Short Wave - These Scientists Are Using AI To Listen To Frogs

Episode Date: July 18, 2025

If you were a miner in California during the Gold Rush, you might have dined on a California red-legged frog. The largest native frog in the western United States, this Golden State denizen used to be found as far inland as the Sierra Nevada mountains and south, into Baja California. But today, they're listed as threatened under the Endangered Species Act. Conservationists have worked to translocate new populations of the red-legged frog back to California in hopes that their numbers can be restored. But how do they monitor those populations' growth? Enter AI.Want to hear more stories about critters or conservation? Let us know at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey, Short Waver, is Regina Barber here with just back from parental leave science correspondent Nate Rot. Hey, Nate. Hey, Regina, I am overjoyed to be back. But mostly, I am overjoyed to talk to you today about frogs and more specifically an AI acoustic model to find them. Okay, you definitely had me at frogs and you lost me just a little with AI acoustic model. Yeah, that's fair. It's pretty technical.

So let's start like this. So what you're hearing right now, Regina, is a recording from a pond outside San Diego in Southern California. Wait, I think I heard a fair amount of frogs in that audio clip. Yeah, so unfortunately, in the words of Obi-1 Canobi, these are not the frogs you're looking for. I think that was like these aren't the droids you're looking for, but I appreciate the effort. Yeah, you know, any Star Wars reference I can squeeze in is great. So what you're hearing there are tree frogs, they're chorus frogs. Your classic, you know, rib it, ribbit, ribbit ones.

Okay. But what scientists are trying to hear in these thousands of hours of recordings have collected is the California Red-Legged Frog, a federally threatened species. Here's Susan North, the director of stewardship for the Nature Conservancy in California. The California Red-Legged Frog is the largest native frog in the western United States, meaning it can grow up to a whopping five inches in size. Five inches?

Okay, so that's like a little bigger than the palm of my hand. And that's a big frog. Yeah, it's a big frog that used to have a really big range. So they used to be found up and down, California, like as far inland as this here in Nevada mountains, and down into Mexico on the Baja Peninsula. They were so prevalent, Regina, that they used to be used as food for the miners during the gold rush. Unfortunately, over the last, like, 150 years, it has really declined, and it's now occupying less than 70% of its range. But it wasn't just the miners and their voracious mining appetites.

Non-Native American bullfrogs share in the blame. They will also eat native frogs. Got it. And there's other things, too. The biggest cause of decline in red-legged frog in California has been habitat loss. So wetlands turned into farming plots or parking lots, you know, ponds degraded by pollution. Susan says that's created this huge gap in the frogs range. So pretty much between like north Los Angeles and Sanjolans,

Central Baja, no frogs. Wow. So that's a 260-mile gap in the range of the species, which is significant. And with a gap that size, you know, you're not going to have five-inch frogs recolonizing their range naturally. So Susan's group, the Nature Conservancy, and a whole host of other groups and federal agencies decided to try to bring these frogs back to, you guessed it. Okay, so these are the ponds we were hearing before? Exactly.

So the challenge right now is that they're trying to figure out if this relocation effort is working. And the way to do that is by listening through recordings like this. Today on the show, a border hopping effort to restore the California red-lake and frog. And how new technology is helping scientists cut through a lot of unwanted noise. You're listening to Shortwave, the science podcast from NPR. Okay, Nate, so we have a threatened species of frog that has a bunch of groups decided, to bring it back to Southern California. How did they do that?

So this story is actually kind of wild. And it starts down in Baja, California, in Mexico. My name is Annie Peralta Garcia. I work at Fauna del Noreste and unprofit in Ensenada. So Annie's been in the red-legged frog game since, like, her undergrad. She got her PhD surveying every stream in the northern part of the Baja Peninsula for the frogs in the mid-2010s. It was pretty fun, especially for my husband, that he loves the fieldwork.

And, you know, we found out that they were only found in 10 sites in the southern part of the range. And even though they were there, they were not in high numbers. But at least there were, like, some, unlike that, like, gap we were talking about in Southern California, where there's none at all? Yeah, exactly. So, like, post-graduation, Annie started this nonprofit to do research and habitat restoration for those red-legged frogs in Baja's, like, other less charismatic species. Oh, a shout out to the less. charismatic species out there. Yeah, no doubt, right?

They need love too. So Annie's team has helped raise and stabilize the populations of those frogs at some of those sites, which is really important to the rest of this story. Because genetic testing of the last known red-legged frogs in Southern California showed that they more closely resembled Baja's frogs than the red-leggeds that still exist further up in like northern and central California. Here's Robert Fisher, a biologist with the U.S. Geological Survey. And thinking about, you know, reintroducing frogs and bringing them back and restoring these ecosystems, it became clear we needed to use a Mexican source for the frogs.

Okay, I love this. I love this like international cooperation that's going to happen here. Yeah, so Robert says in 2020, these two sites in Southern California were approved to receive relocated frogs. They've been cleaned out of predatory bullfrogs. The paperwork and permitting nightmare had been navigated. Okay. And the COVID-19 pandemic is just beginning to see. start. It was like planes, trains, and automobiles. It was just a lot of different moving parts. So Annie's team and others collected parts of an egg mess from one of the sites in Baja, put it in a cooler, transported it by car. I'm sorry, Regina, they did not use planes and trains, but they did

use a helicopter. Okay, close. So that's like the actual sound of the helicopter from a video that the team shared. And they moved it to the U.S. border and eventually to Robert's team, which brings them to these prepared sites in San Diego and Riverside counties. I mean, it sounds super, like, fascinating but also like a logistical nightmare. Yeah, Robert described this whole thing as like an impossible dream that he's gotten to see happen. Okay, so these frogs have been like translocated back to Southern California. Do we know that it's working, like if the populations are stable? Yeah, so again, here's Susan North who helped coordinate this effort at the Nature Conservancy.

A really important measure of success comes kind of at about five years of age. That's when they should start breeding. So the first translocation was in 2020. We're now in 2025. And that's really why we were on the edge of our seats waiting to see if we were going to find egg masses or hear frogs calling, which means that they're attempting to breed. Ah, okay, hear them calling. This is where that fancy, like, AI acoustic model comes in. Yes.

So, like, in order to know if they're successful, you need to know if these frogs are trying to mate. And if you're an amphibian-loving scientist like Bennett Hardy is, who works at the San Diego Natural History Museum. I'd love to be out there every night at these ponds and with my tent and camping and being up every night and trying to listen for them. But it's just not feasible. So instead, they set up a series of microphones at both of their restoration sites. And the idea is that these are then recording the sounds of the night that we're not able to be there for. Okay. So there are these microphones surrounding the these ponds recording every night for like weeks?

Weeks. And then we have thousands and thousands of hours of files from these devices. And now we need thousands of hours of ears to be able to sort through all these files, find where the frogs are, when they are, if they are, and get through all of that data, which is a huge task. Oh, my gosh. Yeah, like, this is an audio producer's nightmare. So much tape, so many sounds.

It's horrible. And to make it even more complicated, it's not like you can just pop one of these sound files into an audio program and look for spikes in the sound waves because there are so many other sounds. So Bennett shared audio clips. They've recorded of coyotes. They've recorded songbirds, ducks, and wild turkeys. And they've even heard a screech owl and a great horned owl. What? And of course, there's still the many chorus frogs and bugs and cars that you'd expect to hear in Southern California. So to find those needles in this sound haystack, Bennett says their group partnered with software engineers to create custom machine learning models. A AI tool set that's been trained on the Red Laked Frog specifically. So it can pick that out pretty well in our data. Bennett says it can sort through the data in like tens of minutes. So they even created another.

model, like another AI model to identify the calls of those non-native bullfrogs so they can get ahead of any new threats. And for like Clark Winchell, who describes himself as a mud boot biologist who helped prep these relocation sites for the U.S. Fish and Wildlife Service, this whole thing is mind-blowing. I spent countless hours by myself in a kayak all night long listening for bullfrogs. It's not the most efficient use of time. What they've done with AI on this project and those audio moths is incredible. It's systematic monitoring of two species. And as anyone who's worked in wildlife conservation or heard about it knows, like monitoring species takes a lot of time, effort, and money.

And this helps him save on all of that. Yeah. So let's get to the most important question, though, of this story that I've been thinking about. Have they actually heard any of these California red-legged frogs? Good news, Regina. They have. Excellent. So this is from last winter. And can you kind of hear that grunting sound, like a finger rubbing on a balloon beneath the sound of those tree frogs?

Yeah, as soon as you said that, yes. So that is the sound of the California red-legged frog. And this is the first time it has been heard in these parts of Southern California in 25 years. Wow. So they are mating. They are making babies. Well, they were at least trying to, right? That's what the sound suggests.

So after they heard this, the team sent out folks to survey near the microphone where they picked up this sound, and they did find a new egg mass. So here's Bennett again. We knew right then and there that, hey, this whole thing is successful. It's working. All of our hard work over the last six years and beyond for this species in this region is starting to pay off. Wow. Okay. So now what is going to happen?

Like, are they going to keep moving these red-legged frogs over the border? Yeah, so they're planning to keep doing translocations. They're going to keep on monitoring the populations they've moved. And the hope is that when these populations become self-sustaining, they'll be able to start, like, translocating egg masses from these Southern California sites to new ones, to keep seeding new populations in that big gap we talked about. They're hoping for some natural dispersal, some of the frogs just to move on their own to new areas. And eventually, maybe the hope is they'll be able to be able to.

to get off of the endangered species list. Yeah, that would be great. And, like, Nate, we all love hearing, like, a happy wildlife conservation story. I don't think anybody wants to hear about, obviously, an animal going extinct, but stepping back, is there benefits to all this work outside of just preventing this frog's extinction? I mean, so nobody wants to see an animal go extinct, but also nobody likes mosquitoes, right? And the frogs help with that, especially with diseases like Zika and Dengay and West Nile virus showing up.

in Southern California. But you know, I like always ask biologists his question, like, you know, why should somebody who doesn't know anything about this fraud care? And Clark Winchell had maybe the best answer I've ever heard. So he quoted the famed naturalist Aldo Leopold talking about ecosystems. He has a line, a good tinker keeps all the parts. And I will just say in my day of rebuilding carburetors on old trucks, when you got up done and there's one screw laying on a table, you knew you were screwed. I don't think any scientist has an answer on that, but it is important to maintain biodiversity as best we can moving forward.

I really, really like that analogy. Being compared to a carburetor and pieces left over, I love it. Yeah, gearheads everywhere rejoice. Yeah, it'll definitely stick with me every time I think about why we should care about species going extinct. Nate Roth, thank you so much for bringing this story to us. Now I want to listen for like red-legged frogs like the next time I go visit my family in California. Heck yeah. Let me know when you do it and we'll camp out, inefficient as it may be.

This episode was produced by Hanna Chin, edited by a showrunner Rebecca Ramirez, and fact-checked by Tyler Jones. Robert Rodriguez was the audio engineer. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. Thank you for listening to Shorewave from NPR.