The engraving from "The Naturalist's Library, Vol. V Part II" depicting "fishing with horses" in the Amazon. (Robert Schomburgk)

"Let me tell you the story."

Kenneth Catania, a biologist at Vanderbilt University, takes a breath. He hasn't known this tale long, but it's quickly become one of his favorites.

Alexander von Humboldt, a famous German scientist, was on an expedition to document the geography and ecology of South America. He traveled for the better part of five years, debunking myths about mystical lakes, observing celestial events, naming new creatures, and investigating questions large and small. He once spent three whole months studying bird feces.

"And one part of his quest was to get eels to do experiments on electricity," Catania said, "since people were very excited about electricity at the time, but they didn't understand the nature of it." (You might recall a certain Founding Father flying a kite in a storm for the same reason.)

Eventually, Humboldt arrived at a village by a muddy stream in the Amazon rainforest, where locals said they could catch him an electric eel.

"They told him, 'We'll fish with horses,' " Catania recounted. Illustrations from the time show fishermen herding dozens of horses into a shallow pool while eels leap at the big mammals' throats, delivering a massive shock with each strike. The spectacle allegedly sapped the eels of their electricity, allowing Humboldt to wade in and catch one to take home.

The story was celebrated when it was first told in 1807, but later scientists were more skeptical. One biologist wrote in the Atlantic Monthly that he considered the story "tommyrot." Catania himself gently suggested that it was a "tale." For one thing, this behavior had never been documented in eels. For another, Humboldt's original account of the incident doesn't even mention the creatures leaping. Within 200 years of Humboldt writing it down, the entire anecdote was deemed a flight of 19th-century fancy.

Until Catania accidentally re-created the incident in his lab.

"There's a lot of good puns here," he joked. "But it was really shocking."

Catania's study, published Monday in the Proceedings of the National Academy of Sciences, shows that Humboldt's account wasn't so fanciful after all. But he didn't set out to prove Humboldt right. Catania specializes in animals with weird behaviors: worms that emerge from the ground in response to "grunts," moles that smell in stereo. And, of course, eels.

"I was moving the eels around with a metal net — which is not really the wisest thing to do, but I was wearing gloves — and I noticed the eels would occasionally go on the offensive," Catania said.

As the net approached, the bigger eels would suddenly reverse course so they were swimming toward it. Then they'd launch out of the water and strike the handle of their net with their chins.

The behavior was odd, to be sure, but Catania didn't think much of it. He added it to his notes, marking it as something to look into later.

Then he noticed the data from the recorder he used to document the eel's electrical pulses. At exactly the same time they were leaping, the eels emitted a volley of incredibly high-voltage pulses.

"That really got my attention," he said. So he decided to dig deeper.

It didn't take much research to discover Humboldt's account of the same behavior more than 200 years ago.

"The eels, stunned by the noise, defend themselves by repeated discharges of their electrical batteries, and for a long time seem likely to obtain the victory," Humboldt wrote in a later account of his travels. "Several horses sink beneath the violence of the invisible strokes ... and stunned by the force and frequency of the shocks, disappear under the water. We had little doubt that the fishing would terminate by killing successively all the animals engaged."

A book on the fish of northeastern South America, written by Humboldt's friend and protege Robert Schomburgk, bore an illustration of the gruesome encounter that looked exactly like what Catania had encountered with his net.

So Catania set up another experiment, this time using a toy crocodile head bedazzled with light-emitting diodes to simulate an "attacker."

"This behavior is exactly what you'd expect the eels to have done from Humboldt's story," he said. The eels positioned themselves as high as possible along the crocodile's head, sending increasingly powerful pulses of electricity into a conductive strip affixed to its front.

These results explain why an eel would risk jumping out of the water to stun his attacker: It's taking advantage of a very basic principle of electronics.

The way Catania describes it, an eel is like a battery, with a positive pole at its front and a negative one on its tail. Electricity always wants to flow from the positive to the negative pole, an easy task when the eel is totally submerged and the current can be conducted by the water. But when the eel leaps up to shock a horse — or a crocodile head — the electricity must travel through the body of the attacker in order to reach the negative pole. This means that the shock administered to the attacker is much more intense.

An electric eel emerges from the water to shock a simulated predator. As the eel increases the distance between its chin and the water, the electric shock administered — shown by LEDs affixed to the crocodile's head — becomes more powerful. (Courtesy Ken Catania via PNAS)

It's a pretty sophisticated behavior — especially when you consider how long it took us humans to figure out how electrical circuits work. And it can be useful to eels during the Amazon's dry season, when streams turn to mud patches, cutting off routes of escape.

"It seems obvious that electric eels are the product of selection for increasing power output," Catania wrote, resulting in "an extraordinary animal that has fascinated scientists for centuries."

This doesn't necessarily mean that Humboldt's spectacular story — and the accompanying images — were wholly true, he cautioned. Just that they were possible. And, much as Catania would like to go back in time and talk with Humboldt himself, he's content to live with that level of uncertainty.

"It makes it more interesting that there's still a mystery," he said.

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