Science Friday - How Cannibalistic Tadpoles Could Curb Invasive Cane Toads

Episode Date: June 4, 2025

South American cane toads were brought to Australia in 1935 to help eradicate native beetles that were destroying sugar cane crops. The toads didn’t care much for the beetles, but they did spread across the coast of Queensland and beyond, with no natural predators to stop them. Their own deadly toxin devastated local reptiles along the way, and they now number over 200 million.Invasive biologists have long tried to curb Australia’s cane toad population. The newest approach uses CRISPR gene-editing technology to create cannibalistic “Peter Pan” cane toad tadpoles: tadpoles that don’t fully mature and instead feast on the tens of thousands of eggs that the toads produce.How was this approach developed, and how do these researchers think about making a potentially massive change to the ecosystem? Biologist Rick Shine, who has led the effort, joins Host Flora Lichtman to discuss it. Later, science journalist Elizabeth Kolbert talks about her experience reporting on similar monumental efforts to control nature—and what they say about us.Guests: Elizabeth Kolbert is a staff writer at The New Yorker and author of Under a White Sky: The Nature of the Future (Crown, 2021). She’s based in Williamstown, Massachusetts.Dr. Rick Shine is a professor of biology at Macquarie University in Sydney, Australia.Transcript is available on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Hey, this is Flor Lickman, and you're listening to Science Friday. Today in the show, engineering cannibalistic tadpoles to solve an invasive species problem. This is the first time that I felt some optimism that we could actually do something about controlling canoads. Cain toads are one of Australia's most destructive invasive species. Picture a football-sized brown toad with poison skin, so animals like dog, snakes, even crocodiles will die if they try to eat them. And scientists have been trying for decades to reduce their population. Now researchers have turned to the gene editing technology CRISPR to create cannibalistic tadpoles

that never grow up and eat their brethren's eggs. You heard that right. So how does this work and could this be the solution to Australia's cane toad problem? Here to tell us more is Dr. Rick Schein, an evolutionary biologist at Macquarie University in Sydney, Australia, who's leading that effort. He's also the author of the book, Cain Toad Wars. Rick, welcome to Science Friday. Thank you. Let's start with some history.

How did cane toads end up in Australia in the first place? Well, cane toads are native to a big area in South America. And people had the brilliant idea that they might be great to control beetle pests in sugarcane plantations. So they were brought to islands in the Caribbean. Then to Hawaii, where they were growing a lot of sugar. and in 1935 the Queensland government sent a chap called Regmont Gombory over to collect 101 cane toads, brought them back to Australia, and their offspring were released in the sugar cane fields of northeastern Queensland.

And what kind of damage have they done and what makes them so destructive? The big problem we have is that Australia is so isolated from the rest of the world that we don't have any native toad species. There are toads over most of the world, but Australia didn't have any native toads. And toads have got a really distinctive set of chemical defences, poisons, called buffatoxins. And if you haven't evolved side by side with toads and you're a predator that eats frogs, you come along when you see this great big, brown, warty, new kind of frog that's turned up in your backyard, you eat it and you do have a heart attack almost instantly.

So we've had massive mortality of some of the big apex predators in Australian tropical ecosystems. Creatures like our goannas are big varanid lizards, some of the biggest lizards in the world. Qualls that are a marsupial predator at the size of a cat, some of our big lizards and snakes as well, and crocodiles. So there's been devastating mortality of some of the apex predators. And have cane toads themselves evolved since arriving in Australia? Yeah, look, it's a remarkable story of rapid evolution. We used to think that evolution was a slow process, but invasive species have shown us that under the right conditions,

with new kinds of evolutionary challenges, a species can change incredibly fast. So the cantoes at the invasion front in Western Australia are very different in their body shapes, their physiology, their reproductive biology, their behavior. Including cannibalism, am I right? Yeah, so remarkably, the cane toads within Australia display intense cannibalism when they're tadpoles. If a cane toad tadpole is sitting in a pond and it detects a female toad has come along and laid her eggs into that pond,

then the toad tadpoles race across and they eat those eggs. They kill them. And so it's incredibly intense cannibalism. It can wipe out 99% of any newly laid eggs. They're very abundant in Australia, and so cane toads have to compete with other cane toads for food. If you're a tadpole and there's 20,000 new eggs just being laid in your pond, you're going to be in big trouble in terms of food supply unless you get rid of the competitors. That's not true in the native range.

And so the toad tadpoles in South America and Hawaii really are not very cannibalistic at all. So your team created something called a Peter Pan. tadpole. What is that? Okay, so we named it after the fairy tale character who never grows up. And that's exactly what we're creating here. We've gone in using CRISPR. We just inject a newly laid toad egg and we knock out a genetic pathway that produces a hormone that a tadpole needs to turn into a baby toad. If it doesn't have that hormone, it simply can't make that transition. And as a result, those gene knockouts simply live in the pond forever. They never turn into a baby toad.

They simply live there as tadpoles. And if any other toad comes along and lays her eggs in that pond, the Peter Panes are there ready to eat them all. And so you've used CRISPR to make this genetic modification to the cane toad. Yeah, look, absolutely. I think it's really critical to point out that this is just a gene knockout. A lot of people are very concerned about using CRISPR for biocontrol. They worry about transgenics, where you take genetic material from one organism and you put it

into a different organism. You create something that doesn't occur in nature. But this is a simple mutation. This is just like something that happens probably in every clutch of cane toads that's ever been produced in the last few million years. There's always a few sad little tadpoles that don't transform. We're just making that happen on a major scale now. that we have a whole lot of non-transforming tadpulse.

And then what's the ultimate goal? How do they help with this invasive's problem? Well, the fundamental problem with cane toads is to stop them from recruiting, from a pond, from reproducing. You can go out there and kill adult toads, but a single female can have 20,000, 30,000 eggs. So you really have to stop that breeding. And the peter pans can do that. If you've got a pond with peter ponds in it, then there can be no toad breeding from that pond until the Peter Panes finally die of old age. And so it's a relatively efficient way to stop upon being a source of new toads.

You know, you can go in there and trap tadpoles and the like, but that takes a heck of a lot of work. The Peter Panes will do that work for you. What does success look like? I think success will look like a pond with Peter Panz in not producing all those little recruiting metamorphs hopping around the water's edge, whereas the pond next door, where we haven't deployed Peter Panes, will have several thousand metamorphs. So that'll be the first step.

And then where are the rest of the challenges? I mean, can you scale this up to put Peter Panes in every single pond in Australia? I mean, every puddle that a cane toad might reproduce in? At the moment, the plans are all about coming up with a system that will work in selected sites where you can put the effort in. And in particular, we'll be looking at the semi-desert regions where they're restricted to a very small number of artificial ponds. They're the only places that toads can breed. And so we can take them out of the equation.

We may be able to eradicate cane toads from hundreds and hundreds of square miles. Similarly, on small islands where there's only a few ponds, we could get rid of toads there that would create real sanctuaries for biodiversity. So I think we'll start out small. we'll pick the areas where the method is most likely to be powerful, and then we'll start looking and thinking and dreaming and hoping that we can find a way to scale it up to cover a much larger area. Obviously, there's a history of humans intervening in the natural environment

and things going wrong, like how cane toads got to Australia in the first place. What are the things that you're worried about, that you're sort of testing for, that you're on guard for with this? Yeah, look, I think there's a very long and humbling history of scientists who think they understand it all well enough and they go ahead and do something and it has unintended consequences. And the cantoads are the classic example of exactly that problem. So we're moving very slowly and carefully. We have an enormous amount of background information about the ecological functioning of cantoes and their impacts on native fauna. But what we'll do is continue to run a whole series of trials beginning,

the lab and moving out to outdoor enclosures and finally to isolated water bodies where we deploy Peter Panes and we look very carefully at every imaginable variable that we can think of and make sure that we're not accidentally creating problems. So far, it's looking great. But we have to proceed slowly and carefully because clearly, you know, I don't want to go down in history as the person that repeated the folly from 1935. You know, I read that you started out as a snake biologist and got dragged into this cane toad field because they were eating your study subject.

Do you feel like Sisyphus with this? Yeah, look, people have been trying to control cane toads in Australia for a very long time, and it's failed miserably. You know, the toads are doing very well. well, and continuing to spread it. They've gone almost the entire way across the continent. This is the first time that I felt some optimism that we could actually do something about controlling canoads. Clearly, right from the outset, when I made the decision to expand the work, to look at the toads that were about to massacre my beloved snakes, I was hoping we would find ways

to buffer impact, and we did that. We've shown that you can train predators, and that enables them survive despite the presence of toads. But I always hoped we'd have something that might contribute to toad control, and I never thought it would happen. And it seems to be happening. So I'm not going to, you know, break out the champagne yet, but I think it's looking very encouraging. Rick, thanks so much and good luck. Thank you. Dr. Rick Schein is a professor of biology at Macquarie University in Sydney, Australia. Don't go away after the break. New Yorker writer Elizabeth Colbert is here to talk about other similar monumental efforts to control nature and what they tell us about ourselves.

We want to think of some nice, easy fix that's going to get us out of it. And often that fix turns out to be either extremely elusive or even non-existent. Using CRISPR to genetically modify animals to kill animals that we brought in to kill animals obviously raises a lot of deep questions about our role in the environment, unintended consequences, rapidly changing technology. What should we make of all of it? To get some answers, we called up someone who's thought about these themes a lot. Elizabeth Colbert is a staff writer at The New Yorker.

She's reported on CRISPR and Cain Toads and virtually every other giant environmental problem and are sometimes out there attempts to solve them. She's the author of Under a White Sky, The Nature of, of the future. Elizabeth, welcome back to Science Friday. Thanks so much for having me. Can you put Rick's Peter Pan work in context? There's a history to this approach with cantoes, right? Well, I visited several years ago a group also in Australia. And what they were trying to do was genetically engineer with CRISPR, a cantoed that would be less toxic. What cantoes do is they produce a toxin, and then they produce an enzyme that really bumps up the toxicity of their

toxin by many, many times. And that is what ends up being so dangerous for other animals. When they get agitated, they produce this enzyme. And enzymes are genetically encoded, and the feeling was if we can prevent them from doing that, we will produce a less toxic cantoed. How successful were they? I think that they've managed to produce a couple of these less toxic toads, but I don't think it went farther than that. Is there controversy around this? How do other scientists or ecologists think about using CRISPR genetic modification to control invasive species? Well, there's a lot of interest in it, absolutely, because invasive species are a very significant source of extinction. And genetic engineering has many possible pathways that it could be helpful, as Rick's work shows.

The problem that you encounter pretty quickly is you're dealing with huge, huge numbers. And in the case of the cane code, you know, probably hundreds of millions. And so deducing at that kind of scale becomes very difficult. And it takes you or drives you quickly to this idea of gene drive, which is not just genetically modifying an organism, but genetically modifying it in a way that you basically push this characteristic or this packet of genetic modifications into the next generation and into the next generation after that. And you sort of override the rules of evolution that way. And that raises tremendous concerns because you could, for example, there are gene-drive mosquitoes out there.

They've been engineered. And the notion is you could put them out. out in the landscape and they could potentially make a big difference in places that suffer from a lot of malaria. But you could also potentially, you know, do in all the mosquitoes in the world or all the mosquitoes of that species because your gene drive is so powerful. At least that's, you know, theoretically possible. So you do run into this question of in order to be effective, it has to be very powerful. It has to spread. But in order to engineer that spread, then you're engineering and something that is potentially very dangerous and quite controversial. So you often write about these monumental efforts to correct something in nature,

from eradicating invasive species to people working on geoengineering solutions to fight climate change. I mean, what is your point of view on these projects? Like, I guess I'm asking, how should I feel about them or how do you feel about them? Well, I think that's a, you know, a very good question and a very hard question. And I guess I try to go into this as a reporter. I'm agnostic. I'm just interested. And I think that the point that people would make is it's really, you know, compared to what? So we get a lot of concern about tweaking these organisms and putting them out on the landscape. But Mark Tizard, who was the geneticist who was working on this project, whom I interviewed several years ago, I remember very distinctions. distinctly him saying to me, well, look, the cane toad itself, it's just a package of genes, but that whole genome shouldn't be there, right? So the question of whether you should tweak that one gene or whether you should have, you know, 200 million examples of this whole genome that doesn't belong in this landscape, you know, are we arguing about the wrong things? And those are,

those are really interesting and tough questions. And I'm sure they're different in every situation. You know, there's no one-size-fits-all answer either. Yes, and I think one of the reason that these efforts haven't really gotten off the ground yet is this scale problem. It's so huge. And if you think about this effort to, you know, introduce these sort of Peter Pan tadpoles to the landscape in order to curb the cantoed population, you know, you're probably talking about releasing literally millions of these. So we're really talking about a monumental effort. And cantoads also have a way, as Rick Schein's group, has, you know, demonstrated of evolving very fast. They actually have undergone some really, really fascinating evolutionary changes, including this cannibalistic behavior. So they're this super interesting genetic experiment. They themselves are evolving and they can evolve potentially defenses against what you genetically engineer to try to get rid of them. That's fascinating. One more question. What is reporting on these themes taught you about?

us? Well, I think that that is a, I guess ultimately that is the question, isn't it? What kind of creature are we? And I think what it teaches us is that these natural systems are incredibly complicated. And we intervene in them often unconsciously. In the case of the cane toad, it was conscious, actually. It was just a huge blunder to introduce them, but they were purposefully introduced. But often invasive species are introduced, you know, in someone's luggage or on the sole of their shoe. And they have huge consequences that we can't anticipate. And then we have to try to sort of backfill.

And that is very, very difficult because these systems are so complicated. And once we realize we have a problem, often, you know, it's a question of having, of the genie being way, way out of the bottle. And we'd want to think of some nice, neat fix that was as easy as bringing in this organism, which turned out to be super easy, and we want to think of some nice, easy fix that's going to get us out of it. And often that fix turns out to be either extremely elusive or even non-existent. This is like the stuff of Greek myths. Exactly. Thank you for all your work on this, Elizabeth. Oh, thanks for having me. Elizabeth Colbert's staff writer at The New Yorker,

and author of Under a White Sky, The Nature of the Future. And that is about all we have time for. Lots of folks helped make the show happen, including John Denkoski, Annie Niro, Jason Rosenberg, Rasha Eredi. I'm Flora Lichtman. Thanks for listening.