An accidental island sits in the middle of the Panama Canal, and for more than a century, it’s drawn scientists from around the world. In this two-part series on Barro Colorado Island in Panama, Nate meets biologists Rachel Page and Erin Spear of the Smithsonian Tropical Research Institute to explore the questions hidden in one of the world’s most closely studied tropical forests.

In this episode, Nate follows Rachel into the rainforest at dusk, where a puddle full of tiny, love-struck frogs must get loud and brave to survive.

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Transcript:

NATE: Hi, I’m Nate. This is The Show About Science, and today, we’re headed to Barro Colorado Island in Panama, home to one of the most closely studied rainforests on Earth. And for Erin Spear and Rachel Page, two scientists who work there, the commute is almost as unique as the island itself.

ERIN SPEAR: It’s awesome. It’s like the absolute best commute I have ever had and probably will ever have in my life.

NATE: That’s Erin, and the only way for her to commute to the island is by boat.

ERIN SPEAR: And it’s really honestly surreal every time I get on the boat to go to the island because you’re in the iconic Panama Canal.

NATE: This boat ride is anything but ordinary. Here’s Rachel.

RACHEL PAGE: It’s kind of a beautiful thing. Like, you’re on the Panama Canal. There’ll be these huge tankers. You’ll be in your much, much smaller boat. And then you travel half an hour to this island.

ERIN SPEAR: You’re approaching the island and you see the hills, and there’s these super tall, iconic rainforest trees with all these vines coming off of them. And there’s just this gorgeous, absolutely, like, dense, dense, dense forest with 300-foot-tall rainforest trees. All that space between the ground and the top of those trees is just crammed full of life. So much life that it feels claustrophobic when you’re in the forest.

NATE: Near the end of the journey, a small cove comes into focus.

RACHEL PAGE: And in that cove, you’ll see some docks, some very small boats, and then these lab buildings.

ERIN SPEAR: Just these little buildings peeking out, and you just know, like, I’ve gone somewhere magical where anything could be possible. It’s just, yeah, otherworldly.

NATE: This magical, otherworldly place is Barro Colorado Island, BCI for short, and it’s home to a research station run by the Smithsonian Tropical Research Institute, or STRI. It’s been a destination for scientists for over 100 years. And it’s our destination for the next two episodes.

But there’s something else you should know about BCI. It wasn’t always an island, and it wasn’t supposed to exist.

RACHEL PAGE: So I guess going back in history a little bit...

NATE: Rachel told me that before there was a canal, BCI was a hilltop, part of the tropical forest. Then, in 1914, engineers dammed the Chagres River to build the Panama Canal. The whole region was flooded. The tropical lowlands became a lake, and that hilltop became an island.

ERIN SPEAR: And so you feel like you’re at the top of a hill, if that makes sense, but surrounded by water.

NATE: Being cut off from the mainland protected the forest and made it possible for scientists to begin studying it soon after.

ERIN SPEAR: And no other tropical forest has as much information or data about it than BCI.

NATE: That century of research has made BCI a magnet for scientists chasing all kinds of questions.

RACHEL PAGE: Evolutionary questions, ecological questions, questions of animal behavior, which is what I’m interested in.

NATE: Rachel and Erin first came to BCI early in their careers with questions they hoped the rainforest could help answer. Today, they’re studying two very different sides of the same rainforest. Erin’s story will take us into the forest by day, but we’ll save that for the next episode.

Rachel’s story begins at dusk.

RACHEL PAGE: Dusk is when things start, like, really waking up in the rainforest. Like, you’ll start to hear all these sounds of insects.

NATE: As the sun goes down, more of the forest starts to come alive. And somewhere, near a muddy little puddle, this curious sound starts echoing through the forest.

[FROG CALL]

NATE: That’s the túngara frog.

RACHEL PAGE: These frogs have such stunning sounds, but they are tiny, brown and a little nondescript. They’re not, like, the most flashy frogs. Like, you think of, like, a red-eyed tree frog, for example, this iconic frog of the rainforest. But these guys are—they call them mud-puddle frogs. And they do. They love mud puddles.

NATE: On any given night, the puddles are where all the action is. Only the male frogs make the calls. First, one arrives to get the party started, and then a few more jump in and start calling.

[FROG CALLS]

RACHEL PAGE: If you listen really carefully, there’s that one male that’s the closest, and then you can hear these two other males calling a little bit more in the distance.

NATE: OK, yeah, I can kind of hear that. It’s a really unique call.

RACHEL PAGE: Yes. Yeah. And actually, a cool thing: It’s onomatopoeia. The Spanish name for that frog is túngara. And “tung” is that whine, this frequency-modulated sweep: tung. And then they add these chucks to the end. And so túngara would be a whine with two chucks. And the females love the chucks.

NATE: That’s really the whole game here. The calls, the chucks. It’s all to impress the ladies. Even more males pile in to compete, every one of them looking for love. Or, well, a mate.

The more males that show up, the more each one tries to out-chuck the others.

RACHEL PAGE: So the male frogs, when they’re acoustically competing with other males for mates, they’ll up that simple call. They’ll start adding these chucks to the end of the call. And that’s more attractive to the females.

NATE: So are those at the end kind of the chucks?

RACHEL PAGE: Yes. And can you count how many? It’s a little hard. They’re fast.

NATE: Three? Is that—

RACHEL PAGE: Yeah. Yeah. OK. So, doom da-da-da.

NATE: Yeah, OK. I hear that. And the lady frogs love those when they add them at the end.

RACHEL PAGE: They do. They really do.

NATE: Pretty quickly, that mud puddle has attracted over 100 screaming frogs.

RACHEL PAGE: Absolutely tons of them. Yeah.

NATE: And those individual calls have become a chorus. A very loud chorus.

[FROG CHORUS]

RACHEL PAGE: I know. That sounds like an arcade, doesn’t it?

NATE: Like, or a siren or something. I don’t know.

RACHEL PAGE: Yes. Yeah.

NATE: There’s 43 seconds of this. I’m gonna pause it here.

RACHEL PAGE: So that is the sound of a really nice, really big túngara frog chorus. So many, many males calling together. And then the females come in, and they’ll check out this one, and they’ll check out that one, and they’ll check out this other one. So it’s very much female choice in the system.

NATE: With the female frogs checking them out, this is the moment to impress.

RACHEL PAGE: And actually, as they assess the males, the males will even up those number of chucks that we were talking about. So if they were making, like, one chuck or two chucks, they’ll go up to, like, three or four. Up to seven is the maximum we’ve ever recorded, which is pretty wild.

And if all goes well, the female chooses a male. They couple together in amplexus. They move off a little bit to exactly where they want to deposit those eggs. And then they make this foam nest.

And actually, several couples, several pairs, will come together to make a foam nest. So they’ll have these communal foam nests. Actually, if you go the next morning, for example, it almost looks like meringue or something, like this beaten egg-white kind of thing. These little foam nests of successful túngara matings that happened the night before.

So that’s the best scenario for the frogs.

NATE: Genes passed on. Mission accomplished. That’s the best-case scenario.

The worst scenario, though, is this.

[BAT WINGBEATS]

NATE: Unbeknownst to the frogs, they’ve attracted some unwanted attention from a winged predator.

NATE: OK, so what is that?

RACHEL PAGE: So that is my favorite bat in the world, Trachops cirrhosus. It is the frog-eating bat, and that’s the sound of its wingbeats as it flies through the tropical rainforest in Panama.

NATE: For this bat, that túngara frog chorus sounds like dinner.

RACHEL PAGE: Exactly. So say your chorus has attracted some froggy bats. They’ll come, and they will find nice perches around the puddle, around the pond. They’ll just hang on a nearby tree. They’ll hang by one foot or sometimes by two feet. And they’ll just, like, listen.

And they do these extraordinary ear motions. So when the frog calls, like, the ears will go—it just completely matches the call.

NATE: Wow.

RACHEL PAGE: And they’ll wait a little bit, and they’ll sort of think about it, make their decision, and then they’ll fly down to the chorus and try to capture one of those males.

NATE: So they’re all just, like, thinking, “I hear that. That sounds like a buffet. That sounds great.” It’s kind of like an ice cream truck that you hear outside, and they’re just like, “Ooh, food.”

RACHEL PAGE: Exactly. Exactly. Yeah. With the important difference that the ice cream truck wants you to be listening to it and finding it. And these poor frogs are—they do. They want the females to find them. But unfortunately, that comes at the cost of the predators finding them, too.

NATE: So if the choruses are, like, a beacon to the bats, and there’s also a lot of competition for mates within the different members of the chorus, why would a frog ever join up with the chorus instead of going to, like, a different part of the rainforest and just making calls on its own?

RACHEL PAGE: That’s a really good question. Yeah, that’s a great question.

So it turns out that the mating success of a frog at a chorus is much higher. And this might seem weird. Also, the predation risk is lower. And I think it’s this safety-in-numbers thing.

So granted, you’re gonna be a beacon. Collectively, you’re this wonderful, attractive beacon to all these eavesdropping bats. But even if a few get picked off over the course of a night, you’re still safer in those numbers than you are calling alone.

NATE: OK, so joining the crowd gives the males two advantages: a better shot at a mate and safety in numbers.

RACHEL PAGE: Exactly.

NATE: Yeah, that makes sense. Like, once a bat eats a frog, it’s done. It’s gotta go do something else. It doesn’t need to eat more frogs, and you can continue with your chorus.

RACHEL PAGE: Exactly. Yeah. Yeah. It does digest for a while.

NATE: OK, so you’re a frog in a chorus. There’s a bat lurking in the canopy overhead. You’re chucking away, making a ton of noise. That bat is trying to lock onto you with echolocation.

You’d think that the frogs would be easy targets, but it’s not quite that simple.

RACHEL PAGE: Like, you have to somehow, amidst this huge cacophony, pick out one stream of information. And what’s that one individual frog that you’re gonna target? So it’s a sensory challenge for the bats to do.

NATE: But the bat, determined to catch its prey, flies down toward the mud puddle anyway. It swoops in.

Is there anything that frog can do, or is it doomed?

RACHEL PAGE: On the frog’s side, they do have a few defenses. Like, what they’ll do is stop calling. And actually, as soon as a bat will fly over a chorus, the whole chorus will shut up at once. Like, it’s just this immediate collective silence.

And actually, a frog call—like a truncated frog call, a frog call that stops midway—is a signal to the whole chorus. So that’s a very, very effective way to avoid predation, is just to not give that bat any more information about where you are.

[FROG CHORUS STOPS]

RACHEL PAGE: Yeah. But the bats also are combining their use of the frog call with echolocation, tactile cues, chemical cues. So the frog, if it’s still there in sort of plain view—which, the bats aren’t really using vision, they’re using echolocation—then you’re still at risk.

So the frogs will hunker down and then even dive down under the pond. And once they’ve dived under the surface of the water, then they’re quite safe.

But that also means that they’re not attracting any females, so they need to, like, get brave again and come to the surface.

NATE: Slowly, the frogs peek their heads out of the water. And when it’s safe, one lets out a call.

[FROG CALL]

RACHEL PAGE: That one super-brave male will start, and then somebody else will chime in, and then it goes again. And you’ll see that over the course of the night, these just bouts of choruses. So the chorus will be on, and then it’ll be off, and then it’ll be on again.

It’s an inherent trade-off. It’s a challenge for those male frogs that you have mating success on the one hand and then, of course, survival on the other. And you have to somehow, every night, weigh those two and make a decision that both gets your genes into the next generation but also keeps you alive to be able to maybe come back to the chorus another night and try it again.

NATE: So sometimes the frog lives to call again.

And sometimes this happens.

[CHOMPING]

RACHEL PAGE: So that’s the happy bat when it successfully catches a frog, and the chomping noises.

NATE: The happy bat is indeed the worst-case scenario for the frog. But being able to hear that chomping is one of the best possible outcomes for Rachel’s research. She’s able to learn so much from these recordings.

But how she goes about capturing these sounds—well, it’s not what you’d expect.

RACHEL PAGE: We’ve started putting these little backpacks on the bats with these onboard microphones. And what’s wild about that is you can hear the entire sequence.

So you’ll hear, like, the túngara frog chorus. You’ll hear the bat’s own echolocation. You’ll actually hear its wingbeat sounds as it’s flying as well. And then you can hear these crash landings. And sometimes the bat will be successful, and sometimes it’ll miss the frog.

And you can hear that all acoustically, like, through these recordings that we recover afterward.

And the pinnacle, like the success for the bat, is if you hear that whole, like, approach, crash landing, and then a few seconds later you hear that chomping noise. You know that the bat was successful and it managed to get that prey.

And actually, this is an interesting thing. We’ve done so much work looking at their predation of those frogs, the túngara frogs that we’re talking about right now. But based on the length of those chewing noises, we discovered that they’re actually eating way bigger frogs than we had ever understood before.

So the túngaras, I mentioned, they’re like a gram, a gram and a half.

NATE: OK.

RACHEL PAGE: And they will be eating frogs that are up to 17, 20 grams. Like, really, really big. And these bats are 30 grams. They’re not terribly big bats.

NATE: Wow. OK.

RACHEL PAGE: And they do it in a funny way. They will eat, I think, until their stomach is totally packed, totally full. And then they will take a nap with that frog dangling out of its mouth. And then they’ll wake up, and then you’ll hear the chewing noises continue.

And they’ll do that up to three times.

NATE: Whoa.

RACHEL PAGE: They’ll continue eating that very, very large frog. So that’s a more unusual behavior. But we do see that that predation sometimes happen as well.

NATE: Wait, hang on. So where do they take a nap? Just on the forest floor?

RACHEL PAGE: No, they always perch when they eat. So they’ll, like, go down to the chorus, grab a frog, and then they’ll fly to often a nearby branch, and that’s where they’ll, like, hang upside down and have their meal.

And if it’s a small meal, that’s it. And they’ll go after another frog. And if it’s a large meal, they’ll do this, like, eat-nap-eat thing, which is hilarious.

NATE: So if it’s a larger meal, you’ll just see a bat hanging upside down, fully asleep, with, like, a big old frog hanging out of its mouth.

RACHEL PAGE: Yeah, it’s hilarious. And I think it’s actually a very good strategy. Like, you’ve gone to all that trouble to get this big frog. You don’t want to let it go.

NATE: Yeah.

RACHEL PAGE: But nor can you eat it in one bout. It’s too big.

NATE: Probably the most beautiful thing about this beautiful island is that the dense rainforests of BCI are filled with answers. Answers waiting for curious scientists like Rachel.

And in a way, everything we’ve heard in this episode stems from the answer to the very first question that she came here to ask.

RACHEL PAGE: The very first question of my Ph.D., when I started, was just: Why on Earth would a bat prefer one call type over another within the same species?

So these frogs make these simple calls, these whines alone, this—

[FROG WHINE]

RACHEL PAGE: And then they make complex calls, this—

[FROG WHINE WITH CHUCKS]

RACHEL PAGE: Like that.

NATE: And we already know why the frogs bother adding those chucks. The females love them.

RACHEL PAGE: And you could imagine, for a female, she’s choosing a mate. She needs to be very, very selective about her choice.

But for a bat, for a predator, if you’re choosing a meal, why would you care if it’s one call type versus another call type?

NATE: So why do they care about the chucks? That’s what Rachel wanted to know.

RACHEL PAGE: So that was the question that initially kicked off my Ph.D. And the quick answer to that is those calls that have chucks are so much more localizable.

So if you’re trying to find, in the middle of that crazy chorus, that huge cacophony of many, many calling frogs, a single individual male, those chucks help you a lot.

NATE: In the middle of all that noise, the chucks make one frog easier to pinpoint.

And answering that question only led Rachel to more questions.

RACHEL PAGE: That’s what brought me there. And I have to say, even all these years later, we’re still finding new questions to ask about these bats. So the questions just keep coming.

NATE: On our next episode, we return to Barro Colorado Island, where the questions keep coming. There, we’ll follow Erin Spear into the rainforest to learn about the work of the DEATH Lab.

But it’s not what you think.

We’ll explain next time on The Show About Science.

[CREDITS MUSIC]

NATE: A very special thank you to Ben Marcus, Rachel Page, Erin Spear and everyone at the Smithsonian Tropical Research Institute who helped make this series possible.

Music from this episode comes from Blue Dot Sessions.

The Show About Science is hosted by me, Nate Butkus. This episode was produced and edited by Eric Butkus, with story editing by Jason Paris.

There you have it, folks. The Show About Science is complete. Thanks for listening, and we’ll see you on the next episode.

Dad, you can shut the recording off.