At the turn of the 19th century, German naturalist Alexander von Humboldt was exploring South America when he hired a group of local fishermen to collect electric eels for him. Humboldt had been experimenting with creating batteries, and was eager to find what he called “living electric apparatuses.”
The locals led him to a pool, where, according to Humboldt's account, they proceeded to fish for the eels using a rather unusual method. Rather than luring the eels with bait, they led some 30 wild horses and mules into the muddy pond filled with electric eels, and once in, kept the horses from fleeing by yelling and wielding long, thin reed canes. The resident eels defended themselves against the invading equids by swimming to the surface, where they then pressed themselves against the horses’ bodies and released electric jolts. Two horses died within the first few minutes. Once the eels exhausted themselves, the fishermen easily reeled in several for Humboldt’s research.
Since Humboldt published this account in 1807, no one had ever reported seeing this shocking—pun intended—behavior (the eels leaping out of the water in such a fashion, not the inhumane treatment of the wild horses, that is). Even Kenneth Catania—a MacArthur genius-grant winning neurobiologist and and modern-day electric-eel expert at Vanderbilt University—hadn’t given the story a lot of credence. “I thought, this is a crazy tale from 1800 that’s probably totally exaggerated, if not possibly false,” he said.
That is, until he saw it with his own eyes.
As he was moving his eels from one tank to another using a metal net, he noticed that the eels “would periodically turn around and change from not wanting to be near the net to explosively attacking it by leaping out of the water up the handle,” Catania says. Because he was measuring the electrical output in the aquarium with wires hooked up to a speaker, he could also hear the amount of electricity the eels were releasing shift from the quiet pop-pop-pops used to sense their surroundings, to much higher voltage, crackling volleys. The eels were leaping, and shocking simultaneously. He captured it on video, like the one seen above, using a fake alligator head as simulated predator.
He realized these were the near-mythical leaps Humboldt had described, and published his findings today in the Proceedings of the National Academy of Sciences. He called the discovery serendipitous. “I love to be able to say that about something trying to leap out and shock you,” he said.
Bruce Carlson—a sensory and evolutionary neuroscientist at Washington University in St. Louis, who wasn’t involved in Catania’s research—writes in an email that he was surprised, specifically, by how far the eel was able to launch itself from the water, calling it a "fascinating discovery."
Catania describes these eels as giant swimming batteries, with the long skinny part of their bodies housing the organs that make the electricity, and the rest of their viscera squished into the front. They only have two settings: a low sensory setting, and a high weapons setting.
Electric eels don’t change how much electricity they emit during their high-voltage volleys, but the voltage passing through a predator (or a voltmeter dressed up as a predator) increases as they leap farther out of the water. That's because a current passing from the eel's positively charged head to its negatively charged tail creates a circuit when passing through water, but when moving through air, is more forcefully applied to whatever the eel is attacking.
“[T]his is a beautiful example of how the eel has evolved a fairly simple behavior that exploits the basic physics of electricity,” Carlson says.
The behavior isn’t hunting oriented—Catania never saw them trying to bite the “victims." Instead, it's most likely a defense adaptation that probably developed during dry seasons spent in shallow pools where fleeing is not an option.
“I have been specializing in unusual animals for a lot of my career, and I always underestimate the animals,” Catania says. “They always do something that amazes me.”