Instant Genius - How ultrasound tech could save hedgehogs

Episode Date: May 10, 2026

Hedgehogs are in decline all over Europe. These prickly little fellas are faced with so many threats – in their rural habitats, in our gardens, and on our roads. But new research about hedgehog hearing could help us protect these adorable creatures from going extinct. As part of our Science of Sound miniseries, we speak to Dr Sophie Lund Rasmussen, a scientist who has dedicated her life to hedgehog research and conservation. Also known as Dr Hedgehog, she’s a research fellow at the University of Oxford, and assistant professor at the University of Copenhagen. Sophie tells us all about hedgehogs – what she loves about them, and how we can do our bit to help them thrive. She also explains her recent study on ultrasound, which she hopes could significantly reduce encounters between hedgehogs and cars. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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professor at the University of Copenhagen. Sophie tells us all about hedgehogs, what she loves about them and how we could blow our bit to help them thrive. She also explains her recent study on ultrasound, which she hopes could significantly reduce encounters between hedgehogs and cars. Sophie, thank you so much for speaking with me today. It's such a pleasure to have you on the podcast. Thank you for inviting me. I'm so excited to speak about hedgehogs and your new research, but first of all, I wanted to ask more generally, you've dedicated so much of your life to spiky little hedgehogs. You call yourself Dr. Hedgehog even. Why hedgehogs? Why not badgers or beavers or birds or something else? Why dedicate so much of your professional life to

hedgehogs? Well, I've always loved hedgehogs and I think it was down to an experience. I had when I was very young. We were living in a house with a small garden. And then suddenly at night I heard a horrifying cry. And I couldn't really figure out what kind of call that was, what kind of cried was. I had to react because it sounded like someone was being murdered. And I stepped out into the garden in the middle of the night with my small torch looking. And all I could see were two hedgehogs.

in the middle of the lawn. And I thought, was that sound coming from one of those? And it turned out that it was. It was two hedge sharks mating and the female asking the male to bugger off. And I had no idea about that. But I was just so fascinated by these creatures and seeing them and being able to get really close to them. Because when they saw me, they just completely froze like they do. I just remember lying down on the grass.

looking at them, admiring them, and that was absolutely wonderful. So I've just always loved hedgehogs. But it was actually because I decided to study biology because of my love for animals and animal behavior, conservation. And I was volunteering at a wildlife rescue center where we rehabilitated, injured, sick, orphaned wildlife, and we spend a lot of resources on hand-raising orphaned hoglets, as they're called hedgehog, juvenile hedgehogs. And that's amazing.

I mean, I love it, but it's really hard work, especially when you're hand-raising a lot of litters because it's every like two, three hours. And if you're doing like five litters, you're ready to start all over again when you finish the last one. So I was inspired to write my master's thesis in biology about the survival and behavior of rehabilitated juvenile hedgehogs. Would they do as well as their wild counterparts when they were released back into the wild

when they had reached the age of independence? So I studied the two groups, so wild juvenile hedgehogs and hand-raised juvenile hedgehogs. And when I started to read up on the subject of hedgehogs, I just realized that in Denmark, where I studied only a few publications had ever been produced about hedgehogs. I mean, nobody had studied hedgehogs before. Only one professor, a few articles from his side and one of his master's students. And then I looked at the European level, and I was just surprised to find that, I mean, there was so many knowledge gaps about hedgehogs and not a lot of research producer.

okay, I can really make a difference here. I can fill a niche and make a difference for the hedgehogs. And by that time, we had already started to understand that the populations of hedgehogs were declining. So I thought, this is what I'm going to do. I'm going to dedicate my career and my life to studying hedgehogs. And I've been doing that ever since. And I work around the clock. I'm very dedicated.

I even have a hedgehog tattoo. too. And what is it about hedgehogs that you love so much? Because anyone can see that they're pretty cute. But what fascinates you about them? What do you love about them? So I think they're really interesting and mysterious when they move around in the dark, enter out gardens and interact and interact with other species. And they are just so peculiar, right? Everybody can tell what a hedgehog is. And they're just extremely fascinating in their behavior, in their appearance, and there's so much we don't know about them yet. So they're sort of, you know, little prickly moving enigmas. And I just love to be able to study them, but also be a hedgehog advocate

to defend them and protect them. And we need people to stand up for the hedgehogs. And I'm one of those people. Why should people care about hedgehog conservation? What do these animals mean to European ecosystems? What's important about hedgehogs? Why should people care? Well, they're really useful creatures in our gardens because they eat slugs and insects and snails and worms. And they also scavenge. So they sort of clean up in nature, you could say. And when they're digging in the soil and searching for food. They, of course, also provide a service to the soil ecosystem as well. But would the world end if the hedgehogs disappeared?

No, it wouldn't. I mean, as long as we have other species carrying out these tasks in the ecosystem, nature will survive. But I think the most important role for hedgehogs is actually as nature ambassadors, because we all like hedgehogs or nobody has ever told me to my face that they dislike hedgehogs at least, but people are sort of neutral and up to loving hedgehogs. And this amazing nature experience they provide us with to be able to admire them. They're one of the few species of wildlife that doesn't, you know, they don't run away,

they don't fly away, they don't bite you, you can actually get really close to them and admire them. So I think their role as nature ambassadors is the most important role they have. But also culturally, I mean, we've all heard about Mrs. Tickie Winkles. We all love her, right? And the hedgehog is just portrayed in so many beautiful ways. And even down to like medieval manuscripts with hedgehog paintings and, yeah, artifacts from ancient Egypt. And yeah, we've just been, you know,

fascinated about hedgehogs all the way through history. And so they do play important roles in the ecosystem as well, but their most important role is definitely as nature ambassadors. We know from the research that the European hedgehog is declining massively. In the UK, for example, it's estimated that up to three out of four hedgehogs in the rural areas have disappeared since the turn of the century. I mean, 75% decline. That's massive. So it's really problematic. And the decline is caused by a multitude of different factors. And one of my most important jobs as a hedgehog researcher is to understand what is causing the decline and what we can do to stop this decline. The major cause is believed to be road traffic. One out of three hedgehogs are being killed on the

roads each year. And this is, of course, a huge negative influence on the population. But it's also the laws of natural habitats because we are, you know, using nature to build factories and housing areas. We're just basically, you know, turning nature into residential areas and roads and whatnot. And the intensification of agricultural practices historically have also changed the rural areas to such a degree that they're just not good habitats anymore. In the old days, you had like small fields with hedges between them, and in these hedges, the hedgehogs left, and now they've been removed to create like large fields with monocultures, only one crop, massive use of pesticides, at least in some agricultural practices.

But this is, of course, also a problem. I'm also a bit concerned about hedgehogs in our gardens because since all the natural habitats are disappearing, since the rural areas are no longer really suited for hedgehogs to live, they increasingly inhabit areas with human occupation. And this means that the battle to save the hedgehogs is going to take place in our own backyards. So we have to make our gardens hedgehog friendly, because the hedgehogs are also faced with a lot of dangers in our residential gardens. And we don't do it on purpose.

We don't intentionally hurt the hedgehogs. But our actions can just cause them so much trouble. First of all, it's important if you want to build a hedgehog-friendly garden to make sure they can enter the garden. So if you have a massive fence surrounding your garden, you need to create a hedgehog highway, a hole for the hedgehog to enter. otherwise it wouldn't make much sense to make your garden hedgehog friendly. But if you could have your whole neighborhood connected by small hedgehog highways,

the hedgehogs wouldn't have to cross roads either. They could just travel through our gardens easily and save a lot of energy also when they're out foraging and looking for mates and so on. They also lack good nest sites. There's a tendency for people to have like really boring gardens nowadays, is like short grass and, you know, tiles or wooden terrace and that's, it's not allowed to be natural or messy or anything. And there are no places for the hedgehogs to nest.

They need, you know, natural nest sites. They need a pile of, you know, garden waste, cut off branches and leaves. They are very adaptable. I've also studied them in urban areas where they live under the, you know, wooden terraces. They live under a garden shed. but natural nest sites are of course best. But you can also just build a hedgehog house, a buy one, and offer this to them.

Oh, that's lovely. And make sure they have something to eat, a nice garden full of biodiversity. You know, insects, snails, slugs, plants attracting biodiversity, will enable the hedgehogs to feed naturally, which is the best option, of course. Thank you. Are you one of those media strategy people clicking through slides, scrolling spreadsheets?

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Hey y'all, it's Kelly Clarkson with Wayfair. Ever order furniture online and wonder what if? Like, what if it doesn't hold up? That sofa was four days old. You should have ordered from Wayfair. With Wayfair, there's no what if. Just style you love and quality you can trust. Visit Wayfair.ca. Wayfair, every style, every home. I feel so bad. The poor hedgehogs. There's so much that's out to get them. But what I love about your very recent study about ultrasound is that you've sort of found a possible solution to stop them being hit by cars, which sounds wild, that somehow

ultrasound could save the hedgehogs. I mean, we mostly associate ultrasound with, you know, pregnant people going to see their babies for the fetuses in the womb for the first time, not hedgehog. So how does this work? Can you explain to me, like, how can we join the dots between hedgehogs, ultrasound, and cars? Yes. So I decided I wanted to do something. something about the road kills, because this is, of course, as I mentioned, the worst cause of the decline of the hedgehogs. And to be able to reduce the number of hedgehogs killed on the roads, because it's estimated that one out of three of them are squashed by cars each year, just to be able to reduce that

number by, let's say, 10% or 15% would make a huge impact on hedgehog conservation. So I set out to try and solve that problem. And I thought, I'm not going to be very successful in having cars banned. Okay, so I had to come up with another solution. And I thought perhaps we could somehow develop sound repellers that would make the hedgehogs avoid crossing the roads when the cars are running. I mean, we want them to cross the roads. We want them to search for maids and food and good nests.

sites and our landscapes are completely fragmented. They're completely divided by roads everywhere, right? So we want the hedgehogs to crossroads, but we don't want them to get squashed by cars. So I thought, okay, to develop sound repellence that would be efficient for hedgehogs and hopefully also not bothering people and their pets because then people wouldn't be inclined to using I set out to describe hedgehog hearing for the first time, because it hadn't really, in the European hedgehogs, it hadn't really been studied like probably before with modern day methods. So researchers had historically tried to describe the hearing of the hedgehogs by jiggling keys, for example, and seeing if they responded to that.

But like a really systematic study with button technology had not been done before. So I gathered a team of experts on bioacoustics measuring hearing in animals and experts in, you know, animal experiments, a veterinarian who was specialized in treating anesthetarchics and treating them during anesthesia and so on, a dream team full of great capabilities. And we set out to measure hedgehog hearing for the first time. And that was really interesting because we found that hedgehog hear, between 4 kilohertz and 85 kilohertz. And ultrasound is everything above what humans can hear,

and that's up to 20 kilohertz. So everything above that is ultrasound. And our cats can hear up to 65 kilohertz, our dogs up to 45 kilohertz. So imagine how happy I was when I realized that the hedgehogs can hear up to 85 K-KHz. I was actually dancing around with my arms in the air. I was so excited because that meant that we could actually potentially target sound repellers at hedgehogs that wouldn't bother us, wouldn't bother our pets and would therefore most likely be used. That was just wonderful news.

We had also done micro-CT scans of a dead hedgehog. She died in care. She had been trapped in a rat trap. so her leg was completely destroyed. There was no way of saving her. So she was euthanized and we were allowed to use her for micro-CT scans to look at the inner ear and the anatomy of the inner ear of the hedgehog. This was the first step because we wanted to see whether the hedgehogs were actually equipped to hear ultrasound. And when we looked at the scans, we could tell that the anatomy was very much like that of bats, for example, that can hear ultrasound and they echolocate and so on.

So we thought, okay, there is something to this. We should definitely study this. So we tested hedgehog hearing on 20 individuals. And what we also found was that they heard best around 40 kilohertz. So that's also ultrasound. And this is also really interesting because this could indicate that they're communicating in ultrasound as well. And I've often wondered about this because apart from the scream I heard when I was a child,

I haven't heard that many sounds from hedgehogs. They don't have that many sounds. I mean, there's this whistling sound when the hoglets are calling for their mother. And then they hiss at each other when they tell each other to go away, like, and then there's this major distress call scream, which is a horrible sound. But when you see the hedgehogs interacting when they meet each other in the garden at night, I know they're solitary species, but still you can see that they're interacting. And I've often been wondering whether they only communicate through body language and scent and so on

or whether they are actually blabbering and we just couldn't hear it. There might be something to this. And we've of course already started to look into this and try to record sounds when hedgehogs. meet each other. But it's just been hibernation season. So we've been waiting for them to wake up again so we can, you know, record sessions and gardens where hedgehogs meet each other. But we have started to work on this. And I find that really interesting. Imagine if they're just, you know, blabbering all the time and we just haven't been able to hear it. Oh, you're really like changing my perception of hedgehogs in this conversation because I feel like I had such a preconceived idea

that hedgehogs were these timid creatures that they were so quiet, so shy and they never said anything, they never did anything. They just sort of crulled through our gardens and ate things. But now I'm like, they might be like gossips that are just, you know, having a natter of all their friends, but we just can't hear them. That's wonderful.

So, I mean, it's just yet to be described, but I just imagine why would they hear best at 40 kHz if this is not where they're communicating? I mean, wouldn't make much sense. Yeah, absolutely. Super interesting. So can you tell me a little bit more about the study and how you tested the ultra? Like, what did it look like? Because I think often when people learn about scientific research, you can go, okay, you tested whether hedgehogs could hear. It's difficult to imagine what the scientists were actually doing to the hedgehogs. How did you work out whether they could hear things? How did that work?

I decided I wanted to study hedgehog hearing. And then I thought, oh, how does one study hearing? hedgehog. I have no idea. So I called a friend. I called Jakob Christensen Delskow, who's a bio-acistician at the University of Southern Denmark. And I asked him, so how would we go about measuring hedgehog hearing? Because I had tried to read up on this. And traditionally, hearing and animals have been tested through behavioral tests, where you would actually train the animals to let you know when they hear something and then they'll get a treat. But I thought hedgehogs, they're wild animals. I don't want to be kept in captivity and to be trained for experiments. And I wouldn't be much successful either,

I think. They were going to hate me, you know, and cheat to get the treats, I think. So I just thought, this is not a good approach because I don't want to stress them. I don't want to cause them harm. because as mentioned earlier, I really love hedgehogs. I really don't want to cause them any harm. So it's very important for me that animal welfare is a top priority. So I asked Jacob, so is there another solution? Can we measure this in another way? And he said, yes, there is a method called auditory brainstem response

where you basically measure the brain activity of a sleeping hedgehog when you're playing all sorts of sounds to it because even though it's sleeping, the brain still perceives the sounds when the hedgehog can hear them. So this was what we did. So basically, we tested 20 live hedgehogs.

They had been in care because they were either sick or orphaned or injured and they were evaluated as being ready for release. And then we tested them, of course, at nighttime when they're active. and then we measured their brain activity, and they were sedated for up to one hour each, just to make sure they weren't stressed. I mean, we wanted them to sleep because we wanted to make sure they were doing fine and weren't stressed. And then we used three little electrodes. They're like little acupuncture needles. One was just put underneath the ear, just on the skin of the hedgehog subcutaneous.

and one in the forehead and one in the paw of the hedgehog. And then we could measure the brain activity of the sleeping hedgehog. And whenever the hedgehog started to wake up, like started to move, we stopped the experiment straight away. This also meant that some of the hedgehogs were not tested within the full range of frequencies because we wanted to stop. But this was really not very invasive. They were doing great.

And my most important task during this test was to monitor them when they were waking up. So I was basically just looking out for the hedgehogs, providing all the food and water they wanted, making sure they were doing great. And we, of course, had a specialized vet on the team who checked them all the time. And they were actually, they were doing great. And they were released on the following night. So it wasn't very invasive to them. Of course, I mean, I'm so, so grateful for these 20 individuals to have provided this information for us. They're out in the while now doing great, and they are potentially going to save thousands of hedgehog lives.

I have another question which is a little bit random, but it's just because you've mentioned them a couple of times and I actually don't know anything about them. Hoglets. What are hoglets like? What do they look like? What are the little baby hedgehogs? I just, I want to be able to picture little baby hedgehogs, and I would love you to be able to tell me about them if that's okay. Yes.

So we actually also tested on juvenile hedgehogs in this test because we wanted to test the young and the old, because I just imagined that the older ones would have a weaker sense of hearing like old humans do. So I wondered, you know, the whole spectrum represented. We, of course, only tested on, juveniles that were independent. They were ready to be released.

They were ready to live their own lives. They were not depending on their mothers anymore. And they had been in care. They had been hand-raised, bottle-fed at the rescue center. But when the hucklets are born, they're quite small. I mean, the size of a grape, almost really small. And they're, of course, born without spines. That would be really hurtful.

But they appear after around one hour. The hucklets are sort of pink and have white spines. And then after up to a week, more brownish spines start to appear, they are a bit harder, stiffer. And then the hucklets are sort of developing up until the age of around three weeks where they actually look like adult hedgehogs. They have, you know, adult looking spines. They look normal.

you could say, and their ears and eyes are open and their teeth have appeared and they're ready to face the world. And this is the age where you normally would see the hoglets in the garden because this is the age around three or four weeks of age when the mother starts to take the hucklets out for walks and teach them how to survive in the wild. So this is where you see that wonderful scenario with a mother with like three, four, five hucklets after. her exploring the garden. Oh, so sweet. Yeah, it's really sweet.

And imagine, I mean, they're completely silent, right? Or are they? Because you would just imagine that hoglets would go, Mama, look at it, look at it, oh, we see that. But, and perhaps they do. We'll soon learn that. Oh, that's lovely. That's lovely.

So, Sophie, do you have a favourite fun fact about hedgehogs that you'd like to share? When hedgehogs are faced with danger, they stand completely still. They freeze. This is actually the first thing they do. It's not to curl up. It's to freeze. And this is what they do because they've developed this strategy when faced with danger to avoid the predators that prey on hedgehogs.

And these predators see very well in the dark, but they see movement. So this strategy is wonderful against natural predators. but it hasn't really evolved since the cars started to show up. So the hedge sharks are a bit behind. And then when they realize that they've been discovered, they decide either to run away or curl up. So this is a wonderful strategy against predators, but a really stupid strategy when a car is coming at you at high speed.

And this is, of course, where the trouble lies. So now we know what hedgehog. can hear. And that's wonderful. Now the real work starts to try and develop these sound repellors because we want sound repellers that would actually scare the hedgehogs away from the cars temporarily. But we don't want the repellers to make them freeze because then they'll just stand still in front of the cars, right, and get squished anyway. So we have to discover which sounds actually scare the hatcharks and which sounds. scare them less, but to a degree where they would avoid the device emitting that sound.

And then we need to discover how often this sound should change, because I would also imagine that they would get used to that sound and perhaps start ignoring it if they heard it all the time. So should we develop sound repellents that would change sounds like every day of the week, for example? And then really important is, of course, also to make sure that there are no unintentional, like harmful effects of sound repellers. I have this vision that all new cars will be fitted with this device. Robotic lawnmowers, some of the models can hurt the hedgehogs.

I'm working on that, creating, you know, hedgehog-friendly robotic lawnmowers as well, but some models could still hurt hedgehogs. And also garden streamers. Imagine if these devices were all fitted with sound repellers. We just want to make sure that the effect of these repellers doesn't last after they have stopped emitting the sounds. We don't want to evict hedgehogs from gardens where the robotic lawnmowers are running or the garden streamers, because the gardens are the most important habitats for hedgehogs nowadays. So we want to make sure that they are only sort of.

of avoiding these devices, perhaps not even leaving the gardens, and then re-entering the gardens when they're not running. And the same goes for the cars. We want the hedgehogs to cross the roads when there are no cars present. So we want to make sure that we don't completely isolate the hedgehogs and local populations because then we'll have an issue with inbreeding, for example, and this could of course also be detrimental to the population. So it's a huge responsibility to take on to try and design sound repellers. And it requires a huge research project. So I'm hoping that the car industry is going to reach out to me now that we've published the study

and we've found that they hear high-frequency ultrasound and that we could potentially target these repellers because I need funding for the research. And for me, it's a no-brainer for the car industry to support this because it's their devices that cause this decline in hedgehogs because the cars are running over hedgehogs. But it could also be beneficial to traffic safety in general because have you ever run over a hedgehog before? I haven't. No, but I'm glad to hear that because people have told me when they've experienced that, it's a horrible experience. and some are trying to, you know, avoid the hedgehogs, and this could actually lead to incidents.

So there was a new story a few years back with a guy in Germany that was actually killed because he avoided a hedgehog and ran his car into a tree. And I also witnessed the situation where a young boy cycling home from soccer practice at night was actually run over by a car because the driver was trying to avoid a hedgehog crossing the road. Fortunately, it was only the bicycle that was really injured. But I mean, if we could reduce the amount of encounters between hedgehogs and cars, I don't think it's realistic to completely avoid them, but reduce this, we could also increase the traffic safety for people, which is, of course, also really important.

Yeah, absolutely. I only have one more question for you, Sophie. And it's that we've talked about so many different things about hedgehogs. But if listeners take one thing from our conversation, what do you hope it is? The hedgehogs are in massive decline all over Europe. We need to stop this decline if we want our hedgehogs to survive, if we want our grandchildren to see the hedgehogs at night in the garden. But it's not too late.

The most important thing you and I can do is to make sure our gardens are hedgehog friendly. So please have a look at hedgehog.org. to see how you can make your garden hedgehog friendly because we can actually, if we work together, make a difference and help save the hedgehogs. Thank you for listening to this episode of Instant Genius, brought to you from the team behind BBC Science Focus. That was Dr. Sophie Lundrasmussen.

To learn more about hedgehogs, you can check out her YouTube channel at Dr. Hedgehog. If you liked what you just heard, please consider subscribing to Instant Genius wherever you like to listen to it. podcasts. If you'd like to watch our presenters and guests speaking in person, then you can also check out our YouTube channel at ScienceFocus. The current issue of BBC Science Focus magazine is out now. Pick up a copy wherever you buy your favourite magazines or download us on your app store of choice.

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