Steve Mirsky:Welcome to Scientific American Science Talk posted on June 27, 2016. I'm Steve Mirsky. On this episode—

Kenneth Catania: It's almost as if these electric eels have taken a course in physics. I mean they've got pretty much every trick in the book down.

Mirsky: That's Kenneth Catania. He's the Stevenson professor of biological sciences at Vanderbilt University. And he's the author of a recent paper in the Proceedings of the National Academy of Sciences titled: “Leaping Eels Electrify Threats Supporting Von Humboldt's Account of a Battle with Horses.” Not your typical journal article. So we had frequent 60 second science podcast contributor Cynthia Graber get in touch with Catania to find out about these fantastical battles between electric eels and horses. Here is Cynthia.

Cynthia Graber: To set the stage for this research who was Alexander Von Humboldt and what is his legendary story about attacking eels?

Catania: So he was a famous naturalist and he went on many different sort of adventures and fieldtrips to sort of study nature. And one of the most famous and longer ones was to South America. And at that time period—this would've been around 1800—he was really interested in following up on this mystery about what is electricity and are the kinds of things that animals are giving off, is that the same thing as the electricity that you get out of a Leiden chart for example?

So this is one of the many big questions and mysteries of the day. And electric eels were one of the few potential sources of this seemingly magical energy.

Graber: And so what did he do and what did he see?

Catania: So he really wanted to get these electric eels. And he had had some brought to him by fishermen that had been—I guess he used the word—enfeebled. This is of course written in German originally. And so they had sort of been through a rough time. And he was looking for fresher specimens. He traveled to another place where there was a drying up pool that remained from a stream in the dry season. And he hired some local fisherman to collect these eels.

And the fisherman said, "We're going to fish with horses." And after a little while the fisherman came back herding about 30 horses and mules. They made a huge noise as they galloped into this pool and there was this kind of epic struggle or sort of battle you could almost say between the horses and the eels. The way he described it the eels emerged from the mud, swam to the surface. Some of them pressed themselves against the horses, shocked the horses. It was really a spectacle.

Two of the horses died, probably according to Humboldt, from being stunned rather than from electrical shock. A couple of them escaped but most of them were kept in the pool by the fishermen who were surrounding it and waiving reeds and yelling and so forth.

Graber: And so this is a story basically of eels leaping out of the water and attacking the horses?

Catania: So he didn't actually describe them leaping out of the water. But one of the really interesting twists to this story is one of the most detailed images of this sort of encounter does show the eel leaping out of the water. And that is something that was illustrated by somebody who was sort of an admirer and friend of Humboldt's who came along later – a younger guy. And he wrote a book that was a volume of what's called The Naturalist's Library. And it was on the fishes of British Guiana.

He has a front piece—this illustration—that was a really detailed illustration of Humboldt's story and it included this eel leaping out of the water. And that was just amazing for me to stumble across because it turns out it's exactly what I was seeing happening with these eels in the laboratory.

Graber: But did you…? When you first heard this story though did you think it was accurate? I mean contemporary scientists it seems like for the past 200 years have discounted this story.

Catania: It's hard to know what most people thought about it. Certainly a number of people who had studied electric eels had said this seems exaggerated or this seems crazy. But most people—and I would put myself in that category—really just didn't know what to make of this. This just seemed to me like an exaggeration from the 1800s. Somebody that was clearly I would say a nuance of what Humboldt was doing was sort of being an adventurer to come and tell these amazing stories to people back in Europe.

And that was sort of what made him so famous. So it seemed like a huge exaggeration to me and I didn't know what to make of it. I didn't think it was likely. And to me the biggest thing to me that was kind of crazy about this story is why would electric eels go on the offensive against horses? Why would they attack the horses and why not just swim away right? So an animal that's threatened that live in the water has the option to escape you would think. But it turns out to be a lot more nuanced than that.

Graber: Right and so this brings us to what happened in your lab. So tell me the story about what happened.

Catania: This study is the fourth study that I've been doing on electric eels. And this has basically been a lot of serendipity in the sense that the eels have been doing interesting things one after the other. This particular behavior that I've observed I noticed when I was—I was doing something that wasn't really maybe the smartest strategy. So I was moving electric eels to experimental chambers with a net that had a metal rim and a metal handle.

Sort of one of the back stories about eel senses and electric fish senses is that things that are conductive are interpreted often as living beings. So eels will attack a small thing and try to eat it—a small conductor.

Graber: Even a small metal net.

Catania: Well it would have to be probably smaller than a net. I'm talking more like a fish-sized thing. So a small prey-sized thing for them would be fish. For a large eel it could be relatively big. It could be five inches long or more but not something like a big net. And not something that had the shape sticking out of the water. That would not you would think be interpreted as prey. And in fact it wasn't interpreted as prey. It was apparently—based on the eels' behavior—this conductor approaching them, sticking out of the water is apparently sort of the hallmark features of some terrestrial animal that's entered into their domain.

Graber: So you have this metal net and the eel thinks that it's a predator. So then what happened?

Catania: Something really weird was going on. So I would use this net to transfer the eels from one tank to another. And the eels, especially the large ones, would change from trying to escape from the net to turning around and doing sort of an explosive attack at the net. And that was basically following the rim to the handle and then leaping up the handle while pressing against it. I could sort of joke that this was really shocking right? [Laughter]

Graber: Did it actually shock you?

Catania: Well I was wearing a rubber glove always and that's why the net being metal really wasn't something I worried a whole lot about. It was just a convenient net. But one of the things that I was doing that really added to the interest here was I happened to be recording the output of the eels with electrodes in the water playing to a speaker. So that told me interestingly they were not only leaping up the handle of the net but they were coordinating that with these high voltage shocks.

So that was—what I did at that point was something I often do. I just wrote down on a Post-It® note this is interesting. I should come back and look at this closer and start to study it.

Graber: So based on what you said earlier people didn't really think that eels necessarily behaved this way. So you weren't literally shocked but were you figuratively a little bit shocked?

Catania: I was figuratively very shocked. I mean electric eels just in my experience do not jump out of the water. It's just not something that you see. So to see them doing this—you know at first I thought well I've got a net. And if you go to a pet store and you try to pick a fish and you see the guy come out and try to catch it for you sometimes that fish will jump out of the water. So I think probably other people had seen something like this. And you know it didn't seem completely crazy at first.

But as time went on and I realized it was directed at the handle of the net and then recording the high voltage I realized this was really an attack of the net, and a pretty impressive one.

Graber: So then what did you do?

Catania: So then I decided to collect some data and start to investigate what goes on with the voltage—essentially the electrical potential—as they make this kind of attack? And the convenient thing or the great thing about this behavior is in the old days when people studied sort of what the eels is giving off they would take the eel out of the tank and put it on a table 'cause it turns out they're air breathers. So you can do that. But that's not a very pleasant way to get natural recordings.

With what's going on with the electric eels doing this defensive behavior they can—you can sort of voluntarily attach them to a volt meter. As they leap out of the water all you need to do is really connect the conductor to a volt meter that then is connected back down to the water to complete the circuit. And you can record the voltage sort of being delivered to their target as they leap out of the water. And that was really interesting because it turns out as they get higher out of the water essentially the voltage you record at the chin that's pressing against this thing goes higher and higher as they get further out of the water.

And that actually makes a lot of sense 'cause the electric current has two ways that it could go. It could either flow back into the water around the eel or it can go through the thing they're touching. And as they get higher out of the water sort of that alternative path back to the water around the eel is essentially kind of squeezed off in a sense. It becomes more and more resistive to current flow back to the water. And so the electric current is really sort of pushed. The electric voltage increases sort of at the point that the eel is touching this thing.

In essence there's kind of directing the electricity into their target by leaping out of the water.

Graber: Wow. And so this is why they would do that as opposed to kind of nipping at its heel in the water.

Catania: Well exactly. This is a way for them to maximize the effect. And one of the things that I think is so interesting about this is that the gradual increase in voltage as the eel comes further out of the water explains how it could've evolved. Because it would be really unlikely of course that some ancestral eel just suddenly leapt out of the water and touched a threatening predator with its chin while it was halfway out of the water.

But because getting closer to the predator increases the electric field in the water then touching it increases this more. Rising out of the water bit by bit gives you a gradually increasing effect. And that explains how it would've incrementally evolved from some ancestor.

Graber: And what's going on in the Amazon that would lead to that kind of behavior?

Catania: Well that's a great question 'cause that's the other side of the coin which is basically there is a rainy season when areas are inundated with water. And then there's a dry season when many fish including electric eels are caught in small pools and drying up streams and in fact their air breathers potentially to be specialized for those kinds of environments. And so that's the kind of situation where you'd be very vulnerable to predation potentially. So that probably explains why this had to evolve as opposed to the retreat which seems more logical if you're in a big pool of water.

Graber: And what does this tell us about Humboldt?

Catania: I think that Humboldt probably observed exactly what is going on that I've seen in the laboratory here. They were probably leaping up on the side of the horse. I originally thought he probably exaggerated and now I don't think he exaggerated one bit. If anything I might have even understated what was going on. I would've loved to have been able to see that except for the poor horses. [Laughter]

Graber: [Laughter] Seriously. I love this story because I love the idea of kind of looking back in history what we understood then, what we understand now. And the other thing I love about it is it looks as if evolution has really allowed eels to use their power to maximum effect.

Catania: That's absolutely right. I mean it's almost as if these electric eels have taken a course in physics. They've got pretty much every trick in the book down and who knows what else they can do? I'm absolutely amazed with these animals.

Mirsky:That's it for this episode. Get your science news at our Web site: www.scientificamerican.com where you Game of Thrones fans can read Brian Switek's article on how the outsized wolves featured on the show had real life counterparts in the Western Hemisphere up until about 10,000 years ago. Those extinct wolves probably averaged about 130 pounds compared to less than 90 for today's diminutive wolf survivors.

And Cynthia Graber cohosts the food related science and history podcast gastropod. So look for that. I don't know if they've done an eel episode yet or a horse episode for that matter. And follow us on Twitter where you'll get a tweet whenever a new item hits the Web site. Our Twitter name is: @sciam. For Scientific American Science Talk, I'm Steve Mirsky. Thanks for clicking on us.