Animals

Electric Eels Attack by Leaping Out of Water

<p>Photo: Electric eel. Credit: Scott, Flickr</p>

When 19th century naturalist Alexander von Humboldt (1769–1859) reported 200 years ago that he witnessed electric eels attacking and killing horses no one really believed him, but now a new study finds that eels can wage such an attack on mammals, including possibly humans.

When eels encounter a partially submerged potential threat, they will often attack by raising up out of the water, pressing their chin against the individual's side and administering a series of powerful electrical shocks, according to study, published in the journal Proceedings of the National Academy of Sciences.

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"The first time I read von Humboldt's tale, I thought it was completely bizarre," author Kenneth Catania, a Vanderbilt University biologist, said in a press release. "Why would the eels attack the horses instead of swimming away?"

Image: An historic illustration of Alexander von Humboldt's story of the battle between the horses and electric eels. Credit: Public domain

He found the answer to that question by accident, after using a net with a metal rim and handle to transfer electric eels that he kept in large lab tanks.

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"In hindsight, it probably wasn't the best design to use with electric eels," he said, but added that it was a serendipitous choice.

As he scooped up larger eels, Catania found that every so often, an eel would stop trying to evade the net and would attack it by leaping out of the water while pressing its chin to the handle, all the while generating a series of high-voltage pulses. Catania was wearing rubber gloves, so he didn't get shocked as all of this happened.

Curious about the "shocking leap" behavior, he devised a series of experiments to investigate the behavior further. An early observation was that eels ignore most things that do not conduct electricity, which makes sense because living things typically do conduct electricity.

He measured the nature and strength of the electric impulses the eels were producing as they leapt onto a fake target, and found that the voltage and the amperage produced by the eels increased dramatically as an eel moved ever higher.

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Catania explained that when an eel is fully submerged, its electrical pulses when fired are distributed throughout the water. When the eel's body extends out of the water, though, the path that the electric current travels goes from its chin directly into the target. From that point the electric current travels through the target until it can exit back into the water where it travels back to the eel's tail, completing the circuit.

"This allow the eels to deliver shocks with a maximum amount of power to partially submerged land animals that invade their territory," Catania said. "It also allows them to electrify a much larger portion of the invader's body."

The scientist went a step further with his research to visually illustrate the effect.

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He covered a plastic arm and a plastic alligator head with a conductive metal strip and a network of LEDs that light up brightly whenever an eel attacks the fake targets.

"When you see the LEDs light up, think of them as the endings of pain nerves being stimulated. That will give you an idea of how effective these attacks can be," he said.

In their natural environment, where the threats are often real, the eels must contend with receding water in the Amazon basin during the dry season. This causes the eels to feel cornered when land-dwelling predators and others approach, making them more inclined to attack. During the rainy season, the eels' freedom of movement is not as restricted.

Catania thinks that the shocking leap method of attack evolved in stages. At first the eels probably just moved closer to their intended targets. They then would have figured out that making direct contact with intruders increased the power of their electric impulses, as would making contact above the water. Finally, the eels must have learned that the further up the target they go, the more power is directed to the threat.

Concerning von Humboldt, Catania said that "it seems reasonable to suggest that [he] observed a similar eel behavior on March 19th of 1800." On that day, according to the naturalist's dramatic account, electric eels attacked the horses, who lost their footing and drowned in less than 5 minutes.

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