How Baby Bats Learn To Eavesdrop On Dinner
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In Austin, Texas, an hour or so before sunset, hundreds of people gather at this one bridge to wait for the moment when bats take flight. Every time I go to Austin, I make a point to visit the Congress Avenue Bridge, where these bats emerge to hunt all at once, clicking and squealing in a plume of wings. Up to 1.5 million Mexican free-tailed bats take to the sky.

This exodus can last 45 minutes, and it is hypnotizing. I sort of fell into bats by chance, and really the reason I fell into them was because of Austin, because of that enormous urban colony of bats.

Rachel Page is a behavioral ecologist and a staff scientist at the Smithsonian Tropical Research Institute in Panama. But back in the day, she was a grad student falling in love with bats for the first time because of this same colony in Texas. I was mesmerized. How on earth are they not bumping into each other? How can they recognize their own echolocation call? How are they also communicating socially with one another?

There are over 1,400 different species of bats found throughout the world. And the way they navigate is hugely varied. Many use echolocation. That's where creatures emit a sound frequency that bounces off surfaces and tells them where they are in space. But echolocation is not the only sense that they use.

When it comes time to find and hunt their prey, bats will use their eyes, some rely heavily on smell, and some have evolved the ability to eavesdrop on their future meals. And then these frog-eating bats, for example...

They are actually listening in on the mating calls of frogs that are much, much lower in frequency. So they've had to evolve basically like another set of hearing sensitivity. Native to Panama, where she now lives, Rachel's been studying fringe-lipped bats for over 20 years. So they have enormous ears. This helps with their eavesdropping behavior as they listen in for frog calls and other prey sounds. They also, as you might guess from the name, have this fringe on their chin and lips.

And this has been hypothesized as a way to very quickly make chemosensory assessments of prey quality. So whether a frog is palatable or poisonous.

And here's the thing. The bats have gotten really good at telling frogs apart. The yummy ones from the poisonous ones based just on their calls. So if they heard a particular frog call, they would expect, okay, this is palatable prey. I'm flying for it versus this is a poisonous frog. I'm going to stay away.

But what we didn't know is how these acoustic preferences developed. Meaning Rachel had no idea about the young fringe-lipped bats, the juvenile bats. Could they eavesdrop too? And if so, how did they learn? Was this through learning over time?

Was it something that they were born with? Was it something in between? So the goal of this particular study was to really probe those juvenile bats to ask them, what are your preferences for these different frog calls? And how do those compare with adult bats?

Today on the show, how to hunt like a bat. We listen in on frog calls, guess which ones can kill us, and learn how baby bats gain these eavesdropping skills in the first place. I'm Emily Kwong, and you're listening to Shorewave, the science podcast from NPR. ♪

Thank you.

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Okay, Rachel, so the bats you study, fringe-lipped bats, which, you know, big ears, fringe on the lip, all the better to hear and taste, they eavesdrop in order to hunt. How do they do that? These poor male frogs have to make this very loud, conspicuous call. And it's an acoustic beacon. ♪

So not only from a distance does it signal to a female frog that here I am, I'm wonderful, I'm attractive, I'm calling at the top of my lungs. It signals that, unfortunately for the male frog, to predators as well.

So these frogging bats, that's their first signal that, oh, there's prey in the area. And that's what initially brings them right to the spot where those frogs are. But as the bat approaches, it has to have landing control as it comes close. It has to know where the ground is and when to stop. And then it has to pinpoint at the very end exactly where that frog is. And the frog's best line of defense is to stop calling. So if that frog calls,

becomes aware that the bat is on the way, the bats really have to switch deck location at the very end because that's going to help them pinpoint in the final moment of approach where that prey target is. And then the bats actually use another sensory modality right when they're in contact with the frog or toad. And that is chemical cues. Yeah. An important thing to bring up here is that not all the frogs...

that these bats want to prey on are safe. Some of these frogs are toxic. Exactly. So if a bat were to eat that frog, it would, like, get very sick or it would die? What would happen? So...

Once they actually have the frog or toad in mouth, if they've made a mistake, they will spit out that frog and toad. They will spend quite a long time grooming and cleaning and trying to get all those noxious toxins off of them, and they'll be fine. And actually, interestingly, the frog is also fine. So it's a strategy that really does work for both. And we think that that might be why they're so cognitively flexible, is that if they do make a mistake,

the eavesdropping level, at the echolocation level, at all these sequential levels of assessment as they approach the prey, they have a number of possibilities for correcting that mistake. And if it did eat that frog, it would get very sick and it could die. Like some of these frogs actually are so toxic that much larger mammals like a dog or a cat, like they would die eating that. So how are they ultimately able to tell the difference between which frogs are safe and which are not? So...

It looks very, very much like the juveniles are acquiring these responses over time, that this is something that they have to acquire with experience and likely with learning as they grow up. So much like a human child, you're not born knowing that there are dangers perhaps in encountering

You have to be taught by those adults around you that there is stranger danger. You have to be careful. And it looks like that might be what's going on with these frog-eating bats. It's almost like these bats have developed caller ID. Absolutely. It is a little bit like caller ID. So you want to be able to know before you take that action that this was one, oof.

That was one to avoid. Do not approach it. It reminds me so much of when I pick up a spam call hoping it's a real person and then they start talking and I have this sinking feeling in my stomach of like, oh gosh, I fell for it. I fell for it. Yes. You're probably less likely to do that the next time, right? Like it's – it definitely is a learned behavior that over time – You think –

So you supplied some sounds for us to listen to. Can we do that? Can you tell me which sounds to play? And then I'll guess. I'll pretend to be a bat and guess, based on eavesdropping, if this frog is safe to eat or not. Absolutely. So why don't you start with the Tungra frog?

That frog's going to kill me. I wouldn't eat that frog. I would stay far away from that frog. Oh, no. Is that true? That would be a bad choice. No. That is the best frog of the forest. Really?

I feel like that's the filet mignon of frog. What are you doing? It really is. And not just bats, there's so many predators that love that frog. Really, we call them the popcorn of the forest. Like they are bite-sized, they're palatable, they're also numerous. Like I said, I don't think I would make it as a bat. Okay, let's play another frog sound. How about next, let's go for the cane toad. Ooh.

See, I personally am lured to that toad. Oh, no. Oh, no. It's poisonous, isn't it? That one is highly poisonous. Oh, my gosh. So that one is actually so famous for this terrible introduction into Australia. It's also introduced into Florida. It's wreaked havoc with many, many ecosystems. It does happen to be native to Panama. So it is supposed to be right here in our ecosystem. But it's enormous.

enormous and it's highly toxic. Yeah. And eavesdropping, it seems, just has never been so essential. Yeah. Okay. So you and other scientists noticed, though, that

Even though this hunting strategy is very fine-tuned in adult fringe-lipped bats, baby bats, not so much. The big question you wanted to figure out was, well, how do they learn? How did you set about figuring that out? So we had only in the past tested adult bats. We captured a whole bunch of bats, both adults and juveniles this time, and we brought them into a flight cage, which is just basically a big screened room. ♪

So they're at ambient temperature, ambient light. They have all that normal outdoor sounds. And then really we just played the frog calls one by one and quantified what those reactions were. So we were wondering to what degree is that eavesdropping behavior really hardwired versus something that needs to develop through experience over time.

What this experiment showed was absolutely the latter. It seemed that they really, really do need to have experience with these palatable versus poisonous species to develop the acoustic repertoire that the adults have later in life. How do you know that? What were the juvenile bats doing at this frog mating call concert that you set up for them?

It actually is like that in nature. The forest at night in the tropics, it's this cacophony of frog calls. And there are many times, many species calling at once. And for the juveniles, they actually showed interest in those toxic frogs and toads, which really, coupled with our other experiments over time, led us to think that that learning period is critical for them to develop this interest

this fine scale discrimination of what's safe and what's not in terms of prey approach in the forest. You speak of these juvenile bats with so much care. You're so endeared by them. I love these bats. This species in particular is fascinating. So these are not even very close evolutionarily, frogs and bats.

And yet these bats have figured out basically how to interpret these frog calls, that this one is palatable and this one is poisonous. And I can consistently rely on that for correct discrimination between meals that are safe and meals that are quite risky. Is there any last thought you'd like to add on why you think this research is important?

So we're fascinated by eavesdropping behavior. And you find eavesdroppers across sensory modalities, across the animal kingdom. Eavesdropping is not just exclusive to bats. Absolutely. Humans do it too. But no, you find it really in animals.

nearly every taxonomic group that you look at. And I think that the ones that we haven't found it in is just because we haven't looked hard enough yet. So this is the first study to our knowledge where we've looked at how

eavesdropping predators acquire that specific behavioral strategy. So I'm really hoping that the study spurs interest in all of the fantastic biologists all over the world who are interested in eavesdropping behavior, because it would be really, really fascinating to see if the patterns that we're seeing with these bats are similar across other predators that eavesdrop on the communication signals of their prey. ♪

Rachel Page is a behavioral ecologist at the Smithsonian Tropical Research Institute and one of the study authors on this paper. Thank you so, so much for talking to me. It was a pleasure. Thank you.

This episode was produced by Rachel Carlson. It was edited by Burleigh McCoy and fact-checked by Tyler Jones. The audio engineer was Kweisi Lee. Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy. I'm Emily Kwong. Thank you for listening to Shortwave, the science podcast from NPR. This message comes from Thrive Market. The food industry is a multibillion-dollar industry, but not everything on the shelf is made with your health in mind.

At Thrive Market, they go beyond the standards, curating the highest quality products for you and your family while focusing on organic first and restricting more than 1,000 harmful ingredients. All shipped to your door. Shop at a grocery store that actually cares for your health at thrivemarket.com slash podcast for 30% off your first order plus a $60 free gift.

This message comes from Thrive Market. The food industry is a multi-billion dollar industry, but not everything on the shelf is made with your health in mind.

At Thrive Market, they go beyond the standards, curating the highest quality products for you and your family while focusing on organic first and restricting more than 1,000 harmful ingredients. All shipped to your door. Shop at a grocery store that actually cares for your health at thrivemarket.com slash podcast for 30% off your first order plus a $60 free gift.