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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