The shocking science of electric eels

26 September 2017

Interview with 

Ken Catania, Vaderbilt University

eel-hand-diagram.jpg

Electric eel shocking an arm

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In the 1800s, whilst exploring the Amazon, naturalist Alexander von Humboldt documented an attack on a horse by an electric eel, which allegedly leapt out of the water to stun the animal… Yet despite being studied for over 200 years since, this - and a video on youTube were the only documented claims of this behaviour, until, that is, Vanderbilt University’s Ken Catania got his hands on an electric eel: literally. The shock can be ten times the power of a TASER! He told Tom Crawford what - in addition to his own pain threshold - he’s discovered...

Ken -The main function of these high voltage pulses is to activate nerve endings in nearby animals and so, in that sense, it’s a lot like a taser. And then, once you know that, you can start to ask how would you design your taser to creatively activate nerve endings. One thing they can do is freeze up animals for predation; that works very well for fish in water.

But they have this challenge if a predator was to come at them in shallow water which actually happens in the Amazon during the dry season. How would you defend yourself against a predatory cat, or some other predator, or a crocodilian that was coming at you and you only had a limited amount of electric resources to deliver? Well, it turns out the best way to do that without having much of the energy dispersed in the water is to lift out of the water, press against the threat directly, and give off you high voltage. It’s really a shocking behaviour!

Tom - Nice, you beat me to the pun. So just to re-clarify what you’ve said, these electric eels, they literally jump out of the water and then zap their target?

Ken - Exactly. It started by realising when I approached them with a metal rimmed net, they would attack the net by leaping up and shocking it, and that’s the only time I ever saw an electric eel try to come out of the water. So it was a big clue that there’s something special going on with these conductive nets. The eels interpret conductors as living things; they interpret large conductors as threatening living things, and this is clearly a defensive behaviour.

So the research that’s recently published was basically trying to work out the puzzle of the electric circuit that develops as the eel comes up out of the water. And so, I had designed a series of experiments to address each variable in that except for one, and that was the target or, in this case, my arm.

Tom - Sorry Ken, let’s just back up a second. You were purposefully putting your arm into the tank with these electric eels in order to be shocked?

Ken - Eventually… yes. But I got there by really wanting to know the answer to each of the variables in the circuit. I wanted to know the electric motor force of the eel, the internal resistance of the eel, the resistance of the water, and then the variable values of this other resistance that develops as the eel comes out of the water, and the sum of the current can pass back down the eel to the water. I had solved for all of those and it gave the illusion that wow, I’ve really got this puzzle figured out. But there’s a quirk of electric circuitry that if you have two resistors in parallel, which is something that happens when the eel comes out, you need to know both of those resistances to understand how much current flows in the circuit. I was scratching my head about what am I going to do about this? And the final answer was to develop this device that could measure the current through my arm.

Tom - I love how you said there “the obvious answer” was to electrocute yourself.

Ken - Hey, who doesn’t want to get electrocuted by an electric eel? Come on, deep down. I should also say this is quite a small electric eel so I wasn’t taking any crazy risks. You know, Faraday played with electric eels; Humboldt  played with electric eels; there’s a long history of people experimenting with this so-called “hands on” approach. I mean, I wouldn’t do it lightly with a large eel, and I didn’t do it with a large eel.

Tom - You’re saying you were trying to measure all of the variables in the system to be able to calculate the electric circuit, so what was it that you found out?

Ken - Yes. The voltage of the small eel that I was working with was about 200 volts. Eels can get up to 500, sometimes 600 volts when they’re very large and they can get to a couple of metres long. I guess one of the key things that I think this research allows you to do is pretty easily plug in the numbers to extrapolate to these larger animals and we know that people get shocked in the Amazon by this behaviour. In fact, there’s a kind of amazing video that went viral recently showing a fisherman getting shocked by a large electric eel with this leaping behaviour.

Now we have a good idea of how much current goes through a person based on eel size. Specifically, for my case, it was about 40 to 50 milliamps and that was sufficient to be quite a deterrent -I’ll put it that way. If you’ve gone to a really colourful, interesting party and have been tased recently, it would be a lot like that. It would be, for a large eel, it would be 20 times the power of a TASER.

Tom - Oh wow!

Ken - Yeah. It was a lot of fun to do - if you can believe that!

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