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. 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] — 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.Learn more about sponsor message choices: podcastchoices.com/adchoicesNPR Privacy Policy
Chapter 1: Why do people gather at Congress Avenue Bridge?
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.
Chapter 2: How did Rachel Page become interested in bats?
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.
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Chapter 3: What unique hunting techniques do frog-eating bats use?
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 sensitivities.
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.
Chapter 4: How do bats differentiate between poisonous and non-poisonous frogs?
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?
Chapter 5: How do juvenile bats learn their eavesdropping skills?
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.
Support for NPR and the following message come from Jarl and Pamela Moan, thanking the people who make public radio great every day and also those who listen.
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? Yeah.
Chapter 6: What challenges do bats face while hunting?
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. Wow. 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 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 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 these some of these frogs actually are so toxic that much larger mammals like a dog or a cat, like they would die eating that.
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