How Baby Bats Learn To Eavesdrop On Dinner

Most bats use echolocation to navigate and hunt, but some use their ears for another trick: eavesdropping. "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," says behavioral ecologist Rachel Page. https://stri.si.edu/scientist/rachel-page But how the bats knew this eavesdropping trick was a mystery. So she set up and experiment with baby bats and a speaker. Have a question about the animals all around us? Email us at [email protected] mailto:[email protected] — we'd love to hear from you! Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:00Speaker 00

  This message comes from Capella University. That spark you feel, that's your drive for more. Capella University's FlexPath learning format lets you earn your degree at your pace without putting life on pause. Learn more at capella.edu.

• 0:15Emily Kwong?

  You're listening to Short Wave from NPR.

• 0:21Emily Kwong?

  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.

• 0:33Emily Kwong?

  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.

• 0:57Speaker 02

  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.

• 1:06Emily 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.

• 1:20Speaker 02

  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?

• 1:32Emily Kwong?

  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.

• 1:48Speaker 02

  But echolocation is not the only sense that they use.

• 1:53Emily 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.

• 2:05Speaker 02

  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.

• 2:19Emily Kwong?

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

• 2:25Speaker 02

  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.

• 2:56Emily 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.

• 3:05Speaker 02

  So if they heard a particular frog call, they would expect, OK, 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.

• 3:20Emily 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 learn?

• 3:27Speaker 02

  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?

• 3:49Emily 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 gain these eavesdropping skills in the first place. I'm Emily Kwong, and you're listening to Shore Wave, the science podcast from NPR.

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• 5:38Emily Kwong?

  OK, 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?

• 5:51Speaker 02

  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.

• 6:14Speaker 02

  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 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 decoal location at the very end because that's going to help them pinpoint in the final moment of approach where that prey target is.

• 6:56Speaker 02

  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.

• 7:06Emily 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.

• 7:16Speaker 02

  Exactly.

• 7:17Emily Kwong?

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

• 7:24Speaker 02

  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 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 decoal location 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.

• 8:08Speaker 02

  Some of these frogs actually are so toxic that much larger mammals, like a dog or a cat, they would die in eating that.

• 8:16Emily Kwong?

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

• 8:22Speaker 02

  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 don't, 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, with these frog eating bats.

• 8:56Emily Kwong?

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

• 8:59Speaker 02

  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.

• 9:09Emily 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.

• 9:19Speaker 02

  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.

• 9:27Emily Kwong?

  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.

• 9:41Speaker 02

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

• 9:45Emily Kwong?

  That frog's gonna kill me. I wouldn't eat that frog. I would stay far away from that frog.

• 9:54Speaker 02

  Oh, no. That would be a bad choice. That is the best frog of the forest.

• 10:00Emily Kwong?

  Really? You're like, that's the filet mignon of frog. What are you doing?

• 10:06Speaker 02

  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.

• 10:18Emily Kwong?

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

• 10:22Speaker 02

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

• 10:29Emily Kwong?

  See, I personally am lured to that toad.

• 10:31Speaker 02

  Oh, no.

• 10:31Emily Kwong?

  Oh, no, it's poisonous, isn't it?

• 10:33Speaker 02

  That one is highly poisonous.

• 10:35Emily Kwong?

  Oh, my gosh.

• 10:36Speaker 02

  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.

• 10:56Emily Kwong?

  Yeah, and eavesdropping, it seems, just has never been so essential.

• 11:00Speaker 02

  Yep.

• 11:01Emily Kwong?

  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?

• 11:17Speaker 02

  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.

• 11:35Speaker 02

  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.

• 12:16Emily Kwong?

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

• 12:25Speaker 02

  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...

• 12:50Speaker 02

  This fine scale discrimination of what's safe and what's not in terms of prey approach in the forest.

• 12:57Emily Kwong?

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

• 13:03Speaker 02

  I love these bats.

• 13:04Emily Kwong?

  Yeah.

• 13:05Speaker 02

  This species in particular is fascinating.

• 13:08Emily Kwong?

  Oh.

• 13:08Speaker 02

  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 for correct discrimination between meals that are safe and meals that are quite risky.

• 13:29Emily Kwong?

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

• 13:35Speaker 02

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

• 13:47Emily Kwong?

  Eavesdropping is not just exclusive to bats.

• 13:49Speaker 02

  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, 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, that eavesdrop on the

• 14:33Speaker 02

  communication signals of their prey.

• 14:39Emily 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.

• 14:52Speaker 02

  It was a pleasure. Thank you.

• 14:56Emily Kwong?

  This episode was produced by Rachel Carlson. It was edited by Burley McCoy and fact checked by Tyler Jones. The audio engineer was Quacey 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 Short Wave, the science podcast from NPR.

• 15:23Speaker 00

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