Who's the daddy? There isn't one.

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Okay, so technically, Warren Booth is an entomologist.

His main research focus and the the stuff he gets funding for looks at bugs.

But he has this hobby.

My little side project is

maintaining 120 snakes in my basement.

And I breed snakes.

I've been keeping him breeding snakes for about 30 years.

So back in 2010 or so, Warren got a phone call.

I was contacted by someone asking if I could do a paternity test on a snake.

This person's BOA constrictor had had a bunch of special babies.

Like they were born with a kind of albinism that made them pink and yellow, which is pretty, but also apparently kind of valuable.

Warren says they can go for thousands of dollars.

So the snake's owner wanted to know like who had fathered these expensive babies.

And even though Warren at the time was doing his postdoc on bugs, he had developed this scientific tool that could actually answer this exact question.

I thought my career was over because there was no faculty jobs coming up.

And to be asked to do paternity tests on a snake, I thought was just the end of the world.

But Warren figured, sure, let me figure out who this snake's dad is.

Why not?

So I ran the tests and it turned out there was no father.

It was parthenogenesis.

Parthenogenesis.

It is a form of asexual reproduction.

In this particular case, it's where an animal that normally goes through sexual reproduction decides to do something a little more DIY, like no male involved.

So a female creates an egg, and then instead of that egg merging with a sperm, it seems like it somehow merges with another egg-like cell.

And it's not cloning, like the egg and the egg-like cell have a mixed-up version of the female's genes.

But the end result is that the female makes an embryo all by herself.

And Warren was pretty surprised to see that this had happened with this boa.

Like, as he understood it, this form of parthenogenesis was pretty rare in vertebrates.

It was unusual enough and special enough that Warren actually figured that it was worth writing up.

He thought like people should know that a boa constrictor can do this.

So even though he was a bug guy, he pulled together a scientific article about this woman snake.

So I'm writing the paper to publish this, and she calls me up and says she did it again.

A year later, there were 10 or 12 more babies.

A dozen more fatherless snakes, which means that not only was this the first documented case of parthenogenesis in a BOA constrictor, but the BOA had done this unusual thing twice.

And this paper kind of blew up.

It went worldwide.

You know, the first phone call I got was from the BBC, and it just went crazy from there.

And as a result of that, I then kind of got known for doing this parthenogenesis work in snakes.

So literally a couple of weeks later, somebody else contacted me about parthenogenesis in a different species of snake.

And that's continued, you know, almost monthly since then.

All these other examples of parthenogenetic reptiles started coming out of the snake holes.

Just yesterday, I was contacted about a potential case in Florida pine snakes.

I had a freezer full of parthenogens just chilling out.

I had 100 and something parthenogens that were sitting in that freezer.

Just your average, ordinary researcher with a basement full of snakes and a freezer full of parthenogens.

But the point is, Warren realized that a lot more species are capable of parthenogenesis than he'd originally thought.

And it's not just reptiles.

Have you guys heard about the pregnant stingray?

Earlier this year, a stingray named Charlotte went kind of viral because she got pregnant, seemingly with no male involved.

Who's the daddy?

Who's the daddy?

The $64,000 question.

Who's the daddy?

We think Charlotte is her own daddy.

And this kind of parthenogenesis keeps popping up in species of birds, sharks, rays, lizards, crocodiles, like examples in every vertebrate class, basically, except for mammals.

And this isn't totally new, like researchers have known for decades that different classes of vertebrates were able to do parthenogenesis, but they're starting to think that they really underestimated how common it is.

So then the next question is, if a lot of animals can do this, what does that mean?

What we're interested in now is kind of understanding why it's happening and then what are the implications of it when it does happen.

So, this is Unexplainable.

I'm Bert Pinkerton.

And today on the show, we look at the weird, wonderful phenomenon of parthenogenesis, and we try to figure out why it happens.

I sort of accidentally stumbled into a study on parthenogenesis.

Christine Dudgeon is a biologist who studies sharks and rays, and she was trying to study some zebra sharks in an aquarium in Australia when this one zebra shark named Leone had not one, but two rounds of parthenogenetic eggs.

Parthenogenesis had been observed in this species before, but what was special about Leone was that she was kind of switching things up up because in the past she'd had some babies the old-fashioned way, like sexual reproduction.

And now she was going it alone.

And while this kind of thing had been seen in insects, Christine was really surprised to see it in a shark.

What had never been documented before in any shark at this time was an animal switching between reproducing sexually to then reproducing parthenogenetically, because in all the previous cases, cases, the documents were of animals who essentially reached maturity in an aquarium setting and had never had exposure to a male.

Christine started thinking, Leone the shark seemed to be able to toggle parthenogenesis on and off, like flipping a switch, basically.

So perhaps rather than it just being this kind of anomalous thing, like a mistake, which was the prevailing concept, you know, perhaps this is actually some sort of strategy.

And this is all speculative, but the hypothesis that Christine is playing with is kind of the evolutionary equivalent of a Hail Mary pass.

So for a shark like Leone, sexual reproduction is a better option than parthenogenesis, right?

It gives her babies more diverse genes and that makes them stronger.

But if there are no males around and sexual reproduction is off the table, then maybe something is triggered in some animals' bodies and they're basically like,

well, I might as well do something.

Because as hockey great Wayne Gretzky famously said, you miss 100% of the shots you don't take.

We're just going all in on sports metaphors today.

But so in some species, like chickens, Parthenogenesis takes care of another problem.

In that case, what happens is that, you know, if you don't have a male, you can make them.

A mom making a son that she then mates with is uncomfortably incestuous, but maybe it's better than nothing.

And in the case of sharks, the babies that come out of these parthenogenetic births are all female, so the females can't make themselves a mate in this way.

But Christine still thinks that parthenogenesis could be useful here.

My current thinking is that what the adaptation is, is that it essentially extends the life of the egg cell.

If the egg cell stays inside the mom and no male shows up, the egg cell dies with the mother.

There's a 100% chance that she doesn't reproduce.

You miss 100% of the shots you don't take.

But if the mom turns that egg into a female baby, then that female could outlive her and carry their genetic information out into the world.

And then hopefully, the female would then find a male to reproduce with to then maintain that genetic diversity.

But most of the examples that scientists are working from are animals in captivity, right?

Like sharks and aquariums, chickens in cages, like scenarios where females are kept apart from males because of human intervention.

Which left me wondering, like, are there similar situations in nature where females are kept separate from males for long periods of time?

And Christine says, maybe.

I mean, the ocean is a massive place.

These are animals that are migrating thousands of miles every year.

They have their mechanisms to come and find each other for sexual reproduction, but maybe they get lost occasionally.

And there's kind of a bigger picture question here, too.

I mean, one of my research interests being sort of the evolution and maintenance of biodiversity.

And so these are questions that, you know, go back into time.

How did new species come about?

One method researchers talk about is when a species gets blown onto an island, say, or just moves into a new area and there are no males around for a while.

Could they default to parthenogenesis to kind of extend their timeline for a bit?

I think there's some interesting concepts there to explore.

But again, before we get too excited with all this, like Christine is very clear that we need a lot more evidence here.

Especially evidence of animals doing parthenogenesis in the wild, which we currently have very little of.

So this is very much speculating, but it's a compelling idea, right?

To me, yes.

To others, maybe less so.

After the break, Warren Booth is skeptical that parthenogenesis is a strategic tool.

And he's got a bunch of super weird snakes to explain why.

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John,

you are not the fox.

Warren Booth, the guy we met at the beginning with all the snakes in his basement, he does think parthenogenesis is interesting, but he doesn't fully buy Christine's hypothesis that vertebrates use parthenogenesis as a hail merry pass to keep their genes going for another generation.

And he's skeptical, in part, because of his own first-hand experience with parthenogenetic baby snakes.

So take that snake that he wrote that first paper about, right?

The boba constrictor that had two litters of beautiful albino babies.

Warren was curious about those babies.

I asked for one because I wanted to be able to see if we could raise it up.

So the owner sent Warren one of the parthenogenetic baby snakes in the mail.

Yeah, just overnight the empathetics and insulated styrofoam boxes with heat packs.

It's like the stork.

If instead of baby humans, it brought baby snakes.

And if sometimes you had to go pick up your baby at a very inconvenient location because you weren't home to sign for it.

Anyway, this baby snake came by FedEx, and Warren was able to raise it, but it was kind of odd.

It was shorter than similar age, sexually produced individuals.

It behaved slightly differently, kind of tweaked.

And when it reproduced, it behaved totally differently.

The snake was able to get pregnant, but it was super weird about it.

Normally, Warren snakes are kept in a tank with a hotter end and a cooler end.

And when they're pregnant, they kind of bask in the heat.

But not this snake.

It stayed at the cool end instead of being under the heat.

The gestation was longer than expected.

And when it produced, it produced a pretty small litter, but half of those offspring were still born.

And then there was the parthenogenetic ball python family from the UK.

There was a snake that had been born by parthenogenesis and her daughter also born by parthenogenesis, so first and second generation Parthenogens.

And they were both sent to Warren, who tried to raise them.

That second generation Parthenogen didn't do well and it died relatively quickly.

Rest in peace, little snake.

But the mother.

She produced a clutch of eggs.

This time, there was a male in the picture, so her eggs were fertilized.

But much like the albino boa constrictor baby from before, this snake was also super weird about things.

She sat in the cool end instead of the the hot end, and she produced six eggs, of which five essentially went bad within the first couple of days.

Warren says this all fits a bigger pattern that he's noticed.

A lot of parthenogens just die as embryos, and those that do make it don't do all that well.

In his experience, parthenogens die within a handful of years before reproducing.

And this actually makes sense if you look at the genetics here, because with the snakes that Warren was working with,

the parthenogenetic babies actually wind up with less genetic variation than their parents have.

So it makes them the most inbred thing that you can think of in a vertebrate system.

So they're not that great.

And that's why Warren doesn't think it really makes sense to think of this as a reproductive Hail Mary Pass, right?

Like, at least in the snakes that he's looked at, He thinks these offspring are just too inbred to meaningfully carry along the torch to another generation.

Instead, he thinks that this ability to sort of randomly, occasionally make yourself reproduce is just a vestigial thing that popped into some ancient vertebrate ancestor and has been passed along from generation to generation.

My feeling is that these are very ancient traits that are not detrimental.

They're not beneficial.

But as a result, they're just kind of meandering their way along through lineages.

They're not being lost because they don't kill the female.

Right.

So therefore, it's a trait that is just maintained.

I ended up taking this to Christine,

and she was very happy to agree that Warren might be right.

It could be sort of an evolutionary artifact.

But she still doesn't think that Warren's kind of weird snakes totally undermine her hypothesis.

Because she says, sure, yes, most parthenogens produced this way are flops, right?

Like most of these kinds of parthenogenetic embryos die.

There's something wrong with their genes or there's too much inbreeding and they're gone.

Plenty of others don't make it to adulthood for the same reason.

But the whole point of a Hail Mary Pass is that it's a long shot.

It's probably not going to make it, but it's still better than not doing anything at all.

You miss 100% of the shots that you don't take.

That is right.

If you are a parthenote embryo and you're the one that actually makes it through to adulthood, maybe you just really won.

Like you got all the good genes, right?

But we don't know everything.

There are some extraordinary strategies out there that we're yet to find.

I do think we're just really scraping the top of this.

I'm so bad with metaphors.

That's not the right one of it.

You know, it's just the tip of the iceberg.

And, you know, there's a lot more going on that we really need to uncover.

So.

Very possibly Warren is right.

And parthenogenesis is just a weird little mistake in vertebrates.

But it's also possible that Christine's right, and there's something interesting going on here.

I think it's at least worth exploring.

Because, after all, you do miss 100% of the shots you don't take.

This episode was reported and produced by me, Bird Pinkerton.

It was edited by Jorge Just and Brian Resnick.

Meredith Hodnott runs the show.

Noam Hasenfeld did the music.

Christian Ayala did the mix and the sound design.

Melissa Hirsch did our fact-checking.

And Manding Nguyen is The Fact That House Sparrows Take Dust Baths.

A huge thank you to Alexis Sperling and Friedzen Hollis, who took a bunch of time to explain parthenogenesis to me.

And this podcast and all of Vox is free, in part because of gifts from our readers and listeners.

So if you want to, you can go to vox.com slash give to give today.

You can also support the show by leaving us a review.

They make a huge difference.

And if you have thoughts about parthenogenesis or ideas for the show, we are at unexplainable at vox.com.

I really love hearing from you all, so please email us any kinds of thoughts.

Unexplainable is part of the Vox Media Podcast Network, and we will be back next week.

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