Hark | 5 | Common Sense

fast forward hundreds of millions of years to 1946. Just down river from the very spot where that ancient moss lived and died, another Trailblazer is born.

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. 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. 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. So without Dollyphyton, there might never have been a Dolly Parton. Welcome to Threshold, I'm Amy Martin, and in this episode, we're going to explore the intertwined acoustic lives of plants and insects. So I want to start by naming the dung beetle in the room here. Both plants and insects are things that people often ignore.

We just 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.

accurate. Insects make thousands of different kinds of sounds, and they also have an impressive variety of ways to listen. Dr. Natasha Mhatre: The fun thing about insects is that they have evolved hearing at least 17 times independently, so they've invented ears over and over and over again. This is Dr Natasha Mhatre. 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. Dr. Natasha Mhatre: Grasshoppers have them on the abdomen, so in one of the segments of their body. Crickets have them on their forelegs, so they move the ears as they walk. Cool! Natasha says one kind of moth even grows an ear on its mouth.

So the thing it eats with is also the thing it hears with. Dr. Natasha Mhatre: I would not want to be this moth because, like it would sound awful. Arthropods currently make up more than 80% 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. Dr. Natasha Mhatre: 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. Is it fair to say that you have a special relationship with tree crickets?

Dr. Natasha Mhatre: I have a special relationship with tree crickets. Tree crickets punch way, way above their weight class in terms of making themselves heard. They are really small and they can be really loud. Dr. Natasha Mhatre: They're super cool. They live pretty much everywhere. There's tree crickets in Australia, there's three 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 this, you know, it's lots of species, so you can probably find a tree cricket somewhere close to you. 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. 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 crickets are actually moving their wings up to 100 times per second, turning the sound of their wing beats into an acoustic blur, at least our ears. There's quite a bit of variety in tree cricket song. Some make sustained trills. Others like to lay down a groove, 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. 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. Dr. Natasha Mhatre: 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?

Wow. So we got like, a bunch of judgmental lady crickets. Dr. Natasha Mhatre: Totally. Well, they're like, if I'm gonna spend the time walking up to you, wherever the hell you are and finding you, you better be worth my while. And I'm gonna tell that by the quality of your, your plucking. Dr. Natasha Mhatre: That plus just how long you go like crickets that you know have a little bit of stamina do better. Yeah. Evolution is cruel.

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 stratulate Their wings, the leaf turns up the volume on their songs. Dr. Natasha Mhatre: Think of making your wings artificially bigger.

Natasha ran experiments to test the cricket's 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. Dr. Natasha Mhatre: 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. And once they've chosen the leaf they want, they demonstrate an incredible level of skill. Dr. Natasha Mhatre: 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. 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 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. Dr. Natasha Mhatre: This one guy didn't get to the center, and he's like, this is no good. Then he went and chewed the second hole and went straight for that. Amazing. That just seems like fairly complex problem solving. I couldn't do that.

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. Dr. Natasha Mhatre: The female climbs on top of the male. She eats from that gland while he mates with her. She is dining while mating.

Dr. Natasha Mhatre: Yep, and it's like, you know, 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 in whatever that nuptial feeding is. And the males are the true multi-taskers here, because they continue to sing while all of this is going on. Natasha says she doesn't really know why. Dr. Natasha Mhatre: 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. Huh. Yeah, it just almost seems like logistically difficult to do so many things at once. Serve a meal, have sex Anything to keep her there. and keep singing. After talking with Natasha, I realized I was hearing tree crickets all the time without knowing what they were, so I decided to track one down on a September evening in a little nature preserve not far from my home in Missoula, Montana.

I'm tiptoeing through this field. With my microphone out and my headphones on, I let my ears guide me through the tall grass. Definitely a case of the whole the elusive, whatever you are. I take a quiet step... That is a squirrel. And then another... Okay, maybe you're in here somewhere. And finally, I see it. 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. It's very light green. I've heard it all my life here in Montana, and never stopped and tried to figure out what it was. It's beautiful. 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 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 all 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.

how much work it takes to make this kind of show. I mean, the the time, the dedication, the determination that's required to tell these, these in depth stories that really make people think and feel, and give people a sense of what it's like to really go to places where the stories are happening, to talk to the people who are part of them. It creates this rich, immersive listening experience, And I'm telling you, that kind of reporting, this whole kind of show, is not easy to make. It's

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stories that inspire climate action. Recent episodes feature insightful conversations with leaders like Netflix's first sustainability officer, Emma Stewart, who discusses how the global entertainment giant uses its platform to promote climate awareness. You'll also hear from CNN chief climate correspondent, Bill Weir, about the importance of integrating climate change into news coverage. Each episode dives deep into the challenges and opportunities that climate change presents to entrepreneurs

and innovators. Listen to Climate Rising every other Wednesday on Apple podcasts, Spotify, or wherever you get your podcasts. 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. 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.

Dr. Heidi Appel: Well, for most of my career, I've been studying how plants defend themselves against insects, and they do that through chemistry. Dr Heidi Appel is a biologist currently at the University of Houston, and she specializes in chemical ecology. Dr. Heidi Appel: Plants actually evolve to ward off their own pests, including microorganisms that cause disease. 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. So this is Heidi's world- plants, insects and chemicals.

Dr. Heidi Appel: 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. Dr Rex Cocroft is an expert in insect communication. We met him in our last episode, and 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.

Dr. Heidi Appel: And I, you know, 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. 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. Dr. Heidi Appel: He paused and I paused, and we kind of looked at each other, complete strangers, you know, until now.

They were both stopped in their tracks with the same thought. Dr. Heidi Appel: Oh, you don't suppose, do you that the plant can use vibration information for its own purposes? Dr. Rex Cocroft: That's when I think, we both began to wonder about from our own perspective, like, well, could any of this information be relevant to the plant, and could the plant be using any of it? Dr. Heidi Appel: That was the aha moment.

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. that's really what kicked off this study, and we've been collaborating closely ever since. It was already known at that point that plants responded to sounds made by humans.

Dr. Heidi Appel: Plants will respond to single tones or to music in all kinds of ways. Dr. Rex Cocroft: If you play a tone, just a pure tone, like you could increase crop yields. There's a whole range of plant traits that will be altered. Dr. Heidi Appel: The real mystery for Rex and me, because we were trained as ecologists, was why would plants have that Dr. Rex Cocroft: And that's where our work came in, is that ability?

Playing music or electronic tones to plants is we said, well, what's an ecologically relevant acoustic very human centric. It's asking, how do plants respond to these stimulus for a plant and what would be a relevant response? 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?

recordings of tree hoppers. So he knew this was a conspicuous sound produced when an insect was doing something very biologically relevant to the plant- eating it. Dr. Rex Cocroft: If there was a caterpillar on the plant feeding on a leaf, then that's mostly what you're going to hear. And Heidi knew about the chemicals plants produce when this is happening. Dr. Heidi Appel: When a caterpillar bites at plant, it

gives 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. 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. Dr. Rex Cocroft: To think of the plant as a kind of active listener. I never thought about that. 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 caterpillary way, to give the plant the experience of being eaten, but without any chemicals being transmitted. Dr. Heidi Appel: We want to just separate the effect of the vibrations. 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.

Dr. Heidi Appel: 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. Uh huh. So it wasn't just the drool or some other element of caterpillar chemistry at play. The plant did, in fact, seemed to be responding to the vibrations. Dr. Heidi Appel: And that surprised us. We thought, oh, maybe it's a fluke. Ever the careful scientists, Heidi and Rex interrogated their results.

Dr. Rex Cocroft: Well, it was interesting. I in no way believed it at that point. They didn't believe it yet, because they didn't know if the plant was alerted by the vibrations of chewing in particular. Dr. Heidi Appel: 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 their environment. 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 and a leaf hopper call. These little insects are related to the tree hoppers 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. Dr. Heidi Appel: So we thought that would be a pretty good test to see how specific this plant response to feeding vibrations is.

So we've got wind, tree hopper 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. 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. Dr. Heidi Appel: We don't identify the samples as coming from one treatment or another, so they come out as a bunch of numbers. Those numbers then go into a spreadsheet that decodes the data. Dr. Heidi Appel: So you're sitting at a computer, you've been sitting for four hours getting all the data aligned and in the system.

And then finally, the results were revealed. The plants that had received the caterpillar vibrations had produced more defensive chemicals than the others. Dr. Heidi Appel: I remember sitting there screaming, there's a difference here and not here! And I think I might have even gotten up and done a happy dance. And of course, right away I'm texting Rex. The plants were listening out for the vibrations that mattered the most. Dr. Heidi Appel: So yeah, it was very, a very exciting moment.

And, you know, I I feel lucky if I had, I've had one of those this exciting in my whole scientific career. And if I never have another one, I'll die happy. Because this really hadn't, no one knew this before. You added something new to our scientific knowledge of plants.

Dr. Heidi Appel: Yes, we did. Because even though people knew that plants could respond to these other synthetic tones or music, right, the fact that he could discriminate between a biologically relevant sound and one that wasn't, was a real big advance in our understanding. 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.

Do you feel a little bit like Galileo looking through the telescope for the first time of like, oh my gosh! Dr. Heidi Appel: It does cause me to look at the world very differently, but I don't think we're going to have the impact that Galileo. Maybe not, but Rex and Heidi's discovery did make a big splash. It was covered in newspapers around the world, and 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 dialog with animals, just seemed too out there. Dr. Rex Cocroft: 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.

Now that we know that plants can 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 every day. 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. Dr. Rex Cocroft: The soundscape, or the vibroscape, if you will, of a living plant is one of the most complex soundscapes that there is. Leaves capture the airborne sound environment. Sounds in the soil can move up through the roots.

Dr. Rex Cocroft: And then you have all of the incredible sounds or vibrations created by insects and other animals that are in contact with the plant. Understanding this, the question shifts from why would plants respond to sound to why wouldn't they? 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, turns out plants are also listening. Does it give you more of a sense of kinship with them?

Dr. Rex Cocroft: I think it does. I would say that 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. 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 evidence based work to try to answer them.

Dr. Rex Cocroft: I would say, in some ways, plants do almost everything that animals do. They just do it really differently. 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 rests on all these other living things that I often ignore, but the work of these scientists helps me feel it.

Dr. Heidi Appel: 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. It feels like it makes plants less other. Dr. Heidi Appel: It's this common sense of being alive and being interconnected, which I think is really important to our development as humans. Maybe someday we'll learn that plants do have supernatural powers, but in the meantime, just the natural seems wondrous enough.

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. 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 Weiske is our

executive director. You can find out more about our show at thresholdpodcast.org.