Frogs singing submerged

Now here’s a question - if you breathe air but you live and socialise underwater, and you rely on sounds to attract a mate, how can you sing and not drown? As they explain to Chris Smith, it took the musical ear and tenacity of Ursula Kwong-Brown to convince her boss, Darcy Kelley, that there was something special about the sounds these Xenopus frogs were making, and that led the duo to the answer. Darcy first…

Darcy - For many years, I've studied a frog that started out - as all frogs did - came from the sea and went on to land, and then at some point it went back under water. And it's been a mystery for over 120 years of how they can actually create sounds underwater and how those sounds manage to contain the information that they need in order to communicate with each other socially.

Chris - Why is it a problem for a frog to make a sound underwater?

Darcy - The way we make sounds is we have air flowing over our vocal cords to create these sounds. But when you're underwater you can't afford to breathe or else you'll drown. So how do you manage to make the sounds that are so important in your social life without actually having air move through the vocal organ?

Chris - And what had scientists speculated might be the mechanism? Well there's a long history of it - it started about 120 years ago; there were all these theories: they have rods in there maybe they knocked against the side of the voice box. The most recent one was that the sounds are actually made by the implosion of little teeny bubbles of air like the noises that are made by snapping shrimp or propellers until this paper came out. That was the accepted version of how the sounds were made, but nobody had actually seen these little tiny bubbles. So we set out to see if they actually exist.

Chris - And, Ursula, did you come to Darcy with the idea, or did she find you?

Ursula -That is a long story. My background is in music and and biology. I was sitting in the lab listening to other members analyse recordings of these male advertising calls that I heard a musical interval and I said, "why are you singing a perfect fourth?" - Dun dun - Here comes the bride! That's a perfect fourth. Even more astoundingly, it was a harmonic perfect fourth: they were sounding at the same time, and I had never heard this before in any species. And so that's when I came to Darcy and said "I want to know what this is. I want to know how it happens and also how it matters to the frogs!"

Chris - And Darcy, when Ursula came to you with this did it mean anything to you?

Darcy - Well the first thing we said was, "Ursula, you're on crack!" Seriously, we found it very hard to believe because we couldn't actually hear it ourselves. But Ursula went and taught herself how to code. She showed us. We believed her.

Chris - You got she got a demo, you can play us, Ursula, to demonstrate this for those of us who are not musically minded like you...

Ursula - Yes. So this was the very first frog call in which I heard the musical interval. [Sounds]

Chris - What have we just heard there?

Ursula - So first we heard the frog's advertising call. And then we heard the exact same pitches played on the piano. Dun dun.

Chris - Now what does that mean the frog must be doing. Why did that jump out at you - or croak out at you I suppose I should say - as significant and important?

Ursula - One of the first questions we had to address was whether or not this was real. It is so rare to see two simultaneously-sounding frequencies like this that everyone we brought it to thought, "this is an artifact of the glass tanks you're recording in. There's no way this could be real." It wasn't until we recorded with laser vibrometry, which is measuring vibrations at the surface of the animal, that we could really prove that it was real.

Chris - And is that what you did. You actually went and looked at the apparatus the animals are using to produce these sounds in order to work out how they're doing it and why it's so unusual?

Darcy - Yeah. Because they produce these sounds underwater you can actually isolate the vocal organ and put it in a dish and stimulate the nerves that would normally make the muscles contract and have sounds in the dish. We called this "vox in vitro". And then to study the vibrations we actually went and recruited a scientist who worked on the courtship song of spiders; you know male spiders vibrate their webs and you can turn that into sound by shining light at it and having the light reflected back at you. So that's called laser vibrometry. So we called him up and we said we want to figure this out. He hopped on a plane with his laser and came out. We started doing experiments on the vox in vitro.

Chris - What does that reveal about the mechanism though?

Darcy - It means that it has to be intrinsic to the vocal organ itself. It means that you don't need airflow and it means that the acoustic qualities - these two harmonic intervals - have to be shaped entirely within the larynx. 

Ursula - This larynx in a dish is creating these two frequency peaks - this musical interval - and at first we were like, "this is amazing. This is proof that this was not an artifact of the glass tank. This is it. The larynx is making it!" But when we tried to ask what about the larynx is shaping these, it was really difficult to get rid of those frequency peaks. We put large glass beads on top of it, inside of it, drilled holes in the top. Almost nothing made a difference to these frequency peaks. They were so strong until we took a pin and actually poked the elastic cartilage inside, disrupting the central lumen which is normally divided into three chambers and making it into one. And that got rid of the two frequency peaks.

Chris - The intriguing thing you said was you don't need airflow to get this sound because, when I'm speaking to you my vocal folds are opening and closing producing pressure changes which then resonate through my mouth, and those are the vibrations that we call speech. So these animals are clearly not doing that. So describe this apparatus then, with the three chambers, that you had to physically perturb in order to get rid of the sound. What's that actually look like?

Darcy - At the front of the larynx there are these discs - there are pieces of cartilage that are flat, held very tightly together. And when they move apart at a certain key speed, they cause the larynx to vibrate and you can record that vibration either on the surface or as a sound wave. And the key feature that enables the larynx to vibrate in two modes right, with creating simultaneously two different sound pitches, is the cartilage that runs through the larynx that separates the central chamber from the side chambers that surrounds the discs that create the sound. So it's intrinsically vibrating at these two harmonic intervals.

Chris - So how does the sound get out of the frog underwater what's actually vibrating and how does it transmit the sound into the environment? Is it mechanically coupled to its whole body then?

Ursula - It is indeed mechanically coupled to its whole body. I could shine that laser on the frog's littlest toe and I would get the exact same frequency peaks, because it's using the entire body of that frog to radiate that signal.

Chris - And so this blows out of the water the idea that bubbles are collapsing, because you could demonstrate with this series of experiments it's not collapsing bubbles. It is literally the apparatus itself making these vibrations and then turning the whole frog basically into a resonant chamber to get the sound out into the environment?

Darcy - Boy you're great! Yes, exactly! And furthermore we never saw any bubbles and we should have been able to see it in the equipment that we had when we did these experiments. Yeah. So it's it's quite fantastic really; it's a new way of creating sound underwater that's extraordinarily efficiently coupled to the medium.