How Baby Bats Learn To Eavesdrop On Dinner

Summary

Short Wave Podcast Episode Summary

Title: How Baby Bats Learn To Eavesdrop On Dinner
Host: Emily Kwong
Guest: Rachel Page, Behavioral Ecologist at the Smithsonian Tropical Research Institute
Release Date: May 14, 2025

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Introduction: The Spectacle of the Congress Avenue Bridge

Emily Kwong opens the episode by painting a vivid picture of the annual emergence of Mexican free-tailed bats from Austin’s Congress Avenue Bridge. "An hour or so before sunset, hundreds of people gather at this one bridge to wait for the moment when bats take flight... Up to 1.5 million Mexican free-tailed bats take to the sky. This Exodus can last 45 minutes and it is hypnotizing" (00:16). This natural phenomenon not only attracts spectators but also serves as a backdrop for introducing the intricate behaviors and learning processes of bats.

Rachel Page’s Fascination with Bats

Rachel Page shares her serendipitous journey into bat research, sparked by Austin's vast urban bat colony. "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" (00:58). Her passion evolved during her graduate studies, leading her to specialize in the behavioral ecology of bats, particularly focusing on how they navigate and interact within their environments.

Navigational Marvels: Echolocation and Beyond

The episode delves into the complex navigation systems bats employ. Emily Kwong explains, "There are over 1400 different species of bats found throughout the world. And the way they navigate is hugely varied. Many use echolocation" (01:33). Rachel Page expands on this, highlighting that while echolocation is a primary tool, bats also utilize other senses. "But echolocation is not the only sense that they use" (01:49). This multifaceted approach allows bats to not only navigate but also communicate and hunt effectively.

Eavesdropping: A Sophisticated Hunting Strategy

A significant portion of the discussion centers on how fringe-lipped bats in Panama employ eavesdropping to locate and assess their prey—frogs. Rachel Page outlines this strategy: "These frog-eating bats... 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" (02:06). The bats' ability to discern between the calls of poisonous and non-poisonous frogs is critical for their survival.

Chemical and Acoustic Sensitivity in Prey Selection

Rachel Page explains the physiological adaptations that facilitate this selective hunting: "So they have enormous ears. This helps with their eavesdropping behavior as they listen in for frog calls and other prey sounds... they have this fringe on their chin and lips... to very quickly make chemosensory assessments of prey quality" (02:26). These adaptations enable bats to not only hear the frogs but also chemically assess whether the frog is safe to consume.

The Learning Conundrum: Innate vs. Acquired Behaviors

A pivotal question addressed is how juvenile bats develop the ability to distinguish between safe and toxic frogs. Rachel Page poses, "Was this through learning over time? Was it something that they were born with? Was it something in between?" (03:29). This inquiry sets the stage for discussing her research on whether these discriminatory behaviors are innate or learned.

Experimental Insights: Testing Juvenile Bats

To explore this, Rachel Page describes her experimental approach: "We captured a whole bunch of bats both Adults and juveniles this time. And we brought them into a flight cage... we just played the frog calls one by one and quantified what those reactions were" (11:14). The findings revealed that juvenile bats exhibit less discrimination between frog calls compared to adults, suggesting that experience and learning play crucial roles in developing this skill.

Behavioral Observations: Juveniles vs. Adults

During an engaging segment, Emily Kwong and Rachel Page conduct a playful experiment simulating the bats' decision-making process based on frog calls. For instance, when testing the tungra frog, Emily humorously predicts its toxicity, but Rachel Page corrects her: "That is the best frog of the forest... we call them the popcorn of the forest" (09:00). Conversely, the cane toad elicits a correct reluctance to approach due to its known toxicity. This interaction underscores the necessity for juvenile bats to learn through experience which calls signify safe prey.

Cognitive Flexibility and Learning Mechanisms

The conversation highlights the cognitive flexibility of bats, allowing them to adapt and correct mistakes when identifying prey. Rachel Page states, "If they do make a mistake at the eavesdropping level... they have a number of possibilities for correcting that mistake" (07:21). This adaptability is likened to humans learning to avoid spam calls after a bad experience, emphasizing the importance of learning in survival behaviors.

Implications for Broader Biological Understanding

Rachel Page emphasizes the broader significance of her research: "We're fascinated by eavesdropping behavior... you find it in 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" (13:32). Her study is pioneering in elucidating how eavesdropping predators acquire their strategies, potentially inspiring similar research across various species.

Conclusion: The Importance of Learning in Predator-Prey Dynamics

The episode concludes by reaffirming the critical role of learning in the efficacy of predators like fringe-lipped bats. Rachel Page expresses hope that her findings will inspire further studies into eavesdropping behaviors, enhancing our understanding of complex ecological interactions. Emily Kwong wraps up by thanking the guest and producers, leaving listeners with a deeper appreciation of the sophisticated lives of bats.

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Notable Quotes with Timestamps

• Rachel Page [00:58]: "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 [01:24]: "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?"

• Emily Kwong [02:20]: "So they have enormous ears. This helps with their eavesdropping behavior as they listen in for frog calls and other prey sounds."

• Rachel Page [03:06]: "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 gonna stay away."

• Rachel Page [07:02]: "So once they actually have the frog or toad in the mouth, if they've made a mistake, they will spit out that frog and toad."

• Emily Kwong [09:16]: "You're probably less likely to do that the next time. Like it's. It definitely is a learned behavior that over time you think?"

• Rachel Page [13:32]: "We're fascinated by eavesdropping behavior... you find it in nearly every taxonomic group that you look at."

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Key Takeaways

• Eavesdropping as a Hunting Strategy: Fringe-lipped bats utilize auditory cues from frog calls to locate and assess prey, distinguishing between safe and toxic species.

• Learning Over Innate Behavior: Juvenile bats require experience and learning to develop the sophisticated ability to discriminate between different frog calls, highlighting the importance of environmental interaction in behavioral development.

• Cognitive Flexibility: Bats demonstrate the ability to correct mistakes in prey identification, akin to human learning processes, showcasing their adaptability and intelligence.

• Broader Implications: Understanding how bats and other predators learn to eavesdrop can shed light on complex ecological interactions and inform conservation strategies.

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This episode of Short Wave offers an insightful exploration into the cognitive and behavioral ecology of bats, emphasizing the intricate interplay between innate abilities and learned experiences in shaping survival strategies. Through Rachel Page’s research, listeners gain a deeper appreciation for the nuanced ways in which animals interact with their environment and each other.

Transcript

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[Emily Kwong]

You're listening to shortwave from NPR 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.

[Rachel Page]

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.

[Emily Kwong]

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.

[Rachel Page]

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?

[Emily Kwong]

There are over 1400 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.

[Rachel Page]

But echolocation is not the only sense that they use.

[Emily Kwong]

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.

[Rachel Page]

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.

[Emily Kwong]

Native to Panama, where she now lives, Rachel's been studying fringe lipped bats for over 20 years.

[Rachel Page]

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.

[Emily Kwong]

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.

[Rachel Page]

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 gonna stay away. But what we didn't know is how these acoustic preferences developed.

[Emily Kwong]

Meaning Rachel had no idea about the young fringe lipped bats, the juvenile bats. Could they eavesdrop too? And if so, how did they lear?

[Rachel Page]

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?

[Emily Kwong]

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 can gain these eavesdropping skills in the first place. I'm Emily Kwong and you're listening to Short Wave, the science podcast from npr.

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[Emily Kwong]

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?

[Rachel Page]

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 froggy 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. 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 becomes aware that the bat is on the way, the bats really have to switch decolocation at the very end, because that's going to help them, like, pinpoint that in the final moment of approach where that. 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.

[Emily Kwong]

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.

[Rachel Page]

Exactly.

[Emily Kwong]

So if a bat were to eat that frog, it would, like, get very sick or it would die. What would happen?

[Rachel Page]

So once they actually have the frog or toad in the 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 that 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 at 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, they would die in eating that.

[Emily Kwong]

So how are they ultimately able to tell the difference between which frogs are safe and which are not?

[Rachel Page]

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. So much like a human child, you're not born knowing that there are dangers. Perhaps, in encountering strangers, 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.

[Emily Kwong]

It's almost like these bats have developed caller id.

[Rachel Page]

Absolutely. It is a little bit like collar 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.

[Emily Kwong]

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.

[Rachel Page]

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?

[Emily Kwong]

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.

[Rachel Page]

Absolutely. So why don't you start with the tungra frog?

[Emily Kwong]

That frog's gonna kill me. I wouldn't eat that frog. I would stay far away from that frog. But what is that true?

[Rachel Page]

That would be a bad choice. That is the best frog of the forest.

[Emily Kwong]

Really? I feel like that's the flamingo. You want a frog? What are you doing?

[Rachel Page]

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.

[Emily Kwong]

Like I said, I don't think I would make it as a bat. Okay, let's play another frog sound.

[Rachel Page]

How about next, let's go for the cane toad.

[Emily Kwong]

See, I personally am lured to that toad.

[Rachel Page]

Oh no.

[Emily Kwong]

Oh no. Is it poisonous? It's poisonous, isn't it?

[Rachel Page]

That one is highly poisonous.

[Emily Kwong]

Oh my gosh.

[Rachel Page]

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 and it's highly toxic.

[Emily Kwong]

Yeah. And eavesdropping, it seems, just has never been so essential. Yep. 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?

[Rachel Page]

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.

[Emily Kwong]

How do you know that? What were the juvenile bats doing at this frog mating call concert that you set up for them?

[Rachel Page]

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 going to critical for them to develop this fine scale discrimination of what's safe and what's not in terms of prey approach in the forest.

[Emily Kwong]

You speak of these juvenile bats with so much care. You're so endeared by them.

[Rachel Page]

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.

[Emily Kwong]

Is there any last thought you'd like to add on why you think this research is important?

[Rachel Page]

So we're fascinated by eavesdropping behavior. And you find eavesdroppers across sensory modalities across the animal kingdom.

[Emily Kwong]

Eavesdropping is not just exclusive to bats.

[Rachel Page]

Absolutely. Humans do it too. But no, you find it really in 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.

[Emily Kwong]

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.

[Rachel Page]

It was a pleasure. Thank you.

[Emily Kwong]

This episode was produced by Rachel Carlson. It was edited by Burleigh McCoy and fact checked by Tyler Jones. The audio engineer was Kwesi 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.

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