Scientific American 2019-04

April 2019, ScientificAmerican.com 69

SOURCE: “POWER TRANSFER TO A HUMAN DURING AN ELECTRIC EEL’S SHOCKING LEAP,” BY KENNETH C. CATANIA, IN

CURRENT BIOLOGY,

VOL.

27, NO. 18; SEPTEMBER 25,

2017

the eels. Bringing an arm close to an eel resulted in a compelling

demonstration of the leaping defense. The lights flashed bright-

er as the eel rose farther out of the water while shocking the arm.

But exactly how and why did this happen?

Getting the answers to these questions required working out

the so-called equivalent circuit and then determining the volt-

age, or electromotive force, of the eel’s electric organ. I would

also need to calculate how much the materials in the circuit re-

duce the flow of electric current through it—a property known

as resistance. So I designed experiments to measure each vari-

able in succession, starting with the eel’s electric organ. At

slightly more than three feet long, the largest eel in my lab had

an electric potential of 382 volts and an internal resistance of

only 450 ohms, allowing for currents of nearly one ampere if

there were no other resistances. That is quite an electric punch—

far greater than a Taser’s.

When an eel emerges from the water, pressing its lower jaw

against a target, the usual current path for electricity from the

eel’s head to its tail is progressively shut down—because air is a

poor conductor—and is replaced by a path through the target.

Remarkably it is similar to a volume-control knob—the eel pro-

gressively turns up the volume in the target as it rises from the

water. This observation explains how the behavior could have

gradually evolved because each increment in height provides an

advantage. But how efficient is the eel at turning up the volume?

When working out the details, I ran into the most basic of

circuit problems: calculating the electric current in a circuit

containing two resistors arranged side by side. It is a favorite

challenge in circuit puzzles (that is, physics exams) because you

cannot calculate the electric current in the circuit without

knowing the value of both resistors. I was able to solve for one

resistance—the path from the eel’s head to the water—by taking

measurements from eels attacking metal plates connected to a

voltmeter. The other resistance was the arm—the eel’s target.

After collecting data for all the other variables, I could only

guess at this last value: the complex resistance that developed

between the eel’s jaw, a living target and the surrounding water.

It was hard to stop working on the circuit without the final

answers. In addition, just as my first paper documenting the

eel’s leaping attack was published in 2016, a video was posted to

the Internet showing a very large eel leaping onto a surprised

fisherman in South America (he was temporarily immobilized

and then recovered, similar to the aftermath of being Tased).

Suddenly the circuit I had been studying out of curiosity had

real-world consequences.

There was nothing for it but to use my own arm to determine

the last variable and test the predictions from all the previous

measurements. I used a very small eel with an electromotive

force of 198 volts and an internal resistance of 960 ohms. I built

a device that measured the current through my arm during the

eel’s attack, allowing me to finally solve the circuit. I can also re-

port with conviction that eels are very efficient at turning up the

volume of their attack.

I may have started this project thinking I would teach about

electric eels, but in the end, it was the eels that taught me. It is

the same lesson I relearn every time I investigate a new species:

the animals are always far more interesting than I could possi-

bly imagine, in ways I could never have predicted at the outset.

It keeps me up at night—in a good way—to contemplate all we

have yet to discover.

MORE TO EXPLORE

The Shocking Predatory Strike of the Electric Eel. Kenneth Catania in Science, Vol. 346,

pages 1231–1234; December 5, 2014.

Electric Eels Use High-Voltage to Track Fast-Moving Prey. Kenneth C. Catania in Nature

Communications, Vol. 6, Article No. 8638; October 20, 2015.

Power Transfer to a Human during an Electric Eel’s Shocking Leap. Kenneth C. Catania

in Current Biology, Vol. 27, No. 18, pages 2887–2891; September 25, 2017.

FROM OUR ARCHIVES

Natural-Born Killer. Kenneth C. Catania; April 2011.

scientificamerican.com/magazine/sa

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