Radiolab - Snail Sex Tape

Episode Date: March 6, 2026

In this episode, we consider a creature we often don’t think much about—the snail. And not just snails, but their sex lives. Which, as it turns out, is epic. There is persuasion and subterfuge, spaghetti penises and co-copulation. And this very surprising habit—erm kink—of making tiny arrows (actually!) and stabbing each other with them. Known as a “love dart,” these limestone daggers aren’t just a strange trick of nature—they have a deep evolutionary purpose.  Special thanks to Menno Schilthuizen and Aaron Chase.EPISODE CREDITS:  Hosted by - Molly Webster Reported by - Molly Webster Produced by - Mona Madgavkar, Annie McEwen, Molly Webster Sound design contributed by - Mona Madgavkar, Annie McEwen Fact-checking by - Diane A. Kelly and Edited by  - Alex Neason EPISODE CITATIONS: Videos -  A love dart being DARTED! (https://zpr.io/rYhLwXhaxQQP)  – Molly has watched this video so many times Articles -  Changes in the reproductive system of the snail Helix aspersa caused by mucus from the love dart. (https://zpr.io/xxjuCcTyiVJV) by Koene JM, Chase R. J Exp Biol. The snail's love-dart delivers mucus to increase paternity. By Chase R, Blanchard KC. Proc Biol Sci. A love-dart at the heart of sexual conflict in snails (https://zpr.io/X2ANHPaEg5sr)  by Foote C  ** This article has an image of eight different love darts, and it’s what Molly shows to Soren in the episode (this image is one of her favorite research finds!)  Books -  “Nature’s Nether Regions: What the Sex Lives of Bugs, Birds, and Beasts Tell Us About Evolution, Biodiversity, and Ourselves” (https://zpr.io/ktMvJbZciCdD)  by evolutionary biologist Menno Schilthuizen. Signup for our newsletter!! It includes short essays, recommendations, and details about other ways to interact with the show. Sign up (https://radiolab.org/newsletter)! Radiolab is supported by listeners like you. Support Radiolab by becoming a member of The Lab (https://members.radiolab.org/) today. Follow our show on Instagram, Twitter and Facebook @radiolab, and share your thoughts with us by emailing [e-mail]. Leadership support for Radiolab’s science programming is provided by the Simons Foundation and the John Templeton Foundation. Foundational support for Radiolab was provided by the Alfred P. Sloan Foundation.

Wait, you're listening. Okay. All right. Okay. All right. You're listening to Radio Lab. Radio Lab. From W. N. Y.

See? See? Yeah. Am I recording? I'm recording. Does your mic have fancy green lights on it? Does yours not? No. It tells me how loud it is.

And I can also mute it. Can you hear me? You can't hear me anymore. No. I can't still. You can't? Yeah. Wait, what does that button mean then?

Button. Button. I'm Molly Webster. This is Radio Lab. Button. Okay. And today I am joined by our executive editor. Cotton.

Soren Wheeler. What are we doing? Why are you here? Well, you're here because I'm going to take you on a walk. Oh, you're going to take me on a walk? Where? In a lowland area along the Kinabatang River. We're going to follow this guy, Menno.

Menos Schildhaus. He's an evolutionary biologist. It seems like a nice. guy to go on a walk with. Menno is going to take us deep into Borneo. It's tropical humid limestone forest, so it's really wet. There's lichens, there's ferns. Orchids and vines and climbers.

And he is looking for something. Okay. And actually, I also went on a walk to look for the something. My walk was not in Borneo. It was in Brooklyn in February. So less orchid. Dog poop. More cigarettes.

Trash. Here's a pigeon. So this is something that you can find in both Borneo and Brooklyn. Yes. In Borneo, it's down by the riverbanks. And it's the limestone cliffs. In Brooklyn. Oh, no, I'm right here.

You just make your way through one single door to a store. And then right there, there is a glass tank. I'm like pressing my face up against the glass. Really put my face to the surface of the rock. And then sort of glistening in the sun. Oh, my goodness. I see these miniature. Tiny, tiny.

Snails. Snails? Mm-hmm. It's got a little brown shell with whorls. Is the thing that we went on a walk to see. Right. I've got two little antennas. And I just can't imagine a penis coming out of that face.

Oh. Wait, wait, what? That, there's a penis in the snail's face? Wait. So it turns out that penis in the face. Oh, my God. Is actually like maybe one of the, the least strange things about snails.

Soren, you look at snails and you think like... I don't think anything. I haven't ever thought. Okay, fine. You don't even think about snails. And I'm here to tell you that there is so much to see when you look at a snail, including a sex life that I don't think any of us saw coming. And so that's what we're going to do today.

We're going to do snails. They're going to do each other. And we're just going to watch. in I'm in okay and oh dear our guy meno is going to be our guide exactly so let's go okay um well to begin with I wasn't originally interested in snails I mean as a schoolboy I was I was mostly interested in insects and birds but you can study things in snails that you cannot study in insects and that often has to do with the fact that they move so slowly, so you can

see... You can catch them. You can catch them. You can mark them. So I've put numbers on snail shellshells and found them back a year later or sometimes two years later sitting on the same tree that they were sitting on when I marked them. No way. That's cute. You know, people do that with insects too,

but the chance of finding them back is much smaller than with snails who mostly sit where you left them. So they call this the the rate of dispersal. So that's basically the average distance between where an animal is born and the place where it has its first reproduction. Okay. And in snails, that's usually between one and five meters.

Oh, really? That's not very far at all. It's not very far at all. And that's the consequence of that is that you, and that's really what got me hooked on snails, is that you can see evolutionary patterns on a human scale, which normally you would, would need entire continents for. So in Crete where I did my PhD, I would walk through the, through the mountains. And with every step, I would see subtle changes in the way the snail shells shells looked. So if you cover a few kilometers, you can see snail shellshells around you,

sitting on the rocks changing from smooth to ribbed and from large to small. So you basically can walk through evolution. So when he's taking this walk, there's like a giant cliff of rock. And in one tiny patch, there are a species of tiny snail. And one day, some of those individuals wander over to a very nearby patch of rock. And then they settle there and evolution acts on them. And then some of them leave and go to another nearby patch of rock. And so basically, you can walk patch to patch to patch and see how evolution has shaped these snails. Well, I'm fascinated by the idea that you can walk along this, but I'm like, yeah, what do you see when you go? A slightly different shell or a different antenna?

It's slimy or less slimyer? Oh, you're looking at the size, whether it's a flat shell or a tall, spire-shaped shell, whether it has any ribs. But sometimes the shells between different species are very similar. They're just smooth and spirally, and you can't really tell one species from the other very easily. and you have to start to dissect. And with slugs, of course, you don't even have a shell,

so there you always have to dissect. Wait, so snails and slugs are as closely related as they look. Yeah, slugs have evolved from snails, but during that process, they've lost their shell. Actually, many slugs haven't completely lost their shell. So usually you need to dissect them

to really be sure what you're dealing with. And then you start meeting all this complex, this whole complex genital world, of snails. Straight to the genitals? Yes. I should have known given that you were... I was like, snails have genitals?

Yeah. When you have species that are very closely related and very similar on the outside, usually their reproductive organs, their genitals are wildly different. So genitalia are the organs that evolve the fastest among all organs in an animal's body. And the result is that anything that is possible... in evolution is going on in genitalia. That's really where the rubber hits the road. So just to set the scene here, the snails that Meno is talking about are hermaphrodites,

which means that they are both male and female at the same time. So they have both male and female parts inside their little slimy snail body. So snails are very asymmetric in the way their bodies are shaped. And the result of that is that snails usually. have their genital opening on their right-hand cheek. So you have these eye tentacles and a little bit behind the right eye tentacle. There's a little opening, very hard to see. And that's the place where both the penis and the vagina sit.

I like it. It's on their right-hand cheek, not even their butt cheek, just on their face. On their just, yeah. So when they made, they have to bring these openings together. So usually they made face-to-face or actually cheek-to-cheek. Dancing cheek-to-cheek. Exactly. Then they have to get these two openings together,

and then both of these animals avert their penises, which are inside their bodies, but they basically averts like a finger of a glove. So like if you have a glove or something, and you pull your hand out, and the finger goes inside the glove, that's how it is tucked in. And then when the snail gets to another snail it wants to mate with, it puffs that finger back out again.

Yeah, well, that's true for the penis. The vagina stays where it is. And the penis of the one partner pushes into the vagina of the other partner and simultaneously, visi versa. So when you see the mating, you see basically two fingers connecting them through that little opening. Okay. That's the first step. But it does take several hours usually.

Is hours? Yeah. Wow. Many snails mate for, you know, two, three, four, five, up to seven hours. I've seen snails mating for an entire night. They usually made during the night, so I can take the whole night for them to get all this process underway. It reminds me of some sort of like dial-up internet. Yeah, yes, exactly.

You're like, okay, we're connected. Now we're waiting to connect. Almost as long as setting up this interview took. Yeah. Yeah. So. Oh my gosh. And they are just connected by these fingers the whole time.

Yes. Again, it depends on the species. This is really where the evolution of genitalia kicks in. There are these fingers. There are also sort of tongue-like structures. There's one family of slugs that has these with which they lick each other. And these tongues are attached to the penis. There are also species that don't insert their penises.

into the partner, but they keep the penises dangling on the outside and the sperm is transferred by a sort of handshake from one penis tip to the other penis tip. There's a species of tiger slug from southern Europe where this happens at the end of a penis that is almost a meter long. What? So that would be like 25 inches, more even, I think. That's two feet. No more.

35 inches. Yeah. Yeah. It's three feet. It's about three feet. And these, so these are actually content. for the longest penises compared to body lengths in the animal world. Well, yeah, it seems like it would just tip over a slug.

Yeah, well, it's like spaghetti. I mean, like cooked spaghetti, it's very flexible. So they hang from a tree and these penises dangle down and they entwine. And at the tip of the penis, the sperm package is transferred. Oh. So the sperm package is quite large and quite nutritious, and it has millions of sperm in it, sperm cells. and it's not they don't produce it beforehand.

They produce it while they're mating, so they have to wait for that sperm package to be ready and filled with sperm. And that travels then through the penis into the sperm receiving organ of the partner. And again, this happens in both directions simultaneously. But it's only like one out of a thousand sperm gets to go to where the eggs are. The rest is being digest is being eaten. Oh, they can eat the sperm. Yes, they can actually just kind of absorb it and like use it to live off of, use it as a food source.

They could do that or they can like store, they have a way to store sperm. If they're just not, now is not the right time for me to, I'm just working on my snail career and I don't really want to get pregnant. They're often coming after. It's funny, but like if you are a snail who spent the better part of seven hours getting ready to swap sperm, You don't want it to be eaten, you know. And so to prevent that from happening, the sex game becomes less of a partnership dance. And more of a duel.

A duel. Which involves all these weird organs that they use in their reproduction, like sperm storage organs and sperm digesting organs. darts glance and darts sex and it's a whole circus of sexual extravagance. That is coming up after the break. Okay. Hey. Hey.

I'm Molly. You're soaring. Yes. And I think we are about to have a duel. I promised a duel and a duel you shall get, which is like there are two snails. They are about to swap sperm, but they want to better their odds. for that sperm to become babies, and so at this moment, they bring out their love darts.

Love darts. Love darts, they are produced in an organ called the dartsack, and the dart is a little limestone needle made from the same material as the shell, shaped like a little dagger. Oh, really? Yeah. Can I Google it? Can I share my screen with you?

Sure. Screenshot. Okay, ready? Whoa. It looks like whittled bone, like white bone. It's almost like a tool. Like you'd imagine like you'd find in like an archaeological site was early needles that early humans use. Yes.

There'd be like early humans crafted this thing. Crafted a needle out of bone. Yeah. Yeah. Oh yeah. It's beautiful. But the one on the far, the top right one legit looks like an arrow. Well, there are species in which the dart looks surprisingly like an arrow you've drawn.

on when you were a kid. Even like a little feathery thing that's in the back of the air, there's a little like, what do they call that? Those are like, Fletchings. Fletchings.

In other species, it's more like a flat knife shape. And I've studied a species in Borneo, which is more like a hypodermic needle with holes along the side. So they're about a centimeter long. One centimeter. Yeah.

So, yeah, that's almost half an inch. They are present in a snail, even if they're not made. and you can feel them crushing when you're eating a snail. So you'll be like eating snails and then you'll go, oh, just hit a love dart. Well, I would. Most people wouldn't. They would just think there's something crunchy.

I'm talking about you specifically, you know? Yeah, yes. Yeah. Yes, I would. My gosh, I've never actually had snails, but now I... Okay. Well, I don't know how if I would feel if I was eating the love dart, maybe bad? Yeah.

No, you eat that entire reproductive system as well. including the love dart. But so the dart is not a penis. It's not like delivering sperm. The dart is not a penis. Okay. The dart plays a specific role. So what they do with the love dart is they expel this dart with considerable force into the partner.

Like they shoot it. They shoot it. Yeah. Yeah. Some species shoot it. Some species push it. But like the escargo that we eat, it really shoots it with force and it goes fast. So there's a muscular organ that's very fast and forcefully pushes it into the skin of the partner.

And in some species it is withdrawn after that. And in other species it's sort of a disposable dart and it stays in the body. Yeah. And where are you shooting the dart toward? Like what do you want to hit? It looks like you mostly want to hit the skin very close to that genital opening. But sometimes... So the head, the face.

Yeah, the cheek. Okay. And what is it doing with the dart? Is it shooting a material? Substances is just like, hey, I want your attention. Well, that was a mystery until quite recently. People thought, yeah, it was some sort of stimulation. You know, just like when you see lions mating,

they're also biting into each other's head. And sharks also do that. So why not shoot a dart? It could be something like that. And then people thought maybe it's what they call a nuptial gift. So a donation of calcium to help the partner build the eggs for the offspring that they're going to produce. Because eggs use calcium? Because the eggs need calcium for the shells. But then research in the 1990s showed that there was barely enough calcium in a dart to produce

the shell for one egg. So that also wasn't the answer. But research by Ronald Chase and Joris Coona showed that there were actually hormones being delivered into the body of the snail that is being shot. Hormones? Yeah, yeah, yeah. So the dark gland produces this mucus that has inside of it essentially a sex hormone and the arrow is either covered in or full of this sticky goo. These hormone-like substances get into the bloodstream of the animal that receives the dart.

And it turns out that those hormones, they produce involuntary spasms in the female organs of the partner, which increases the uptake of sperm. Wow. Okay. So it's a kind of manipulation. You're saying that when this hormone hits the system of the partner, what you could call the vagina, and it like makes it seize or like cramp or something? It's a little bit more complicated than that. unfortunately. So it's actually that behind the vagina there are these two pockets.

One is a storage room, the other is a digestion room. And when the hormone hits and causes these like tiny spasms. Sounds almost like a little orgasm or something. Yeah. But it causes muscles to contract. And what the hormone does is it sort of creates or it causes the entrance of that digesting organ to close off. And it also kind of pushes sperm out of the storage chamber, and all of that can lead to the

sperm, you know, being kind of pushed toward an egg. So if you shoot your dart right, you probably will gain more offspring than if your, if your dart misses its target. There's also species in Japan which don't shoot a laugh dart, but they stab multiple times. And every time they step, the dart is recharged with new hormone. So they have this, they sort of dip it in the glands in their body and then they push it in again. And then they pull it out and dip it again and push it in again. So there's a snail where this has been seen, so the snail would be seen stabbing its partner 3,300 times in rapid succession.

So probably it's there. It's also a matter of more is better. So they just keep pumping this hormone into the partner and vice versa. And the partner lived through 3,000 stabs. Yeah. Yeah, yeah, it's just a single. It's going into the same wound all the time. So it's not making new wounds.

It's not like Caesar. Did you say it's not like Caesar? Yes. So, yeah, there's pictures of these snails that are mating. And this dart is sticking through the head of one of the snails. It's the sort of the fletchings are still coming. coming out on one side and the tip is coming out on the other side. So they're like visible to the naked eye?

Yeah, I find them in my garden after a wet night. They, they, when snails have been mating, and sometimes they, yeah, they leave those darts on the, on the floor. Wait, do you think I've seen a snail dart and just not known it? Could be. And it doesn't kill it? It doesn't kill it. No, they don't seem to suffer too much from it.

Of course, they don't have a brain like we do that can be hit. They just have some ganglia, which are in a different place. So maybe nothing vital is being hit. Maybe the ganglia are in a different place because they're like, we've seen these darts. We're going to move on over to the stomach. I wouldn't be surprised, yeah. Man, I'm just thinking about you in your backyard, just like picking up love darts. You're like, wow, last night was a big night.

Yeah, exactly. It's usually in the spring when it's warm and but rainy. when we have a spring like that, that you see snails mating and you often find, you find these slime spots, which are sort of a telltale sign that they've been having fun,

and you find these love darts lying on the floor, because they do sometimes miss, and sometimes they also expel them from their body after mating. How would they get it out? Like, if I have a love dart through my head, how am I extracting it?

Maybe if it's completely through, you wouldn't be able to get it out. But if it's sort of sticking in the skin, then yeah, I guess just by movements it wiggles out like just like a splinter in your skin. And you as a scientist, I'm assuming that love darts are interesting to you and that finding out how they worked

is of interest to you. So like I'm just curious as to like what insights it gives you. Well, what insights it gives me is. that it's so abundantly clear that the evolution of reproductive organs, it's a complete madhouse of evolutionary novelties and measures and countermeasures and warfare sometimes, but also persuasion. Let's say on the male side, an evolutionary change that allows the male to bypass any control of the female.

This will then immediately be followed by a... an evolutionary countermeasure on the female side that regains control for the females. And that all these changes accumulate on top of each other. And it's very unpredictable in which direction it will go. And the fact that these darts have evolved multiple times, they look different in different species that some species have one, some species have two, some species have four. Some species have this disposable darts, some reuse them,

some stab once, some stab thousands of times. Some don't form darts on their first mating, but only on their second and later. So even within the darts, there's so much variety that it really drives home the fact that this genital evolution, you can see anything in evolution sort of encapsulated in what goes on in these genitals. I mean, I,

evolution aside, and just sort of, I had no, like I thought snails, maybe they just slime up next to each other and then there's a sack of eggs or something. Like, I had no. Yeah, it's like, it's like you just look at these things and you think slime ball. Yeah. You don't think theater of evolution. No, no. Or three foot penis or.

Yeah. Look at what I'd been missing this whole time. I think of one of the things I'm most fascinated by is that I can see it, that like, I could go pick up a little love. dart. It's almost like, it's almost like being, it's so funny to say this to a scientist, and I do really believe in science, but it's like, it's almost like being like, oh my God, fairies do exist. Like it's like being exposed to this world that just feels so other and tiny and magical, but like there's a little remnant of it left behind. And I know that it's real because of that. Yeah. And of course, there's also these medieval marginalia. I don't know if you've

heard about these. what's that? So marginalia are these little embellishments in the margins of medieval books. Okay. Where, you know, there would be little scenes of a hair or a duck or a fox chasing a bird. But there's also lots of marginalia about knights fighting with snails with swords. Really? And yeah.

And people have also thought maybe that has to do with the loved arts. There's a whole literature on this. So it feels like there's. there was almost like a time of the world when we were bumping into love darts more. Mm. Yeah. Okay. Oh, my God, this pet story is busy. Okay.

That's that maybe. Do you think that some scientist or some naturalist or just some person walking along millennia ago found a love dart on the ground and then made up. the myth of Cupid? Yeah, there is actually, some people think that's how the myth of Cupid evolved. Wow. Across the street, stiffing over an explosive bag of trash. Wait, so could I go find a love dart?

Well, they're in Prospect Park in Brooklyn on trees. So there are some local love darts? Definitely. You'd have to wait for them to come out of hibernation, which would be in late April probably. Okay. And then they usually start mating quite soon thereafter. So if you go out early in the morning and look, yeah, on walls, on trees, on tree trunks. And as soon as you see two snail shells that are sitting really close together, touching each other, then they're usually mating.

Then you can look from the side and, yeah, then you might see a dart sticking out. Or maybe a dart is lying or being stuck on the slime that they're sitting on. You've given us a new thing to go into the world and find. The crown's not thought yet. You're right. I mean, the story like this could make people go out and think, I want to see my love darts for myself. And I'm going to go into the park and find snails that are mating.

There's a wet tree. And they will. Not quite yet, but soon. It's a miniature world that I. exist. This would be where you could find a snail or two. This episode

was produced by Mona Medgalker, Annie McEwen and Molly Webster. It was edited by Alex Neeson, fact-checked by Diane Kelly, and reported by yours truly. If you want to read more about snail sex, you should go check out Menno's book called Nature's Nether

Regions. There's a whole chapter on snail sex. Meno Schiltowsen is a evolutionary biologist at Leiden University in the Netherlands. Thank you, Meno. I also want to give a shout out to Aaron Chase. I first heard about love darts from Aaron very recently, and in fact, it is his dad, Ronald Chase, who discovered what the love darts were actually doing to snails.

If you want to read more about how I found out about love darts, check out our newsletter or go sign up for the newsletter, which is at radio lab.org slash newsletters. newsletter. For those of you who are lab members, we are dropping some snail extra content on the lab. If you are not lab members, go sign up now and you can learn all about snug life. And finally, you best believe that the second it thaws outside, I will be out there looking for love darts, and I'd love for you to join me. So keep an eye on social media, and we'll keep you posted. For now, I am Molly Webster. Spring is a

coming. This is Radio Lab. Thanks for listening. Hi, I'm Gabby. I'm from San Francisco, and here are the staff credits. Radio Lab is hosted by Lula Miller and Lettif Nasser. Sorin Wheeler is our executive editor. Sarah Sandbach is our executive director. Our managing editor is Pat Walters. Dylan Keefe is our director of sound design. Our staff includes Jeremy Bloom, W. Harry Fortuna, David Gable, Maria Paz Gutierrez, Sindhu, Ninasambandan, Matt Kielty, Mona McGawker, Annie McEwen, Alex Nieson, Sarah Kari, Rebecca Rand, Anisa Vizza, Arian Wack, Molly Webster, and Jessica Young, with help from Gabby Santis.

Our fact checkers are Diane Kelly, Emily Krieger, Natalie Middleton, Angeli Mercado, and Sophie Semayi. Leadership support for Radio Lab's science programming is provided by the Simons Foundation and the John Templeton Foundation. Foundational support for Radio Lab was provided by the Alfred P. Sloan Foundation.