Season 5: Episode 5

Common Sense

Insects invented song. They’ve developed a multitude of ways to listen. But insects couldn’t have evolved these complex skills without plants. In this episode, we explore the interconnected acoustic lives of insects and plants.

Guests

Credits

Transcript

[00:00] INTRODUCTION

AMY: We’re in eastern Tennessee, half a billion years ago. The land we now call the Smoky Mountains is almost totally covered by a shallow sea, and life on Earth is still mostly confined to the oceans….until a tiny moss does something remarkable. It finds a foothold on some rocks, and stays there. It’s an early pioneer into the barren world above the waves—an adventurer that helps to turn the terrestrial world green. 

AMY: Now fast-forward hundreds of millions of years, to 1946. Just downriver from the very spot where that ancient moss lived and died, another trailblazer is born.

Sitting on the front porch on a summer afternoon

AMY: Dolly Parton.

In a straight-backed chair on two legs leaned against the wall

AMY: Raised in a one-room cabin with eleven siblings, Dolly went on to write thousands of songs, many of them full of references to the wild things she grew up with. This one, called “My Tennessee Mountain Home,” recounts a childhood filled with butterflies, birds, wildflowers, and crickets.

In my Tennessee mountain home

Crickets sing in the fields nearby

AMY: So when a geologist unearthed that ancient moss near Dolly’s home, and learned it might be one of the oldest fossilized land plants ever discovered, he named it Dollyphyton boucotii in her honor. 

AMY: A moss and a musician. A plant and an animal. This connection between flora and fauna is fundamental to the soundscape of our planet and the flourishing of life overall. Because as plants began to emerge out of the sea, some adventurous animals followed suit. They were probably scorpion-like creatures. Arthropods, related to our modern insects and spiders. And they were the very first animals to scuttle across the untouched beaches of the terrestrial world.

INSECT SOUNDS

AMY: This new realm must have been shockingly bright and dry for the plants and the animals. But finding no other life, they went forth and multiplied. And multiplied. And multiplied. After the mosses and lichens came ferns and trees. And about 150 million years ago came the miracle of flowers, painting the world in color, and pollinated by beetles, moths, and other winged things. And as the world got greener, it got louder. Cicadas called from the branches, crickets made choruses in the leaves. 

INSECT SONG

AMY: Insects invented song. They laid down the very first notes of what would later become a symphony of terrestrial sound. But they couldn't have done it alone. Without plants, there would be no animals. No songs at all. Without Dollyphyton there might never have been a Dolly Parton.

MUSIC

AMY: Welcome to Threshold, I'm Amy Martin, and in this episode, we’re going to explore the intertwined acoustic lives of plants and insects. 

INTRO MUSIC

[04:18] SEGMENT A

AMY: I want to start by naming the dung beetle in the room here: both plants and insects are things that people often ignore. We take it for granted that plants spend their lives giving us breathable air, stabilizing our climate, and providing the basis for every bite of food we consume. And we forget that insects are arguably the toughest animals our world has ever known—that they’ve made it through multiple mass extinctions, major climatic changes, and a total rearrangement of the continents. And they’re also just…well…Wil Hershberger says it best.

WIL: Many of our singing insects are handsome if not downright beautiful. From the tiny trigs and ground crickets, through the meadow and conehead katydids, to the true and false katydids, there's almost an endless variety of shapes, colors, and sounds to explore.

AMY: That’s Wil introducing people to the website songs of insects dot com. And I challenge you to visit this site and not fall in love with at least one insect. It is chock full of pictures, videos, information, and sound recordings of a huge array of six-legged singers. There's the slow-tinkling trig…

SLOW-TINKLING TRIG

AMY: The Confused Ground Cricket

CONFUSED GROUND CRICKET

AMY: The Scissor-Grinder Cicada

SCISSOR-GRINDER CICADA

AMY: and…. the Slightly Musical Conehead. Kind of a passive aggressive name, but—sorry coneheads—I think it's accurate.

SLIGHTLY MUSICAL CONEHEAD

AMY: Insects make thousands of different kinds of sounds. And they also have an impressive variety of ways to listen.

NATASHA: The fun thing about insects is that they have evolved hearing at least 17 times independently. So they've invented ears over and over again.

AMY: This is Dr. Natasha Mhatre [MAH-tray]—she’s the one who told us about how spider webs can be ears in our last episode. She’s based at the University of Western Ontario and she researches invertebrate neurobiology: in other words, she's an expert in bug brains, and especially how insects and spiders process sound. She says insects grow ears in all sorts of places on their bodies.

NATASHA: Grasshoppers have them on the abdomen, so on one of the segments of their body. Crickets have them on their forelegs so they move the ears as they walk.

AMY: Cool!

AMY: Natasha says one kind of moth even grows an ear on its mouth. So, the thing it eats with is also the thing that it hears with.

NATASHA: I would not want to be this moth. It would sound awful.

AMY: Arthropods currently make up more than 80 percent of Earth's animal species on land and in the sea, and insects are the largest group of animals in that category. All of which is to say: it would be impossible to do a full survey of all the sonic wonders of the arthropod world, so I decided to focus on just one especially creative specimen—the tree cricket. The delicate, green cousin of the much burlier field cricket.
 

NATASHA: They're actually quite slender. They look like blades of grass. And the males have these, like, lovely, beautiful glass like wings. And when they sing their wings are flipped up, and they're almost completely transparent. They just look so pretty.

AMY: Is it fair to say that you have a special relationship with tree crickets?

NATASHA: I have a special relationship with tree crickets. (laughter)

AMY: Tree crickets punch way way above their weight class in terms of making themselves heard. They're really small, and they can be really loud. 

NATASHA: They're super cool. They live pretty much everywhere. There's tree crickets in Australia. There's tree crickets all over Asia. North America, South America, they're in Africa. I don't think there's any in the Antarctic, but that's it. So they're everywhere. There's lots of species. So you can probably find a tree cricket somewhere close to you.

AMY: Natasha says it's a common misconception that they make their sounds by rubbing their legs together, but tree crickets actually sing with their wings.  

TREE CRICKET: oceanthus henryi

AMY: One wing has a row of pegs on it, called the file, and on the other, there's a tiny little lump, called the plectrum. When the tree cricket rubs its wings together, the plectrum runs down the file, like a guitar pick running down the strings. It's called stridulation, and when I slow the recording way down, you can hear each individual wing beat:

TREE CRICKET: oceanthus henryi slowed down

AMY: But tree crickets are actually moving their wings up to a hundred times per second, turning the sound of their wing beats into an acoustic blur, at least to our ears.

TREE CRICKET

AMY: There's quite a bit of variety in tree cricket song. Some make sustained trills….

TREE CRICKET TRILL

AMY: Others like to lay down a groove…

CHILL TREE CRICKET BEAT

AMY: But all of this high-speed wing strumming is essentially a love song. Males do it to attract females. And the gals are choosy. They listen to the songs not only to locate their suitors among the leaves, but also to assess them.

MUSIC

AMY: Consider the situation of two romantically inclined tree crickets—two almost weightless little beings living in a dangerous world. With predators all around, ready to turn them into a tasty meal, hiding is essential. Their bodies have evolved to blend in among the stems and leaves, and they’re very good at holding very still. But that means tree crickets are hidden from each other, too—until the male starts to sing. His song is an acoustic beacon cutting through the night, broadcasting his location. Instead of a profile picture, he has a profile song. And instead of swiping left or right, the females, with two ears located on two different legs, listen to the serenades, and hop left or right, slowly working their way toward the fellow of their choice. 

NATASHA: You know, the purity of the tone the animal produces tells you something about the condition of the wings. So is this an old animal? Is this a young animal? Are half his teeth missing?

AMY: (laughing) Wow. So we've got, like, a bunch of judgmental lady crickets...

NATASHA: Totally. Well, they're like, if I'm going to spend the time walking up to you, wherever the hell you are, and finding you, you better be worth my while.

AMY: And I'm going to tell that the quality of your...your plucking.

NATASHA: That, plus just how long you go. Like crickets that, you know, have a little bit of stamina do better. Ah...yeah.

AMY: Evolution is cruel.

AMY: So for the males, it’s all about getting heard. And sometimes, to increase their chances, they actually build their own cricket-sized megaphones, using all local, recyclable materials to boot. They’re called baffles, and male tree crickets make them out of leaves. They chew holes into them and stick their heads through, and when they stridulate their wings, the leaf turns up the volume on their songs.

NATASHA: Think of making your wings artificially bigger.

AMY: Natasha ran experiments to test the crickets’ baffle-making abilities, and she discovered that they’re almost freakishly good at it. Even before they start the building process, they're somehow calculating which leaves to use as raw materials.

NATASHA: If you give them a small leaf, they probably won't make a baffle. If you give them a nice big one that really gives them a lot of benefit, they will make it. They don't even need to sing on the leaf to know if it's the big one, they seem to have some way of figuring it out in the darkness. They ignore the small leaf. They go straight for the big one.

AMY: And once they’ve chosen the leaf they want, they demonstrate an incredible level of skill.

NATASHA: They don't baffle kind of willy nilly. They try and go to the center of the leaf, which is the best position, make this perfectly sized hole there, they'll stick their wings through, start singing, oh, that hole isn't just right. They might go trim the edges a little bit and make it the right size, and then they'll sing from it.

AMY: If I wanted to become an acoustical engineer, I would likely study baffles and how they work, and it would involve math and physics. But here are these very small animals, with tiny brains, who've figured it out. Natasha says these skills are innate to some tree crickets. It's not a learned behavior, it's genetically programmed. But some of them do have behaviors that look like learning—like a craftsman critiquing his work, and making improvements as he goes.

NATASHA: This one guy didn't get to the center, and he was like, this is no good. And then he went and chewed the second hole and went straight for that.

AMY: Amazing! That just seems like fairly complex problem solving. I couldn't do that.

MUSIC

AMY: And the work doesn't end there. If a male tree cricket manages to attract a female, then he has to feed her. He produces a secretion from a gland on his back which apparently tastes really good to the ladies. Scientists call it the “nuptial gift,” but that seems pretty euphemistic to me. I think we should call it a honey pot.

NATASHA: The female climbs on top of the male, she eats from that gland while he mates with her.

AMY:  From the female's perspective, she is dining while mating?

NATASHA: Yep, and it's like, how tasty can I make this? The longer she feeds, the more sperm that get transferred into her. So the higher likelihood of him getting babies in the next generation. So there's a lot of investment and whatever that nuptial feeding is.

AMY: And the males are the true multi-taskers here, because they also continue to sing while all of this is going on. Natasha says she doesn't really know why.

NATASHA: I guess they're like, let it carry on, maybe then the sound will continue convincing the female to stay. I don't know for sure.

AMY: Huh. Yeah, it just seems logistically difficult to do so many things at once. Serve a meal, have sex, and keep singing.

NATASHA: Anything to keep her there.

TREE CRICKET (distant)

AMY: After talking with Natasha, I realized I was hearing tree crickets all the time without knowing what they were. I decided to try track one down on a September evening in a little nature preserve not far from my home in Missoula, Montana. 

AMY: I'm tip-toeing through this field. 

AMY: My microphone out and my headphones on, I let my ears guide me through the tall grass.

AMY: This is definitely a case of 'behold the elusive...whatever you are.

TREE CRICKET (closer)

AMY: I take a quiet step…

SQUIRREL

AMY: That is a squirrel.

AMY:  —and then another.

TREE CRICKET (close)

AMY: OK, maybe you're in here somewhere.

AMY: And finally, I see it.

TREE CRICKET (close)

AMY: It's a tiny wisp of a thing, with delicate, lacy wings, clinging to a blade of grass. It is indeed a tree cricket. And for the moment at least, he’s alone. But there's probably a female tree cricket nearby, listening like I am. Maybe she's pointing a leg at him in order to hear him better, just like I'm doing with the mic.

AMY: It's very light green, and I've heard it all my life here in Montana and never stopped and tried to figure out what it was. It's beautiful.

AMY: As I sit here in the grass, taking in the song, I know that I’m a listener within a community of listeners.

There are other people are walking around, I can hear an owl across the clearing.

But what about the branches of the Ponderosa pine tree above me? Or the milkweed plants nearby? Or the blade of grass itself, that the tree cricket is singing on?

They’ve been bathed in this song all summer long. But of course, they’re plants. They can’t hear. They’re not listening…right?

We'll have more after this short break.

Break

[17:59] SEGMENT B

AMY: Welcome back to Threshold, I’m Amy Martin, and let’s return for a minute to that time before there were any plants on land. No grasses. No flowers. No trees. That's how our Earth has been for most of its history. Even long after plants began to grow in the oceans, the world above the seas continued to be dominated by rocks and dust.

AMY: Plants and insects ventured into that forbidding, lifeless terrain, and transformed it together. They've co-evolved for hundreds of millions of years, and they have an endlessly complex and intimate set of bonds. Like any close connections, their relationships can run the whole gamut from quiet coexistence to mutual support to intense conflict. Plants and insects depend on each other. And sometimes they try to kill each other.

HEIDI: Well, for most of my career, I've been studying how plants defend themselves against insects, and they do that through chemistry.

AMY: Dr. Heidi Appel is a biologist, currently at the University of Houston. She specializes in chemical ecology.

HEIDI: Plants actually evolved to ward off their own pests including microorganisms that cause disease.

AMY: When plants get attacked by insects, they can't get up and run away. What they can do is make and release nasty-tasting chemicals, or even poison. And many plants are very sophisticated chemists. They can produce different compounds to target specific kinds of invaders. And sometimes they can even differentiate between intentional and accidental damage. For example, when a moth eats a leaf, a plant might send out a little chemical warning shot, hoping to shoo it away. But when a falling twig rips a hole in a leaf, that same plant ignores it. this is Heidi’s world: plants, insects, and chemicals. 

HEIDI: So sound was not on my radar at all until I moved to the University of Missouri and met Rex Cocroft after a biology seminar.

AMY: Dr. Rex Cocroft is an expert in insect communication, we met him in our last episode. For decades, he’s been researching how some insects send their sounds out through the bodies of plants. So Heidi and Rex both happen to go to this seminar and start chatting. This was in 2007.

HEIDI: I explained what I did, and Rex said, well I study the way that insects use sound transmitted through plants to communicate with each other.

AMY: So here’s Heidi, describing her research on how plants respond to insects. And here’s Rex, describing how some insects use plants to send messages to each other.

HEIDI: He paused, I paused, and we kind of looked at each other—complete strangers, you know, until now.

AMY: They were both stopped in their tracks with the same thought.

HEIDI: Whoa.. you don't suppose do you that the plant can use vibration information for its own purposes?

REX: That's when I think we both began to wonder about from our own perspectives, like well, could any of this information be relevant to the plant and could the plant be using any of it?

HEIDI: That was the aha moment.

AMY: It seemed like a pretty radical idea. Kooky even. Because if the plant was somehow using the vibrations caused by insects, that would mean plants could kind of “hear.” Maybe not the way we hear, but still, detect vibrations—receive acoustic information, maybe even from the animal world—and do something with it.

REX: And we both immediately thought, no, that's very far-fetched and unlikely.

AMY: But it also sounded fun to both of them to explore it.

HEIDI: And that's really what kicked off this study. And we've been collaborating closely ever since.

AMY: It was already known at that point that plants responded to sounds made by humans.

HEIDI: Plants will respond to single tones or to music in all kinds of ways.

REX: If you play a tone, just a pure tone, like booooo... you can increase crop yields. There's a whole range of plant traits that will be altered.

HEIDI: The real mystery for Rex and me, because we were trained as ecologists, was why would plants have that ability?

AMY: Playing music or electronic tones to plants is very human-centric. It's asking: how do plants respond to these sounds we're making. It's very different to ask a plant—or an animal, or another person—what if any sounds are important to you?

REX: And that's where our work came in, is that we said, well, what's what's an ecologically relevant acoustic stimulus for a plant, and what would be a relevant, biologically relevant response?

HEIDI: So that became really the focus of our work. Relevant sounds and relevant responses to plants.

AMY: To explore this, Heidi and Rex needed to identify a distinctive vibration produced by an activity that mattered to plants. And this sound quickly came to mind.

CATERPILLAR CHEWING SOUND

AMY: This is a caterpillar chewing on a leaf.

CATERPILLAR CHEWING SOUND

AMY: Rex was very familiar with these sounds, because they'd been driving him bananas for a long time. He was constantly having to remove very hungry caterpillars like this one from his plants in order to get good recordings of treehoppers.

CATERPILLAR CHEWING SOUND

AMY: So he knew this was a conspicuous sound, produced when an insect was doing something very biologically relevant to the plant—eating it.

REX: If there was a caterpillar on the plant, feeding on a leaf, then that's mostly what you're going to hear.

AMY: And Heidi knew about the chemicals plants produce when this is happening.

HEIDI: When a caterpillar bites a plant, it gives you lots of signals that damage has occurred. So tissue is damaged. Things leak out of cells. That triggers defense responses. And then caterpillars actually drool some. They have some oral secretions when they feed. And that chemistry also influences how the plant responds.

AMY: By combining their expertise, Heidi and Rex could try to answer a novel question: would a plant respond to the vibrations of a chomping caterpillar, even if there was no actual caterpillar present? If so, that would mean that the plant was essentially listening to—or feeling for—the caterpillar. And responding.

REX: To think of the plant as a kind of active listener….I never thought about that.

AMY: They got to work designing an experiment to find out. Rex was in charge of vibes, Heidi was in charge of chemicals. Step one was to figure out how to vibrate individual leaves of a plant in a very caterpillar-y way—to give the plant the experience of being eaten but without any chemicals being transmitted.

HEIDI: We wanted to just separate the effects of the vibrations.

AMY: So, Rex essentially built a little munching caterpillar mimicry machine. Minus the drool. They attached it to some leaves, and Heidi measured the chemical responses.

HEIDI: And we found that the ones that had received the feeding vibrations made more of the chemical defenses than the ones that hadn't received the feeding vibrations.

AMY: Ah ha.

AMY: It wasn't just the drool or some other element of caterpillar chemistry at play—the plant did in fact seem to respond to the vibrations.

HEIDI: And that surprised us. We thought, oh maybe it's a fluke.

AMY: Ever the careful scientists, Heidi and Rex interrogated their results.

REX: Well it was interesting. I in no way believed it at that point.

AMY: They didn't believe it yet, because they didn't know if the plant was alerted by the vibrations of chewing in particular.

HEIDI: Maybe the plants just respond to anything. You know, maybe this isn't a significant response because they'll respond to all kinds of things with the environment.

AMY: So they planned a second experiment. This time, they exposed the plants to three different vibrations, all of them naturally occurring in the environment: the munching caterpillar, a light wind…

WIND (via piezo)

AMY: And a leafhopper call:

LEAFHOPPER

AMY: These little insects are related to the treehoppers we met in our last episode. The vibrations they produce are in a very similar frequency to the vibrations made by dining caterpillars. But these are mating calls. They don't signal any danger to the plant.

HEIDI: So we thought that would be a pretty good test to see how specific this plant response to feeding vibrations is.

AMY: So we've got wind, leafhopper mating calls, and caterpillar chomps. Three kinds of vibrations with similar frequencies, but only one of them signals a threat. So if the plants produce more defensive chemicals in response to the caterpillar, it would indicate that they're listening or feeling for those vibrations in particular. And responding to them.

MUSIC

AMY: Rex and Heidi vibrated the leaves again, measured the chemical responses again, and waited for the results to come in. To help eliminate bias, they put their measurements into a sort of code.

HEIDI: We don't identify the samples as coming from one treatment or another. So they come out as a bunch of numbers.

AMY: Those numbers then go into a spreadsheet that decodes the data.

HEIDI: So you're sitting at a computer, you've been sitting for four hours getting all of the data aligned and in the system.

AMY: And then, finally, the results were revealed. The plants that had received the caterpillar vibrations had produced more defensive chemicals than the others. 

HEIDI: I remember sitting there, screaming, there's a difference! Here, and not here! I think I might have even gotten up and done a happy dance. (laughter) And of course, right away I'm texting Rex. 

AMY: The plants were listening out for the vibrations that mattered the most.

HEIDI: So, you know, it was very a very exciting moment.

AMY: Wow.

HEIDI: And, you know, I feel lucky if I get I've had one of those. But it's exciting in my whole scientific career. And if I never have another one, I'll die happy.

AMY: Because this really hadn't...no one knew this before. You added something new to our scientific knowledge of plants.

HEIDI: Yes, we did. Because even though people knew that plants could respond to these other synthetic tones or music, right, the fact that it could discriminate between a biologically relevant sound and one that was a real big advance in our understanding.

AMY: It's not very often that you get to talk to people who've made major scientific breakthroughs like this. And as a scientist, it's not very often that you get to be one of those people.

AMY: Do you feel a little bit like Galileo looking through the telescope for the first time, like...oh my gosh!

HEIDI: (laughter) Ahm, it has caused me to look at the world very differently. But I don't think we're going to have the impact that Galileo did.

AMY: Maybe not, but Rex and Heidi's discovery did make a big splash. It was covered in newspapers around the world, they got interviewed countless times. It was all pretty shocking to them. Rex says when they started down this path he was very skeptical that they would find anything of interest. The idea of plants being able to engage in a sort of dialogue with animals just seemed too out there.

REX: But it now has become firmly established in my mind that if you're studying communication through plants that one of the potential receivers of these signals is the plant. And that we need to think about that.

AMY: Now that we know that plants detect and respond to biologically relevant sounds—like munching caterpillars—it seems kind of surprising that it took us this long to figure it out. And indeed, from the perspective of many Indigenous people, research like this is not so much a discovery of new information as it is an incorporation of wisdom that’s been there all along. But by testing this question inside the framework of western science, Rex and Heidi helped to bridge that gap between different ways of knowing. They’ve provided hard evidence for plant listening—a whole layer of relationship, really, happening all around us, everyday. Scientists knew that plants and animals compete and cooperate. Now they know that they also converse. They’ve gone from assuming sound was irrelevant to plants to understanding that plants are actually super-receivers of sounds and vibrations.

REX: The soundscape, or the vibroscape if you will, of a living plant is one of the most complex soundscapes that there is.

AMY: Leaves capture the airborne sound environment, sounds in the soil can move up through the roots…

REX: ...and then you have all of the incredible sounds and vibrations created by insects and other animals that are in contact with the plant.

AMY: Understanding this, the question shifts from why would plants respond to sound to why wouldn't they?

AMY: I'm curious if you feel more closely connected to plants because of this. Because you've been listening to insects for decades now and here it turns out that plants are also listening. Does it give you more of a sense of kinship with them?

REX: I think it does. I would say that it it does. There's some physical information out there that I'm really interested in that the plant also has a stake in. So yes, it does provide a perhaps a little bit of a fellow feeling.

AMY: If you hear some wariness in Rex's voice, I think it’s a reflection of the fact that people can get really polarized around plants. On one side are hardcore mystics, who insist on things like plants can read minds and predict the future. On the other are militant guardians of the ivory tower, who seem eager to attack anyone who proposes a slightly out-of-the-box research idea. But there are many people who are working in the exciting and often uncomfortable territory between these two fairly rigid camps. People like Heidi, Rex, and some scientists we’ll meet in our next episode, too, who are asking brave questions, and doing solid, evidenced-based work to try to answer them.

REX: I would say in some ways, plants do almost everything that animals do, they just do it really differently.

AMY: We’re walking around in the world that plants made. Every creature on Earth, from Dolly Parton on down, depends on them for survival. Intellectually, I know this. I know my survival depends on all of these other living things that I often ignore. But the work of these scientists helps me feel it.

HEIDI: When we can identify with other living organisms, it creates an empathy in us that I think will allow us to be better stewards of the planet.

AMY: Yeah, I mean, it feels like it makes plants less other.

HEIDI: It's this common sense of being alive and being interconnected, which I think is really important to our development as humans.

AMY: Maybe someday we'll learn that plants do have supernatural powers. But in the meantime, just the natural seems wondrous enough.

CREDITS

AMY: This episode of Threshold was written, reported, and produced by me, Amy Martin, with help from Erika Janik and Sam Moore. Music by Todd Sickafoose. Post-production by Alan Douches [rhymes with couches]. Fact checking by Sam Moore. Special thanks to Natasha Mhatre, Rex Cocroft, and Heidi Appel for some of the insect sounds you heard in this episode, and to Chris Peiffer at WGTE Public Media. Threshold is made by Auricle Productions, a non-profit organization powered by listener donations. Deneen Wiske is our executive director. You can find more about our show at thresholdpodcast.org.