JUANA SUMMERS, HOST:
Put a microphone outside overnight in Southern California not far from the urban sprawl, and you might be surprised how much you can hear.
(SOUNDBITE OF ANIMAL HOWLING)
SUMMERS: But for scientists listening for the grunts of a federally threatened frog, all that sound is a problem. NPR's Nate Rott has the story of how artificial intelligence is helping cut through the noise.
NATE ROTT, BYLINE: All right. Before we get to the fancy frog-finding technology part of this whole thing, there's a pretty wild border-hopping backstory to tell about why scientists are so keen to hear these frogs.
SUSAN NORTH: The California red-legged frog is the largest native frog in the western United States.
ROTT: This is Susan North, the director of stewardship for The Nature Conservancy in California. And historically, she says, the California red-legged frog used to be found in ponds and waterways from Baja, California, all the way up through the U.S. state. They were so prolific that they were a food source for the forty-niners during the gold rush. The short story that launched Mark Twain's writing career, "The Celebrated Jumping Frog Of Calaveras County" - a California red-legged frog. Today, though?
NORTH: Unfortunately, over the last, like, 150 years, it has really declined, and it's now occupying less than 70% of its range.
ROTT: Habitat loss, disease, overconsumption and invasive species like bullfrogs have killed off populations, creating a 250-mile long gap in the frog's historic range - an area where none are left, centered over Southern California.
NORTH: And with a gap that size, you know, you're not going to have 5-inch frogs recolonizing their range naturally.
ROTT: So North and a group of other scientists and conservationists decided to help bring them back - translocating them, if you want to be technical - from a site in Mexico. Robert Fisher is a biologist with the U.S. Geological Survey.
ROBERT FISHER: It was like "Planes, Trains and Automobiles." It was just a lot of different moving parts.
ROTT: A paperwork and permitting nightmare, as he puts it. But in 2020, the first egg masses were put in coolers and transported by cars and - no, not planes or trains - but helicopters to a pair of sites in Southern California...
(SOUNDBITE OF FROGS CROAKING)
ROTT: ...That had been cleared of invasive bullfrogs which would otherwise eat them. Which brings us to now, many translocations and years later and the acoustic challenge of figuring out if their efforts worked - if the frogs are breeding. Bennett Hardy is a biologist with the San Diego Natural History Museum.
BENNETT HARDY: We want to listen for their calls.
(SOUNDBITE OF FROGS CROAKING)
HARDY: So when frogs are calling, most of the time, that's the male of the species.
ROTT: Either telling other males to bug off or saying to females the frog equivalent of 'sup. Those ribbits you're hearing, by the way, are tree frogs, not the reintroduced frogs we're listening for.
HARDY: 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.
ROTT: So the researchers put microphones around the ponds, collecting thousands of hours of audio files like the ones we're listening to.
HARDY: Now we need thousands of hours of ears to be able to sort through all of that data, which is a huge task. And that's where advancements in technology through AI have become really helpful for us.
ROTT: The team trained an AI model to go through the sound files and find any California red-legged frog calls. It also identified bullfrog calls, letting scientists know if the predators were coming back into the area. And this winter, for the first time in decades, the California red-legged frog was heard again in Southern California.
(SOUNDBITE OF FROGS CROAKING)
ROTT: They're that grunting sound you hear beneath the tree frogs, kind of like a finger rubbing on a balloon.
(SOUNDBITE OF FROGS CROAKING)
ROTT: Soon after, near the microphone that picked up that sound, a new egg mass was found.
HARDY: So we knew right then and there that, hey, all of our hard work over the last six years and beyond is starting to pay off.
ROTT: With time, Hardy says, the hope is that the frogs will start to naturally disperse to other areas on their own. And with funding, they'll do more relocations to give them another hopping hand.
Nate Rott, NPR News.
(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.
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