Thelma the snake confused then astounded her keepers.

This 6m long (20 ft) python had spent four years alone in Louisville zoo in the US, without ever having met a male of her species. But, somehow, she laid over 61 eggs, producing six healthy babies.

Perhaps she’d managed to secretively mate with a male many years before, and store his sperm all this time?

Genetic tests soon revealed the answer.

Thelma had become the first reticulated python in the world known to have had a real-life virgin birth.

She’d made eggs that contained all the genetic information required to make a daughter; without the need for a father, his sperm or DNA. She’d done it fusing her eggs with a by-product of her dividing cells, called a polar body. This object played the same role as sperm would normally, triggering the egg to develop into an embryo. Each of her offspring contained two copies of half her chromosomes. They were half-clones of Thelma.

Extreme reproduction?

Though special, we now know that Thelma and her daughters are far from unique.

Scientists are discovering that virgin births occur in many different species; amphibians, reptiles, cartilaginous and bony fish and birds and it happens for reasons we don't quite understand.

Initially, a virgin birth, also known as parthenogenesis, was thought to be triggered by extreme situations; it was only documented among captive animals, for example, perhaps by the stress, or isolation. A way to continue the bloodline when all other options had gone, when there was no other choice.

Not necessarily. It now appears that some virgin females produce offspring even in the presence of males.

What’s more, they do so in the wild, and may have been doing it for hundreds of millions of years. It may carry advantages, even more so in a modern world where populations of many species are rapidly dwindling, but it raises fundamental questions about the importance of sex.

And other uncertainties remain. Why among vertebrates, can fish, reptiles and birds have virgin births, but mammals, including humans, seemingly cannot? Even here, things aren’t straightforward…

Virgin turkeys

Perhaps the best understood ‘virgin’ vertebrate is the common domesticated turkey. In the 1800s, reports started appearing of virgin births among chickens. Then researchers started studying similar events among turkeys, finding that these large fowl could lay unfertilised eggs that produced live young.

The baby turkeys were always male, however, which was put down to a quirk of bird genetics in which male sex chromosomes are dominant. Soon a parthenogenetic strain of the domestic turkey was developed in which most males appeared normal and reproduced successfully.

The turkeys were considered nothing more than a curiosity; an artificial creation kept in artificial conditions.

But then, in the past 15 years, reports started coming in of a series of weird and wonderful virgin births occurring in captive fish, snakes and lizards.

It seems to be something a wide variety of sharks can do

On the 14 December 2001, for example, one of three captive adult female bonnethead sharks gave birth to a healthy female pup. Each of the prospective mothers had been caught as immature fish from the wild waters of the Florida Keys, US.

None had met a male shark, and all were virgins.

Yet one of them had clearly given birth, reported a team led by Demian Chapman of Stony Brook University, in New York state, US.

Later genetic tests confirmed that no males had been involved, and since then the same has been discovered in four other shark species. "It seems to be something a wide variety of sharks can do," Chapman told BBC Earth.

Giant lizards

In 2006, scientists reported that two different Komodo dragons, the world’s largest type of lizard, had also had virgin births. Both were captive, kept at separate institutions, one at Chester Zoo and one at London Zoo, in the UK.

At the time, researchers speculated that the giant lizard was capable of switching between sexual and asexual reproduction, essentially finding ways to clone itself in extreme circumstances when no males are around.

Then in recent years, scientists have also documented different snake species, including boas and pythons such as Thelma, giving birth in the absence of males.

The question is why would they bother?

A life without males

One possible answer may lie with a wild counterpart, the whiptail lizard. In fact, there are numerous species of whiptail lizard, with many being specially conceived, a result of two species hybridising to form a third.

These unique hybrid species are all female; males have been completely cut out of the reproductive process. Each female produces asexually, creating new generations of females, and so on.

Creating such an exclusive club has its evolutionary benefits; if any of these lizards were left stranded, they could continue to reproduce. Other whiptails that rely on males would see their lineage die out. This is a particular type of parthenogenesis that only occurs in the absence of males, and this may have been the trigger for these lizards. Female whiptails that become stranded on islands may have somehow switched their biology to reproduce alone.

Thelma the snake was thought to have had a virgin birth for similar reasons; without any males around she had no choice but to go it alone. And being well fed, and housed in a large enclosure at an optimal temperature, she had the optimal conditions to make the biological leap into solo parenthood, says Bill McMahon, a scientist who helped care for her.

Perhaps the same was true of the sharks, komodo dragons and snakes?

It's amazing that we do all of this work on reproductive biology and we're still learning something new about the reproductive modes about the animals around us

There is a problem with that idea. Generally, asexual reproduction is thought to come with costs. Essentially it’s the ultimate form of inbreeding – there is no way to create genetic diversity. So animals that clone themselves leave their lineages vulnerable to disease and other threats, which they lack the genetic variety to counter.

For that reason, after the virgin birth of the Komodo dragons, scientists recommended that the species, which is endangered, not be kept in isolation. They feared the genetic diversity of the species might diminish if it started cloning itself.

But in extremis, when there are no males to mate with, it makes some sense.

Wild virgins

Then came another shock: wild vertebrates, as well as captive ones, are capable of virgin births.

In 2012, scientists discovered that another type of snake, the pit-viper, commonly has virgin births in the wild.

Warren Booth from the University of Tulsa and colleagues captured 59 litters from two species of pit viper snake and analysed their "DNA fingerprint", a sort of paternity test. He found that two litters had come about through virgin births, via a process called facultative parthenogenesis.

So the stress of captivity may not be what triggers such an extreme mode of reproduction. What’s more, wild male pit-viper snakes are plentiful. So the females don’t have virgin births simply because they have no other choice.

"We used to call facultative parthenogenesis an evolutionary novelty but it's not as novel as people think,” Booth told BBC Earth. “I've got a box of shed skin from snakes that's overflowing with examples.”

"It's amazing that we do all of this work on reproductive biology and we're still learning something new about the reproductive modes about the animals around us," he says.

Ancient reproduction

Booth suspects that virgin births may actually be an ancient mode of vertebrate reproduction.

Those species that do it best, the boas and pythons among snakes for example, are also some of the oldest. More recently evolved species, such as cobras, fare less well, producing only one or two babies via a virgin birth, which then often die.

Perhaps when these ancient snakes lived, millions of millions of years ago, either so few existed, or it was so hard to find a sexual partner, that they didn’t bother, and cloned themselves instead. The fossil record can’t tell us.

It may also be extremely difficult to discover how many wild species actually reproduce this way. It would be almost impossible to know whether wild fish have had virgin births or not. The only way to prove it would be to harvest DNA from a female shark and her babies, to determine their parentage. So many species are endangered that the approach would not be ethical, says Booth.

So the conundrum remains; why reproduce alone, when asexual reproduction has so many down sides. Especially in the wild, where males are plentiful? And even if virgin births are an ancient, evolutionary hangover, why still do it in the modern age?

Healthy babies

One answer may actually lie within those questions.

If asexual reproduction is disadvantageous, then it wouldn’t have survived for so long, points out James Hanken, an evolutionary biologist at Harvard University, Massachusetts, US. So while genetic diversity is important, it can't be the be-all. That’s supported by evidence from the 'miracle' babies, or parthenogenetic offspring, themselves.

Baby sharks born to virgin mothers are less genetically diverse than those born to two parents. But they appear just as healthy, having been "purged of all the deleterious recessive genes", says Chapman.

Females may also decide to reproduce alone because the act of sexual reproduction can be costly, according to one of Booth's close collaborators, Gordon Schuett of Georgia State University in the US, the first scientist to document facultative parthenogenesis in snakes. Females have to put up with males competing and fighting over them, and it can be hard to find the ideal male partner.

It's fascinating that nature has evolved a way of making this possible

One other idea is that something other than evolution is at work. Perhaps virgin births are triggered by some outside factor; a hormone, or hormonal imbalance? Or even a pathogen, such as a virus, or parasite. There is a species of wasp, for example, that starts reproducing asexually when infected with a certain bacteria.

Booth suspects so. "What we find is that across birds, snakes and sharks, they do the same thing. It appears they evolved it independently, and therefore there's something else driving it."

Schuett is less sure, finding it difficult to accept that a single cause could trigger the same outcome in so many diverse species. But Booth is keen to investigate, testing the genetics of the various tissue samples taken from Komodo dragons, boas, pythons and many more he has stored in his lab. He’ll be looking for a tell-tale genetic signature that reveals the presence of a common virus or some other stimulus.

If no such trigger is found, it could be that the ability to have virgin births is retained in species as some kind of back-up mechanism, to be utilised when sexual reproduction is too unlikely or costly.

One of the big downsides of sexual reproduction is it requires two individuals to be in the same place at the same time

If true, that suggests we may see more if it, as populations of many wild species dwindle, according to Peter Baumann of the University of Kansas Medical Center in the US.

Already scientists are waiting to discover if the anaconda, the world’s heaviest snake species, will join the list of those vertebrates capable of virgin births.

"It's fascinating that nature has evolved a way of making this possible. From an adaptation point of view it does enhance a species' ability to survive long term if it can use this back-up pathway.

"One of the big downsides of sexual reproduction is it requires two individuals to be in the same place at the same time, that becomes an issue when population density is low," says Bauman.

From an evolutionary point of view, sexual reproduction remains the more dominant and successful method at this point of time, but he adds that "there's clear advantages to both mechanisms".

And mammals?

But one final enigma remains. If parthenogenesis is more widespread than scientists first thought, then why can’t mammals do it, including primates, the group that includes humans?

Perhaps they can.

There is no known example of a mammal having a natural virgin birth, either in captivity or the wild.

But in the 1930’s at Harvard University, Massachusetts, US, a scientist called Gregory Pincus started investigating the reproductive systems of mammals. His work later led to him co-inventing the human contraceptive pill.

At the time he controversially claimed to have triggered parthenogenesis in rabbits, a feat that other scientists failed to replicate.

Decades later, in 2004, scientists reported they had genetically engineered a mouse to have a virgin birth. The offspring not only survived, they were capable of having offspring of their own.

Researchers today say that it remains highly unlikely, and perhaps even impossible, for a virgin mammal to naturally produce viable offspring, due to some fundamental aspects of their biology.

But perhaps, someday, somewhere, somehow, a mammal will surprise us all.

Just as Thelma the snake, and all the chickens, turkeys and sharks have done, she will lay down and have a 'miracle' birth, one that will challenge our fundamental ideas about reproduction.