How do insects hear?

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You don't know this, but you're part of a radio show called CrowdScience, so lucky you. I'm in a lab at the University of Lincoln in the UK, with an insect you might be familiar with. You're our first cricket guest, actually, I think. A cricket. Hello, cricket. Can you hear this? Hello, cricket. Can you hear this? And I'm subjecting him to my singing. Hello, cricket. Can you hear this? Hello, cricket. Can you hear this? Hello, cricket. Can you hear this? Hello, cricket. Can you hear this? I'm really sorry if you can hear that. We'll come back to this cricket concert later, but first, a question from a CrowdScience listener. My name is Lauren and I'm in Brussels in Belgium. And Lauren, what's your question for CrowdScience? So my question for CrowdScience is how do animals here, and particularly insects, they're quite different than us? Lauren's curiosity comes from something that happened to her a couple of years ago. About two years ago now, I was diagnosed with sudden sensory hearing loss. It's a sudden hearing loss that occurs normally on just one side of your head. And it happens overnight. So trying to explain that to my friends and family has been complicated, which meant I have done a lot of research into how humans here, so I can explain how my own ear works to them and to myself. And it's crazy complicated. Like, there's so many little moving parts that rely on one another. For example, the reason I lost my hearing is because I'm so reliant on these tiny little hair cells. And when one of them dies, they can't be replaced or regenerated. That seems very irresponsible to have everything relying on one tiny piece of equipment. So I thought there has to be some other better way of hearing. And I thought, okay, let's get away from the mammal kingdom. How do insects here? And can they even hear me? Do you think as a chance that insects don't hear at all? Because I'm not sure. I can't picture an insect ear, right? Yeah, I can't picture the ear, but I can't imagine they can't hear it all because they make noise. And it seems they communicate to each other with noises like a cicada or a cricket. But maybe the way they hear is more reliant on vibration or maybe they have a different receptor for those subwebs than we do. Or maybe they have eight ears. Like how do I do? It has eight eyes. Exactly. Back up. Yeah, yeah. Thank you, Lauren. I agree with you. It does seem like a bit of a design flaw that something small can go wrong in an ear and stop the whole thing from doing its job. So are there better ear options out there? Might insects have simpler and more reliable ears? Let's start with a quick reminder of how we humans hear. Sound makes our ear drum vibrate and those vibrations pass through a few bones into the cochlear, which is a fluid filled structure that looks a little bit like a snail. It's lined with hair cells that turn those vibrations into electrical signals that travel into the brain. And it's those hair cells that have stopped working in one of Lauren's ears. So first things first, can insects actually hear? Some insects can hear, not all insects, but many insects can hear. What's the kind of default? If I met a random insect, should I assume that it can or can't hear? Let's see, beetles. Some beetles can hear, not all beetles. This is Natasha Matry, associate professor of Invertebrate Neurobiology at the University of Western Ontario in Canada. Some ants can hear, some butterflies can hear and not all of them. Car crutches, they can't hear you, so yelling is perfectly fine, but it won't scare them one bit. Okay, so a good chunk can't hear. Those that can hear, is there a particular basic insect that you've studied where you can talk us through its ear? What's going on? It's kind of hard to say which one's the simplest, but let me talk about the one that's the most studied, potentially. It's cricket. I mean, they clearly make a lot of sound because they use sound to attract mates and therefore they have to have ears and they do. So we study them quite a lot for that reason. Their ears are on their forelegs, so just below the knee of the front leg. Just in case you missed that, crickets have ears just below their knees. If you've got a pet store cricket, then you'll see a silvery little disc and that's its ear drum. And it's a stretched membrane, there's a hollow tube behind it and there are cells coupled to that air tube and that ear drum somewhat indirectly. So that's how they're here. Welcome. Hi, nice to meet you, I'm Caroline. Nice to meet you, Caroline. Hiya. To see, oh here, these cricketers in action, we're back at the University of Lincoln in the UK. We're going to add the stuff here. Okay. Visiting Professor Fernando Montelegrés at Pattern. Oh wow, there's a box of crickets on the side and his crickets. I don't know, maybe 40 crickets. Yeah, there are males and females and the males are singing. Now, male crickets make two different sounds. So that's a, that one. Yeah. That is a, it's a, it's a calling song. This is the sound of the male cricket saying, "Oi, you come over here." Okay, so why is one of them doing a calling signal? Oh, because the female probably went away from him. Okay, right. And this is the sound of him flirting. When she's closer, he begins to do the high frequency chip that you are hearing. He's trying to impress the female and persuade her that mating is a good idea. It's called call chip signal. Now, to help answer Lauren's question, Fernando has taken one of the crickets out of the box. Okay, so next to the box of crickets, you have a suspended cricket. Suspended cricket and he has been sedated. In front of me is a sedated dark brown cricket. I can just about make out his ear. It's a silvery bean-shaped membrane just below his knee. And Fernando has set up a machine to shine a red laser on it. This is only recording from one point on the membrane. So every time the membrane by, by grades, move back and forth. The laser detects that and converts this into oscillations. The membrane will act as a microphone, you know. You see, it's natural microphone. The laser detects the vibrating membrane and those vibrations are converted into a sound that we can hear in the room next door. So we can basically eavesdrop on what the cricket's ear drum is picking up. So this is what the cricket is hearing? Yeah, this is what the cricket is hearing. Wow, I mean, it sounds the same to me. Yeah, yeah. It can hear it as clearly as I can hit it. Yeah, when you play it inside sounds or these frequencies, it sounds really clear. Yeah. So a cricket can hear other crickets just as clearly as we humans can. But what about the other way around? How good are crickets at hearing humans? More precisely, hearing my singing. Initially, my voice is difficult to make out. But as I start singing higher and higher, the sound of my voice becomes clearer. And is that because basically when I go all squeaky, I sound closer to a cricket? Yeah. I'm basically a cricket. Crickets, the field cricket, usually the best range for sound automation is between three and six kilohertz, but five is the optimal. According to Fernando's Equipment. my voice peaked at 0.9 kHz, so I didn't quite reach a cricket's 5 kHz, but not a bad effort. Now, I think it's worth saying that we've been listening to the vibrations Fernando's laser has been detecting from the cricket's eardrum. Those vibrations still have to pass through the air tube behind the eardrum before being picked up by sensory cells and then passed on to the cricket's brain. So how much of my voice can our cricket actually hear? That's a very good question. We don't know how much of this the animal is capturing. When it comes to sensory cells that pick up on vibrations, a cricket ear is quite different from a human ear. We have sensory cells in the cochlea, so they are arranged and there's a resolution. We can hear every note and we can detect the difference between one frequency and another frequency that is close to the first one. The human cochlea has around 16,000 sensory cells, while the average field cricket is just 60 to 70 and they're likely tuned to the higher frequencies that other crickets and bats make. We don't know how they will interpret a complex signal. When they hear human voices or many sounds in nature, the birds or anything that you hear in the forest, we don't know how they interpret that. So a cricket ear is a more simple option, but it comes with a major compromise. It's likely that crickets only really hear frequencies that are important to them, so not the sound of humans or my singing, but the sound of other crickets and bats. But we're not done searching yet. Crickets are far from the only option when it comes to insect ears. There's so many interesting ears. Here's Natasha on all-round insect ear expert again. The mantis has the two ears on the two sides of fused into a single ear that's like between their legs. Locusts have ears on the side of their abdomen, moths and butterflies when they have them have ears just at the base of their wing, which is weird because the movement of the wings has to be making a lot of noise. But in my opinion, the funniest ear I know of is the moth that has a ear in its mouthparts. They have a mothpart, which is like a balloon, and this balloon when sound hits it starts to vibrate. And there's another mothpart that sits on top of this balloon, and then the vibration off the balloon is picked up by the second mothpart, and that has cells inside it which then pick up the sound. So why on earth do these insects all have ears in different locations? The vast majority of animals have ears on their heads, so how of insects ended up with ears from anywhere from behind a knee to the edge of a wing? Right. So when we're talking about us and then thinking most animals, what we're talking about is vertebrates, and effectively all vertebrates derive their ears from a single common ancestor. So lizard ears, bird ears, mammalian ears are all most likely from a single common ancestor. They have a lot of similarities. The ears of insects were all independent evolution, so each line of insects made its own ear. And one of the reasons that happened is making ears for insects is actually quite easy. Okay, so what might you need to make an ear? Let's go through the recipe. So what does the ear require? Ingredient number one, a membrane like an eardrum. It requires this thin stretch membrane. Ingredient number two, a pocket of air behind it, has to be stretched over some kind of air sac. And ingredient number three, sensory cells, cells that are attached to it in some way. And for insects, those cells are already there. So insects have these things called proprioceptors, cells that sense movement and vibration all over their bodies. And then they also have air sacs everywhere, because that's how insects breathe, they kind of have these trachea that run through their body. So insects have kind of pockets of air all over their body. Yeah, yeah. So you know how we breathe, we have lungs. Insects don't do that. They don't have lungs. What they have is this like massive, massive system of tubes that run all through their body. They absolutely go everywhere. So it's almost like the entire insect is along. Yeah, yeah, it is. It's not almost as if it is the whole insect is along. That's so strange. And then they can develop an ear sort of anywhere on this lung. Absolutely. They've got air sacs and you've got proprioceptors. All you need to do is make a membrane that stretches over one of these air sacs. And that's really easy. What I mean is it's easy for evolution to generate that variant. As far as we know, insects have evolved ears 17 to 19 times independently, depending on how you count. Wow. And so differently, you just just did it, whatever. So insects coincidentally already have nearly all the ingredients needed to make an ear, which means they've been able to evolve is in incredibly creative places. So far, we've looked at insect ears built around a membrane, but do insects offer any other hearing options. That's coming up next. Fall gatherings never tasted better. Save through November 3rd at Whole Foods Market. Sometimes we all need an extra help in hand, especially with the tricky business problem you've been putting off. Claude is an AI for just that. It's the collaborator that understands your entire workflow and thinks with you, not for you, and Claude isn't just for quick questions, handed a whole task like turning a folder or scattered notes into a report, and it does the work and hands back something finish that you can open and edit. Claude can now drive the dock or dock right in front of you, and Claude design now lives right inside Claude. Describe the website or prototype your picturing, attach your design system, and it comes back in your own design language, ready to edit right on the canvas. It works with your files and the tools you already use, like Gmail and Slack, and whether you're debugging code at midnight, building a financial model, or strategising your next business move, Claude extend your thinking to tackle the problems at Meta for problems worth solving, get started with Claude at Claude.ai/solve. That's Claude.ai/solve, and check out ClaudePro, which includes access to all of the features mentioned in today's episode, Claude.ai/solve. You're listening to crowd science from the BBC World Service, the show that listens to your science questions so you can hear the answers. I'm Caroline Steele, and this week we're answering a question from listener Lauren, who wants to know how do insects hear? So far, we've discovered that insects have evolved to have eardrums in all sorts of places, from below the knee on a cricket to the mouth of a moth. Now, we humans don't have ears on our elbows, but insects with eardrums sound quite similar to our human hearing system, and listener Lauren wants to know about simpler options. There are other ways insects can hear. I wouldn't call them simpler ways. This is Jörg Albert, a biophysicist at both the University of Oldenburg in Germany and University College London in the UK. Some just use like a pole because the forces of sound move, oscillate the particles in the air, and then by friction, these poles are moved around. Of course, then on the inside of the pole or stick or whatever you want to call it, certain cells attach to this stick, and those are neurons, auditory neurons, and they are also moved back and forth. So if some insects have kind of poles that stick out of their body get sort of bashed around by particles. Exactly. So where are these poles on the insects? In contrast to the membrane-based ears, these poles are almost exclusively at the head. They look like antennae, and they're also called antennae in zoology. And which insects hear like this? For example, the frute fly hears with their antennae, they've got antennae ears. Also, the disease-transmitting mosquitoes hear with their antennae, so the males use it mostly to hear the female. Imagine you're a male mosquito in a swarm of other male mosquitoes. It's sunset, and you're looking for a female. There aren't many potential partners out there. So the ratio is probably with one to ten you can approximate it. That's maximum for ten males, one female. But there are a lot of males with the same goal as you. There's lots of competition going on there, being able to hear the female. Now, it's the most important selection advantage of a mosquito. Imagine against the backdrop of hundreds of other male flight tones, there must be able to detect this faint female flight tone somewhere in the distance. It's unlikely to detect a female, and it's even more unlikely to really be able to chase Earth with this noise. The largest noise we haven't even talked about, that is the flight tone that is humming directly behind these antennaeers of the male himself. His own wingbeat, his own flight tone, should be drowning out almost all other sounds. So how do they do it? How do they hear the female over the sound of themselves and the other males around them? That is, I think, one of the most interesting and intriguing questions. York has studied the hearing of male mosquitoes by measuring their auditory nerves, which run from the ear to the brain. If you play the sound of the male wingbeat, the males can't hear their own flight tone. They can't hear it. They're completely deaf to their own flight tone, no response in the nerve, zero. And if you play the sound of the female wingbeat, female flight tone hardly any response. They can't really hear that either. Almost inaudible, at the very cusp of a tearing range. But if you play the sound of the male and female wingbeat together, suddenly the nerve goes bonkers. There's a massive response in the nerve. So what on earth is going on? How do they do that? The process is called distortion. Our ears, as the ears of mosquitoes, are non-linear. So if you pass through two tones, these two tones will generate additional tones, simply because they change, distort, deform, the waveforms of sound, simply by the construction, they can't but do that. So if you play a male wingbeat. Let's take an numerical example. Let's call it 600 hertz, so they beat 600 times per second. And a female wingbeat. The females have a lower tone, so they beat a bit less often, 400 hertz. Then the male mosquito hears something else entirely. Strangely enough, we found that the tone they can hear is something that we don't know where it should come from. It's a 200 hertz tone. And then this was the key to understanding the magic of the mosquito hearing system. Because the 200 hertz are simply the difference between the male and female flight tone. 600 minus 400, so 200 hertz. And even lower pitch, a lower tone that is created exclusively in the ear of the male. And it's called in music terms, the difference tone. So the male can't hear himself or the female. Instead, he hears the difference between the two, the difference tone. And fascinatingly, you can work out the pitch of the male mosquito based on the pitch of the female mosquito. They often have even a certain ratio, 1.5. And you can roughly estimate the male frequency for a species if you know the female that multiply it by 1.5, 400 leads to 600 then. This 1.5 in piano keys is a perfect fifth. The tonal ratio is exactly the perfect fifth. Paul Hindemith, the composer and music theoretician, he considered the most perfect of all tonal intervals. The relative difference between the average male and female flight tones is a particular musical interval, a perfect fifth. And mosquitoes aren't just hearing the difference tone, but lots of musical distortions at once. Many of those distortions, which can be calculated, become numerically identical for the perfect fifth. So that means many create 200 hertz. So this super distortion is, of course, super audible. So in his brain, in his auditory brain, in his ear, that is really almost a pure tone. Mosquitoes present us with a totally different kind of ear, no ear drums, but instead antennas which pick up on vibrations in the air. But these aren't necessarily more simple than human ears. There are 16,000 neurons surrounding each antenna, which is probably more neurons than you're using in each of your ears to listen to me right now. But I want to look at one final group of insects, the insects that don't have is. Well, every insect probably has the ability to detect solid-born vibrations. This is Professor Jane Yak from Carlton University in Ottawa, Canada. Technically, that's not hearing in the way that we use the word hearing, but it is detecting a vibration just through a different medium. So do you sort of mean like, if my phone vibrates when I get a message and I've got my hand on my desk, I'll feel that vibration? That's right. There's a broad umbrella of acoustic sensing in insects, and that includes solid-born vibrations and airborne vibrations and water-borne vibrations. So vibration detection through solid is probably the oldest form of acoustic sensing. It probably evolved way before hearing with an ear drum like we do. Imagine yourself a caterpillar or a spider or anything on the leaf. There's a lot of ways that vibrations are occurring, right? So there's wind, there's rain. Not to mention all of the other insects on the leaves that are making these vibrations, so it's this really noisy environment. Whether they can detect it or not depends on their sensory system, but probably what the insects are having to do is they have filters in their sensory systems, just like we do in our ears, to be able to tell what is relevant, what's not, and so we're really just learning about this thing that I like to call the Vibroscape. The Vibroscape, like a landscape of vibrations. Could you give me an example of an insect that detects vibrations through a solid? So I guess that's something like through a leaf or a tree. I'll give you an example of something we work on in my lab just because it never ceases to amaze me. A lot of caterpillars actually will use complex vibrational signals to talk to each other, to find each other, and also defend territories. They make silk and another of their same species will try and take over their house. Caterpillars build tiny silk shelters on leaves, which are apparently a hot commodity. And what in caterpillars will try to displace each other out of their houses? Yes, yes. And they do. They do. Sometimes these rat battles go for like two hours and they'll just back and forth, and then finally somebody leaves. So it's kind of like I like to equate it to a rat battle because it's nobody bites each other. Nobody hurts each other, but they just keep on doing it until one of them gives up. Because you're a little caterpillar, your mission in life is really to eat and develop into a moth, and you can't spend all day making a racket, you know. You got to get on the life. You got to get on with like, usually it's the bigger ones that win, so the drumming indicates how big you are, basically. Ah, and it's like, oh, I should give up, that other ones clearly stompier and more massive than me. Exactly. And another thing that's like super interesting that we just discovered in passing is that sometime the spiders will actually talk back to the caterpillars, and they make the spiders use these vibrations, they have their own communication system for mating and territoriality. And they're kind of going back and forth, and it's like, are they actually understanding each other? Because they're all living on the same vibroscape. Unlike Fernando, Jane doesn't shine her laser on cricket eardrums, but on leaves. And in a way, a leaf is like an eardrum. It vibrates and helps a caterpillar interpret the world around it. Here are some of the vibrations Jane's laser has recorded from a birch tree leaf. There are various caterpillars. There are predators like ladybugs. There's rain falling on the leaf. But airborne sounds move the leaf too, like traffic noise. So where does vibration sensing end? And hearing begin. Lauren, you wanted to know how insects here? And our whistle-stop tour of insect is, has taken us to some unexpected places, basically anywhere on an insect's body. But I think I'm most surprised by how familiar some insect is, of vibrating membrane, a cavity behind it, sensory cells detecting the vibrations, all ingredients we find in our own ears too. And even insects that don't have is, in my opinion, can still sort of hear? They can detect vibrations, like the caterpillars fighting over their silk houses. So could any of these insects offer a simpler alternative to a human ear? Well, some insect is, do have fewer moving parts, but none of them would be a better alternative for us humans, is they're all perfectly tuned to what each unique species needs to survive. And finally, over to you, Lauren, for the credits. That's it for this episode of Proud Science from the BBC World Service. It was presented by Caroline Steele and produced by Florian Bohr. Today's question came from me, Lauren, and Brussels and Belgium. If like me, you have a scientific question that has been bugging you for a while, do what I did in email [email protected]. Thanks for listening, bye. It's the collaborator that understands your entire workflow and thinks with you, not for you. And Claude isn't just for quick questions, handed a whole task, like turning a folder of scattered notes into a report, and it does the work, and hands back something finished that you can open and edit. Claude can now draft a doc or deck right in front of you. And Claude design now lives right inside Claude. The problems worth solving. Get started with Claude at Claude.ai/Solve. And check out Claude Pro, which includes access to all of the features mentioned in today's episode, Claude.ai/Solve. This week on the interface, millions of kids have grown up on camera, with their childhood started to content. There's a new law in California that gives influencers the right to be forgotten, and it could impact adults too. And an attack on Russian data centers started a rumor that all the evil disinformation bots had been deleted from the internet. But the truth might actually be a lot scarier. For our take, join me, Jess Maddox, and me, Thomas Jermaine, for the interface. Listen on BBC.com or wherever you get your podcast.