National Park Scouts

The Frog That Sleeps Under Ice: Saving the Mountain Yellow-Legged Frog

Host: Jenni · Park: Sequoia National Park · Topic: Mountain yellow-legged frogs, chytrid fungus, trout, conservation

Intro

Intro

A hundred years ago, if you hiked up into the high mountains of Sequoia National Park, up above the trees, up where the air gets thin and the lakes are fed by snowmelt, you could not walk near the water without watching your feet. Why? The ground was covered in frogs. A scientist named Joseph Grinnell came up here in 1915 and wrote in his field notes that you could hardly take a step near the water without almost stepping on one. They were everywhere.

Today, those same lakes are quiet. Most of the rocks are empty in most of the Sierra Nevada. The mountain yellow-legged frog has almost completely disappeared. What happened? And can it be fixed? Today we find out. And the answer involves a fungus, a fish, and a helicopter. Let's go!

Welcome to National Park Scouts, the show where curious kids discover America's wildest places. We are back at Sequoia National Park, one of the most spectacular places in all of California. In our first episode, we met the giant sequoia itself, the biggest tree on Earth. Then in episode two, we went underground, into the Crystal Cave, and met creatures that live their whole lives in total darkness with no eyes at all. Today we are heading up, way up, to the high mountain lakes above 6,000 feet, where the snow doesn't fully melt until summer. And we're going to meet a small frog that, against impossible odds, is making a comeback. One frog, two villains, and a team of scientists who absolutely refuse to give up.

Meet the Mountain Yellow-Legged Frog

It's small, about the size of your palm. Brownish yellow on top, kind of creamy on the belly. Not flashy, not huge, easy to overlook. But here's what it does that is genuinely incredible: it spends up to nine months of the year frozen under ice at the bottom of a lake. Hibernating, basically not moving, underwater, under ice, for nine months. Most frogs hibernate, but this frog is only awake for about three months a year. You get twelve months; this frog spends most of its life waiting for winter to be done.

And when the snow finally melts in early summer, this frog has about three months to eat, grow, and have babies before winter closes back in. That is a very tight schedule. The tadpoles, which are baby frogs before they grow legs, take two to three years to develop. Most frog tadpoles take a few weeks to turn into full-grown frogs. This one takes up to three years because the water is cold, and cold water slows everything down.

This frog has lived in the high Sierra lakes for a really, really long time. It was part of the food web, eaten by herons, river otters, and garter snakes; the frog itself eats insects that would otherwise have no one keeping them in check. It was the most common amphibian up here. And then, in just a few decades, more than 90% of them were gone. Scientists started asking why. And what they found was two villains working at the same time.

Guess That Park Sound

Guess That Park Sound

Okay, scouts, let's play guess that park sound. Listen closely; what do you think is making it? We'll come back to it right after the break.

The Reveal

That is the mountain yellow-legged frog, the star of today's episode. As you heard, this frog does not ribbit; it makes a sound like something rubbing together. You only hear it for a few weeks in summer, when the snow melts and the frog wakes up from its nine months under the ice. Scientist Carlos Davidson shared that this frog mostly calls underwater, not above the surface like most frogs; it's one of the reasons researchers love studying it. Thank you to Tim Zeismer and Carlos Davidson for their work studying and cataloging these frogs and sharing it with us today.

Villain One: Chytrid Fungus

Chytrid is a microscopic fungus that lives in the water, and here's what it does: it attacks a frog's skin. Now, why would that matter? It's because frogs don't just breathe through their mouths and lungs the way we do; they also breathe through their skin. Their skin soaks up oxygen from the water like a straw. That's how they stay alive when they're hibernating underwater all winter. When chytrid infects a frog, it makes the skin thick and crusty. The frog can't breathe through it anymore; it can't absorb water. It's like sealing off the straw, and the frog just slowly stops working.

Chytrid has caused the disappearance of more than 200 amphibian species worldwide. It's one of the most destructive wildlife diseases ever recorded. And when it reached the Sierra Nevada, probably carried in on equipment or moved between areas by people, the mountain yellow-legged frog had never seen anything like it. It had no defense.

Editorial flag: the source transcript spelled this fungus "Kitrid" and "Kittrid"; corrected throughout to its actual name, chytrid.

Villain Two: Trout

The high mountain lakes in Sequoia had no fish in them naturally, historically. The waterfalls and cascades below made it impossible for fish to swim up. The frogs had evolved for thousands of years with no fish predators at all. Then, starting in the 1870s, people began dropping non-native trout into those lakes: brook trout, rainbow trout, brown trout. They stocked the lakes for fishing, and they did it for over a hundred years.

But here's the problem: trout eat tadpoles. And mountain yellow-legged frog tadpoles take two to three years to develop; that's three years of being a small, slow tadpole in a lake that now had hungry fish in it. Almost none survived. The frog's slow-and-steady strategy, perfectly suited for a cold, fishless mountain lake, became a deadly weakness the moment trout showed up. Two villains at once: a fungus killing the adults, fish eating the tadpoles before they could grow up. The frog had nowhere to go, and that's how more than 90% of them disappeared.

The Comeback

Scientists Didn't Give Up

But here's where the story turns. Starting in the 1990s, researchers, zoos, and the National Park Service started one of the most ambitious frog rescues in American history.

Step one, remove the trout. In some of the mountain lakes, park managers stopped adding more fish; in others, they actually removed the trout. When the trout were gone, something remarkable happened: the frog populations started coming back on their own, slowly but surely. That was the first proof that recovery was possible.

Step two, fight the fungus. Scientists at the Oakland Zoo and San Francisco Zoo figured out a treatment, basically an antifungal bath, that clears the infection right off a frog's skin. Tadpoles are collected from the wild, brought to the zoo, cleared of the fungus, and raised in clean, safe facilities until they're big enough and strong enough to survive in the wild.

Step three, the helicopter. The lakes where these frogs need to go are remote; there are no roads, sometimes no trails. The only way to get there is on foot or by air. So scientists load crates of healthy, treated frogs into helicopters, fly them into the backcountry, and land in high mountain meadows, where they open the crates. In 2017, the Oakland Zoo and the San Francisco Zoo partnered with Sequoia and Kings Canyon National Parks and the U.S. Fish and Wildlife Service to release 215 healthy frogs into the park's backcountry lakes by helicopter. They kept doing it year after year, and recently the Oakland Zoo celebrated a massive milestone: releasing their 1,000th mountain yellow-legged frog back into the wild.

And here's the thing: it's working. In lakes where trout were removed and frogs were reintroduced, populations are growing. Garter snakes, which had almost disappeared when the frogs did, are coming back too. The food web is rebuilding itself, one piece at a time. The fungus is still in the water, and it will always be there now, but early research shows that some frogs may be starting to develop a natural resistance to it. The frog is not just being rescued; it might be learning to fight back on its own.

Wildcard

Guess That Number

The mountain yellow-legged frog spends most of its year hibernating under ice. Is it more or less than six months?

More, way more. It hibernates for up to nine months, and is only awake and active for about three months a year. The rest of the time, it's at the bottom of a lake under ice, just waiting.

Most frog tadpoles take a few weeks to grow into frogs. Do mountain yellow-legged frog tadpoles take more or less than one year?

More. Up to three years as a tadpole in a cold mountain lake. That's why the trout were so devastating; that's a long time to be small and slow in a lake full of fish.

The chytrid fungus has affected amphibians around the world. Has it caused the disappearance of more or less than 50 species?

Way, way more; more than 200 amphibian species worldwide. It's one of the most destructive wildlife diseases ever recorded, and now scientists are racing to help this frog build a defense.

Scout Voices

From Our Listeners

August, age 8

I have seen a frog in the wild at our lake house in Northern California. It was on our window, and then I caught it and released it.

Ben, age 8

Once I saw a frog in my front yard. When it saw me, it stayed still so I could pet it. I named it Bob.

Do you want to have your voice on National Park Scouts? Find us on social media or visit our website, NationalParkScouts.com.

What did you learn today that most adults don't know? I think most adults don't know that a frog the size of your palm, one that spends nine months frozen under ice, was almost wiped out by a fungus. But here's what's cool: a team of scientists didn't give up. They airlifted those baby frogs into remote mountain lakes by helicopter to give them a second chance. And you know what? Some of those scientists were once kids who grew up caring about frogs and wild places, just like you. Never underestimate the impact you can have on our world.

Your Mission

If you're at home, find a lake, creek, pond, or even a ditch. This spring or summer, take a walk and look for frogs or tadpoles near the water. Frogs love shallow, calm water with plants nearby. If you find tadpoles, look closely; you might be able to see their legs starting to grow. Don't scoop them up, just watch and count how many you can spot.

If you're at Sequoia National Park, the mountain yellow-legged frog might be really hard to see; they live in high alpine lakes that may be inaccessible for most kids. But you can look for a shady, wet, rocky spot near a stream, the kind of place a mountain yellow-legged frog would love. Even if you can't see the frog, you can find its neighborhood. Dip your hands in for ten seconds and see how cold it is. That's what the frogs live in year-round, and sleep under for nine months. How long could you last?

Outro

The mountain yellow-legged frog spent nine months under the ice, survived two villains at once, and got airlifted home by a helicopter. You might have thought we were high this time, but next time we're going all the way up to the highest point in the lower 48 states: Mount Whitney, 14,505 feet, the tippy top of America. Scouts, your next adventure is waiting.