Listening To Love Songs of African Clawed Frogs

For many, romantic music is a key part of a Valentine's Day celebration. Biologist Darcy Kelley studies the communications of South African clawed frogs, including their underwater love songs. What can their music reveal about communication elsewhere in the animal kingdom?

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IRA FLATOW, host:

This is SCIENCE FRIDAY from NPR News. I'm Ira Flatow. A bit later in the hour, a National Climate Service and a squid invasion off the California coast. But first, Valentine's Day in a quiet, South African pond. The lights are low, and there's music.

(Soundbite of frogs)

FLATOW: Ah, the long song of the South African clawed frog, probably not Grammy material, but Kermit might think otherwise.

(Soundbite of frogs)

FLATOW: Joining me now to talk about her work studying them and what their communications can tell us about other animals is Darcy Kelley. She's the Harold Weintraub professor in the Department of Biological Sciences at Columbia University here in New York. Welcome to SCIENCE FRIDAY, Darcy.

Professor´┐ŻDARCY KELLEY (Harold Weintraub Professor, Department of Biological Sciences, Columbia University): Happy to be here, Ira.

FLATOW: What were we just listening to? Could you hear that okay?

Prof. KELLEY: Yes, I can hear it. It's a familiar song in the lab. And nothing you'd hear in South Africa, actually, because they sing underwater. You have to put a microphone in the water to hear them.

FLATOW: Is that right?

Prof. KELLEY: Yeah.

FLATOW: So this is all going on underwater?

Prof. KELLEY: Yes. So they're singing these duets, these love songs to each other, and everybody else is oblivious of what's going on, basically, underneath their noses.

FLATOW: Huh. So this is a successful love story, then?

Prof. KELLEY: Oh, very successful. These frogs, you know, they've been around for 50 million years. They'll probably be here when we've long left the planet.

(Soundbite of laughter)

FLATOW: What was that song that - can you actually tell what they're saying in those different songs?

Prof. KELLEY: You can tell each song type by its rate and its rhythm. So what you were just listening to was a lovesick male advertising his reproductive state. They actually advertise both to females, in order to attract them, and to other males, in order to have kind of vocal battles underwater.

FLATOW: And so we can actually listen to the dialogue, then, the back and forth between the frogs.

Prof. KELLEY: Yeah, absolutely. And this is one of the few species in which the females actually have a fair amount to say to the males.

FLATOW: All right. Let's see - well, let's see what the next dialogue sounds like.

(Soundbite of frogs)

FLATOW: There's your male advertising.

Prof. KELLEY: That's male advertising. And eventually, in the next clip I think you're going to hear what happens to the male's advertisement call when he hears a call that the female makes.

FLATOW: All right. Let's see if we can play that clip now.

(Soundbite of frogs)

Prof. KELLEY: Yeah. So that is the acoustic aphrodisiac of the female frog. So when a female is about to lay her eggs, she puts out that call, which Martha Tobias(ph) in my lab discovered and which Martha calls, for pretty obvious reasons, rapping. It's like a Geiger counter or machine-gun fire. And it's so exciting to the male, that if you do it in the lab from an underwater loudspeaker, he'll try to mate with the loudspeaker.

(Soundbite of frogs)

(Soundbite of laughter)

FLATOW: No kidding?

Prof. KELLEY: No kidding. He refuses to believe every evidence of his senses, and it just has to be a female because it's putting out that sound.

FLATOW: I guess it's - males are alike all over the world.

Prof. KELLEY: Yeah, they are.

(Soundbite of laughter)

Prof. KELLEY: There's no stopping them.

(Soundbite of laughter)

FLATOW: All right. Let's listen to the next sound, and you'll tell us what we're listening to.

(Soundbite of frogs)

Prof. KELLEY: Now, that is the anti-aphrodisiac. So females, when they're not about to lay their eggs, if a male approaches them singing, or if he tries to clasp them, they produce that song, which is called ticking. And it means get off my back.

(Soundbite of frogs)

FLATOW: Literally.

Prof. KELLEY: Literally. And it's incredibly effective. You'll hear what it does to the male song in a minute. But among other things, it's one of the things that persuades the male that he better let her go.

FLATOW: Wow.

Prof. KELLEY: So there's no rape in the frog world. It's a consensual, totally consensual reproductive system.

FLATOW: And as you say, this is all done underwater.

Prof. KELLEY: All underwater. So these animals, they started out, evolutionarily, on land, like most frogs. But at some point, they - I guess they perceived the advantages of being underwater, where they can't be seen, they can't be heard, and they can live out their reproductive sound lives in isolation and in relative safety.

So there are all these species of these frogs all throughout sub-Saharan Africa, singing underwater. You can only hear them if you actually put a microphone into the water.

FLATOW: But they do come up for air to breathe.

Prof. KELLEY: Oh, yeah. They come up to the surface to breathe. They look kind of - they don't - they come up very - they actually have to come up too often, and the kind of look like little hippopotami. You can see their little eyes hanging out at the surface.

FLATOW: All right, well, let's go to our next sound and see if we can decipher that one.

(Soundbite of frogs)

Prof. KELLEY: So that recording starts out with a male producing the ad call...

(Soundbite of frog imitation)

Prof. KELLEY: Then the female starts to rap, and it's, you know, almost pornographic, right? He gets incredibly excited and keeps on rapping. So there are two frogs singing in that. That's a duet between a male and a female who's desperately trying to find him.

(Soundbite of frogs)

Prof. KELLEY: And he never gives up. As long as she raps, he'll just call and call and call and call.

FLATOW: Wow. And so you say if there was a frog in your lab listening to that, he'd be...

Prof. KELLEY: He'd be very excited, very excited.

(Soundbite of laughter)

FLATOW: He'd attack the speaker.

Prof. KELLEY: He'd attack - well, I don't know if I'd call it attack, but, you know, it's sort of - you know, in some ways it's pathetic, but in the other ways, it's kind of comforting.

(Soundbite of laughter)

FLATOW: Let's not go there.

(Soundbite of laughter)

FLATOW: Let's listen to one more sound.

(Soundbite of frogs)

FLATOW: Kind of tapered off there at the end.

Prof. KELLEY: Yeah. So that's what happens to a male when a female ticks. In addition to being get off my back, it also makes the male shut up. So they have very effective vocal communication. Females can turn males on, and they can also turn males off, at least in the acoustic arena.

FLATOW: Wow. And so why do you study these things? What can you learn from these things?

Prof. KELLEY: Well, you know, these are the animals, actually, that came to science initially because they were the first pregnancy tests for human cryonic anatatropin(ph). These animals are incredibly sensitive to sex hormone, and, you know, the hormones are very ancient, evolutionarily. They're just exactly the same in frogs and in humans.

So they ended up being a really good animal to figure out reproductive biology. They're really great for development. So that's how they came into that world. And in terms of neuroscience, which is what I do, because sounds are so important to these animals, it's much easier to study the parts of the brain that translate what's heard into what's uttered.

So if you have an animal whose whole life is devoted to one extreme thing, it's there in an extreme way in their brain. So it's a lot easier to study.

So, you know, we're interested in speech, what's going on in your brain when you listen to me and what's going on in mine when I listen to you, but it's really hard to study in human beings. It's a lot easier to study in these frogs, partly because they're so single-minded, and also because their brains are a lot smaller.

FLATOW: Now, we keep hearing about frogs being - some frogs becoming vanishing species around the world. Is this one of them?

Prof. KELLEY: No. I'm afraid this is the one that's going to be left when all the other frogs are gone. This frog is actually pretty immune to the chytrid fungus, which is one of the things that is contributing to the amphibian die-off.

Of course, the other thing that's contributing to the die-off is changes globally in the climate. As long as there are standing bodies of water, we're going to have these guys around. So them I'm not worried about. I'm more worried about the other guys.

FLATOW: Yeah, I understand that. Can you find them outside of South Africa there?

Prof. KELLEY: I'm sorry to say that these frogs have been released, sometimes by pet store owners, sometimes by zoologists, in practically every country in the world.

And, for example, the San Diego water system is chockablock with these frogs. You can find them in Chile.

I mean, they're native to South Africa, the ones you're listening to now - to most of Africa, but there's pretty much - most of the continental parts of the United States have one or another of these frog species. They're very successful.

FLATOW: So if you were a science teacher and you wanted to go, let's say, in San Diego, where the frogs are in one place, could you just stick your microphone under in a lake and listen to them?

Prof. KELLEY: Well, you know, you'd have to be there - you know, courtship is tricky, right? You've got to be there at the right time. But you could certainly put traps in, as some of my colleagues have done. You put in one of these - they're called funnel traps. So you put some bones in, you put it in. You prop up the end so the frogs can come up to breathe, and low and behold, you go and pick it up in the morning, and it's full of frogs. So that would be a good way for a teacher in San Diego to go do a little trapping expedition in the waterway.

FLATOW: 1-800-989-8255. Let's get a call in before we have to go. Malory(ph) in Portland, in Oregon. Hi, Malory.

MALORY (Caller): Yes, hi.

FLATOW: Hi, there.

MALORY: Thanks for taking my call. So I work on the auditory system of frogs. Actually, I work with Rana pipiens, which is the leopard frogs, the temperate frog. And they've been able to show that the auditory system of these frogs actually becomes more acutely tuned and sensitive to mating calls during the mating season, and then the sensitivity drops off when they're not mating.

And I'm actually interested in whether there's anything about - since they call nocturnally - whether there's anything about the circadian rhythm, whether they actually get more sensitive during certain times of the day versus other times.

Prof. KELLEY: Well, that's an interesting question. People don't pay very much attention to that. When they're studying the auditory system, you know, they're generally in the lab, studying it during the daytime. Maybe we should tell our graduate students to focus on the night.

But one of the interesting things about the frog and its seasonality is that, at least in Xenopus and also in Rana - I published a paper on this - the cells in the part of the brain that are responsive to sound in the auditory mid-brain contain receptors for sex hormone. So those receptors may, in fact, be contributing to the exquisite sensitivity that you see during the breeding season.

FLATOW: So you see how the sound sort of stimulates them in the brain.

Prof. KELLEY: Yeah, well, there are some species of frogs in which actually hearing sound will stimulate their reproductive system. And there's actually a theory in human evolution that human speech actually came from the songs that males and females sang to each other in the caves at night to ward off boredom.

So, you know, these - sound is very powerful, particularly in the dark, and probably many of the principles that we see in Rana pipiens and in Xenopus are common principles across many species that communicate with vocal signals.

FLATOW: Malory, does that answer your question?

MALORY: Yes, it makes my research even more interesting. Thank you.

FLATOW: Are you going to go out and listen to these frogs underwater somewhere in Oregon?

MALORY: The Xenopus? Oh, yeah, you can find them here. We also have another underwater species called the northern red-legged frog that actually - Rana aurora - that actually, it's also an underwater caller and mater.

Prof. KELLEY: Yeah, Rana aurora is cool. Do they call at the interface between air and water, or are they actually calling underwater?

MALORY: They're actually calling underwater.

Prof. KELLEY: Cool.

MALORY: In deep water, actually, like more than three feet. So...

Prof. KELLEY: Yeah, you know, sound falls off really rapidly in shallow water. So if you want to make a big noise, you're better off in the deep water.

FLATOW: All right, well, I'll let you two scientists pick this up later.

(Soundbite of laughter)

Prof. KELLEY: Nice to talk to you, Malory.

FLATOW: Glad we could make the connections here. I want to thank you very much for taking time to be with us today, Darcy.

Prof. KELLEY: Well, thank you for having me on, Ira, and I want to wish all your listeners a very happy Valentine's Day.

FLATOW: Thank you. You, too. Darcy Kelley is the - Darcy's the Harold Weintraub Professor in the Department of Biological Sciences at Columbia University. We're going to take a break. When we come back, we're going to switch gears, talk about a new service from NOAA. This is a National Climate Service. So stay with us. We'll be right back.

(Soundbite of music)

FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

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