an unusual non-sexual reproduction strategy called parthenogenesis, meaning the two newborn snakes that survived are perhaps clones of their mother.

The mother anaconda is known as Anna, an 8-year-old, 3-meter-long (10 feet) adult weighing about 13 kilograms (30 pounds). She’s a green anaconda (Eunectes murinus), native to the tropical rainforest of South America, and is the heaviest and one of the longest known living species of snake.

In January this year, she gave birth to a number of young babies. This species is ovoviviparous, meaning the females giving birth to live young, as opposed to eggs. Most of these young were stillborn and another died after a few days, which is pretty common in parthenogenesis. However, two juveniles managed to survive against the odds.

But how did this happen in a girls-only enclosure?

“Ruling out a delayed embryo implantation was on the list, if there had been previous exposure to a male. But Anna’s life history was well known. She had been born in the care of a certified reptile organization and had come to the Aquarium as a very young animal with no exposure to adult males,” New England Aquarium said in a blog post.

“The ruling out process had eliminated any other suspects but parthenogenesis, yet testing the DNA was the only way to have a positive confirmation. Aquarium veterinarians sent off tissue samples for analysis,” it confirmed.

Just as the aquarium staff speculated, genetic sequencing on the two young snakes showed they were genetic copies or clones of the mother, strongly suggesting they were born via parthenogenesis. Stemming from the Greek words for “virgin birth,” parthenogenesis is a relatively common process of several insect species and plants. It’s also not unheard of among a select handful of vertebrates, including some species of lizards, snakes, rays, sharks, and even birds.

In simplest terms, it explains the development of a female egg without any genetic contribution from a male. The mechanisms of this can vary from species to species and are not fully understood, although some snakes are known to achieve parthenogenesis by "replacing" the role of males’ sperm with polar bodies, a small cell that contains genetic material leftover from egg cell formation.

However, animals will only generally use this in a last-ditch attempt to reproduced if a female doesn’t come into contact with a male for an extended period because it can lead to a low genetic variation among a population and comes with many risks.

In Anna's case, the risk seems to have paid off. The two offspring are currently being cared for behind the scenes and appear to be doing well.