It was March of 1800, and Alexander von Humboldt wanted electric eels.
While traveling in South America, the German naturalist and explorer became fascinated by these two-meter-long fish, and asked local fishermen to help him collect some. Their strategy was unorthodox, to say the least. They drove some 30 horses and mules into a pool of eels, which allegedly reared out of the water, pressed themselves against the stampeding legs, and shocked the shit out of them. Once the chaos had died down (and two horses had died), the fisherman waded in and safely grabbed five of the exhausted eels.
When Ken Catania from Vanderbilt University started studying the eels a few years ago, he too doubted the stories. Then, one day, while trying to herd a reticent eel into a net, the animal suddenly went on the offensive.
In an explosive burst, it swam up the handle of the net, partially leaving the water and heading straight for Catania’s hand. Pressing its chin against the handle, it delivered its trademark shocks—rapid trains of 600-volt pulses that can stun small fish and incapacitate humans (or horses). “Despite wearing a [rubber] glove, it was pretty intimidating,” says Catania. “I made a little note to myself to come back and study this.”
Over time, Catania got better at eel-wrangling. In the last two years, he has single-handedly shown that an electric eel—not a true eel, but a type of knifefish—is a battery, Taser, remote control, and tracking device, all in one. With different kinds of pulses, it can make a prey animal twitch and so give away its presence, it can paralyze its victim by forcing all its muscles to contract, and it can monitor the movements of the stiffened targets. And when it wants to take down a really big target, like a crayfish, it can curl its body to deliver twice the shock for no extra effort.
Through all these discoveries, Catania’s mind kept returning to that image of a cornered eel hurtling up his net. “I suddenly realized how effective it would be as a defensive strategy,” he says.
The electric eel is a living battery, with the positive pole at its head and the negative one at its tail. When it’s fully submerged, the current it creates passes every which way through the water in between. That’s great for killing a small submerged target, but terrible for shocking a larger standing predator like a wading bird or English physicist Michael Faraday, who, after some (quite literally) hands-on experiments with electric eels in 1838, wrote: “When one hand was in the water the shock was felt in that hand only, whatever part of the fish it was applied to; it was not very strong, and it was only in the part immersed in water.”
But when the eel leaves the water and presses its head against its foe, it creates a short circuit. There’s nowhere else for the current to go except through the target. “At its full height, it’s giving virtually all of the current to the potential threat,” says Catania. “What it’s doing parallels a dimmer switch or a volume control knob,” he adds. By jumping, it dials its shocks up to 11.
“It’s a welcome example of a serendipitous discovery in animal behavior,” says Graciela Unguez from the New Mexico State University. “It pays off to be attentive to the unexpected!”
At first, Catania measured the eel’s volleys using conducting rods hooked up to a voltmeter. But as he says, “I strive to show people as much of the behaviour as possible, and when you have an animal who’s generating hundreds of volts of electricity, you have some fun options.” Such as: embedding LEDs in a prop crocodile head and a prosthetic arm. You can see the results below.
“Each of those flashes represents the firing of the nerves in a potential predator,” says Catania. “In each of those volleys, there are 200 pulses per second. That’s a very good rate for causing pain.”
The eel’s shocking ascent isn’t an attack; these animals swallow their prey whole and Catania never saw them trying to bite the objects he shoved into their tanks. It’s purely defensive, and it’s carefully judged. The eels aren’t just leaping and shocking in a haphazard panic. Slow-motion videos revealed that they forcefully bend their necks to press their chins against their threats. If they lose contact, they cut off their pulses.
It’s easy to see how this behavior evolved. Every little step—from touching the target with its head underwater, to briefly leaving the water, to the full-blown leap—increases the current flowing through the target. But why do this at all, rather than simply retreating? They might have nowhere to go: In their Amazonian home, dry seasons regularly leave them confined to small pools and lakes. They might also want to stand their ground: During dry months, males guard their larvae until the rains return.
Indeed, the events that Humboldt described took place at the end of the dry season when the eels were trapped in a muddy basin. And a striking illustration of the events, commissioned by Humboldt’s friend Robert Schomburgk, shows the eels doing to horses what they did to Catania’s fake crocodile prop. When Catania first saw that image, “I was stunned, heh, no pun intended,” he says. “Here was exactly what I was observing.”
So, sorry about the 1947 article, Alexander von Humboldt. The Atlantic regrets the error.