IRA FLATOW: As you probably know if you listen to the show, we’re big fans of the microbiome, microbes that live in the gut. But one related subject that we don’t talk about as much is what happens at the end of the gut. Yeah, I’m talking about what even scientists call the butt. But what is a butt anyway? And why are so many scientists celebrating it for a whole week?

But I digress. Science Friday’s Daniel Peterschmidt has the story.

DANIEL PETERSCHMIDT: A few weeks ago, we saw a tweet that kind of caught our attention.

Do you mind reading the tweet that you sent out back to me?

MAUREEN BERG: Hold on a second. Let me get it up.

Hello. Does anyone know of any animal that has a few butts? Like more than one but fewer than 100. And I’m generally talking butts as the anus, but I’m open to other interpretations.

DANIEL PETERSCHMIDT: That’s Dr. Maureen Berg, a scientist at the Joint Genome Institute, at Berkeley National Lab.

You probably aren’t sitting around thinking about animal butts, but Maureen is a part of a group of scientists and illustrators who think about them a lot. She got her start in invertebrate biology, and invertebrate butts– or invertebutts– have become one of her passions. She’s even given public talks about them.

MAUREEN BERG: Now I’m known as the invertebrate butt girl on Twitter. So anytime any kind of new animal butt thing comes up, I always get tagged in these. It’s just like a standard procedure at this point. Even though I do no research in this field, just once again, I’m just the loudest person about this.

DANIEL PETERSCHMIDT: Yeah. How do you feel about that, of that being your calling card now on Twitter?

MAUREEN BERG: I’m honored, honestly. It’s–

[LAUGHTER]

DANIEL PETERSCHMIDT: So now people tag her and tweets when certain discoveries are made.

MAUREEN BERG: There’s a recent worm that was discovered that has hundreds of butts.

DANIEL PETERSCHMIDT: That worm, Ramisyllis multicaudata, isn’t like most worms. Its body segments and branches out in multiple places, looking more like a connected series of cracks in a dried-up riverbed than a traditional worm. And at the end of each of these dozens of branches is an anus.

ROSEMARY MOSCO: I’m always looking for comic fodder, and that one kind of wrote itself.

DANIEL PETERSCHMIDT: Turns out Maureen isn’t the only one fascinated with invertebrate butts on Twitter.

ROSEMARY MOSCO: I thought, oh, my goodness. I have to do a comic about an animal with a zillion different butts.

DANIEL PETERSCHMIDT: Science Illustrator Rosemary Mosco put together a chat group, appropriately named Butt Chat, and invited other butt enthusiastic illustrators and scientists.

I mean, what were your initial reactions to just being involved in this project?

AINSLEY SEAGO: A complete lack of surprise.

DANIEL PETERSCHMIDT: Dr. Ainsley Seago, the Curator of Invertebrate Zoology at the Carnegie Museum of Natural History, in Pittsburgh, was one of the researchers who got invited to this butt chat.

And like Maureen, she’d also given a talk about bug butts before. She started a small document with some of her favorite butt facts, threw it into the chat, and the other members started adding to it.

AINSLEY SEAGO: I think at one point I said, oh, no, we’ve opened Pandora’s Butt.

[LAUGHTER]

Because there were so many different pieces of information flying in this chat.

DANIEL PETERSCHMIDT: The group decided they’d team up and use their combined science and illustration powers for good, by making comics about the back ends of the backboneless. They’re calling their celebration InverteButt Week. Like Shark Week or Cephalopod Week, but for invertebrate butts.

MAUREEN BERG: It’s just a chance for some people who do science communication to do the silliest thing that they can possibly think of. We love talking about this stuff. Sometimes you get really tired of only covering the depressing news or only covering the extremely technical details. And this is something that’s both educational and delightful, frankly.

DANIEL PETERSCHMIDT: So we’ve got lots of information but not an answer to the big question– what even is a butt?

MAUREEN BERG: There’s been discussions on Science Twitter in the past about what is a butt. Is it just like the back end of an animal? Or is it the anus?

DANIEL PETERSCHMIDT: Some purist researchers are a little anal about this, and believe that the word butt should only be used when referring to fleshy buttocks. Maureen and Ainsley have more generous views on this.

MAUREEN BERG: You got one end where food comes in and one end where poop goes out. That second end is, in my personal definition, the butt.

It does get challenging when you think about questions like if a bug wore pants, would it wear them like this or like this? But I think we can conclude that what would we, in insect morphology terms, refer to the abdominal apex, is, I would say, with zero ambiguity, the butt region.

AINSLEY SEAGO: So yeah, context is very important on how you define it, but I’m flexible on definition.

DANIEL PETERSCHMIDT: And for a bit more context we have to go back hundreds of millions of years ago, to the Earth’s oceans. Most animals back then didn’t have what we think of as a butt. Most just had a single multi-purpose hole for eating and excreting. The descendants of some of these animals are still with us, like coral and jellyfish.

But as you might imagine, that one-road setup had some serious drawbacks.

AINSLEY SEAGO: And so the idea with that is like you can only eat and then you digest your food and then you can get rid of your waste. Whereas, with us as humans, you can continue to eat as you’re digesting. You don’t need to wait for your whole digestive system to clear out before you eat again.

So the whole even concept of like evolving an anus allows you to basically eat and digest at the same time. So it’s a little bit more efficient.

DANIEL PETERSCHMIDT: The evolutionary marvel of the digestive system, and subsequently the anus, was a big deal for life on Earth. Animals got more out of their meals. Bodies lengthened and grew bigger, and developed better ways to move around, like swimming, walking, and flying, rather than, say, just existing, floating in the water like a jellyfish.

[MUSIC PLAYING]

What’s a butt that you think that more people should know about?

MAUREEN BERG: I mean, my favorite animal butt to talk about is the sea cucumber butt, just because it does a lot of weird things. Like it’s not just one weird thing, it does a lot A lot of sea cucumbers’ butts act as homes for other animals. Like you have the fish, you have crabs, you have a lot of things that live in the butt.

And because maybe you don’t want just any animal living in your butt, a lot of sea cucumbers have anal teeth, to prevent certain animals from inhabiting their butt, essentially. Because what some will do is they’ll get into the butt and they’ll start kind of munching and gnawing on the gonads and stuff. Which is obviously bad. So they kind have it all. They have like eating, breathing, defense, apartment building, like they have it all. And I really admire it.

DANIEL PETERSCHMIDT: It would get like the most versatile butt award?

MAUREEN BERG: Exactly.

FRANZ ANTHONY: I want to talk about the face mite. The best part about them is that they don’t have butts.

DANIEL PETERSCHMIDT: Franz Anthony is another science Illustrator working with Rosemary on the project.

FRANZ ANTHONY: The problem when someone doesn’t have butts is that they can’t poop. So throughout their life, their body just gets longer and longer as their poop accumulates inside. And then, once they die, they just burst open, and then the mite poop is basically all over people’s faces. And I think that’s really, really fun.

DANIEL PETERSCHMIDT: And I don’t like to pick favorites, but Ainsley’s preferred butt might be mine now too.

AINSLEY SEAGO: One of my absolute favorites is the Neuropteran family Berothidae.

DANIEL PETERSCHMIDT: They’re a type of lacewing, small insects with large clear wings. And their larvae live in termite mounds. Which is a pretty dangerous place to grow up. Termites are essentially soldiers. They’re territorial and dangerous. And they don’t want any intruders in their home.

So how do these seemingly defenseless larvae defend themselves when termites approach them?

AINSLEY SEAGO: They turn around and wave their butt in its face, and release an invisible but powerful gas that knocks out the termites almost instantly. So they’re essentially farting them to death as a form of defense. And it’s just one of the most beautiful things that nature has come up with in her infinite wisdom.

That’s the central thesis of InverteButts week, which is let there be joy. It’s OK to have yourself a secret little chortle at an insect that’s farting another animal to death. That’s pretty great.

DANIEL PETERSCHMIDT: I wanted to end on this question, which is, why should we care about butts? And I’ll take my answer off the air.

[LAUGHTER]

ROSEMARY MOSCO: Listen, I don’t think that butts are necessarily the most important thing going on right now in society. But I think that butts are something delightful to think about. And looking at one particular body part of an animal can be a way to look at an entirety of an animal, and look at the way that it experiences the world.

FRANZ ANTHONY: Yeah I think the butt in general is just really funny, because it’s really accessible. Even kids understand it. So it is a gateway for kids to understand bigger concepts.

AINSLEY SEAGO: And adults, too. I mean, I think adults are already so excited about anything goofy and butt related. So I think we all need to hop aboard the butt train and ride it to science down.

[LAUGHTER]

I’m so sorry.

[LAUGHTER]

DANIEL PETERSCHMIDT: Invertebrate Week is currently happening. It started March 1st and it’s going until March 8th. They’re using the hashtag #invertebuttweek, on Twitter. And you can draw your own favorite animal butt and tag it under that hashtag.

We also have official illustrations from Rosemary and Franz and others on our site that you can check out. That’s at sciencefriday.com/butts.

For Science Friday. I’m Daniel Peterschmidt, National Butt Correspondent.

[MUSIC PLAYING]

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