A well-known account of an epic battle between electric eels and horses, long unsubstantiated by scientific evidence, now appears more realistic thanks to new research demonstrating these aquatic creatures are capable of targeting land threats with powerful electrical shocks.

The study, led by Vanderbilt University biologist Kenneth Catania and published online Monday in the Proceedings of the National Academy of Sciences early edition, lends credence to the story of famed explorer and naturalist Alexander von Humboldt, who allegedly witnessed a conflict between horses and eels while on an expedition to the Amazon.

The lack of evidence led many to suggest that von Humboldt had been exaggerating his tale, but last year, Catania made the accidental discovery that eels can have a dramatic defensive reaction when cornered by a land-based threat: they will attack it by coming up out of the water, placing their chin against the object’s side, and delivering a series of powerful shocks.

In the newly published study, he details for the first time the effectiveness of this mechanism and explains the evolutionary advantage it provides to the eels. In short, he answers a question that he said had long puzzled him: “Why would the eels attack the horses instead of swimming away?”

Leaping attacks increase the voltage, amperage of eels’ shocks

As luck would have it, Catania chose to to move his eels from place to place with a metal net. Typically, he explained, the larger eels would try to avoid the net, but at times, they would leap up out of the water, place their chins on the net, and deliver electric pulses.

Fortunately for the Vanderbilt professor, he was wearing gloves and avoided shocking himself, but the behavior seemed strange. Previous research demonstrated that eels view some types of small conductors as prey, and this newly-observed behavior gave Catania the impression that they viewed the larger conductor as a potential predator.

In his previous work, Catania showed that eels use a high-frequency series of extremely short pulses when attacking their prey, causing its muscles to contract similar to a TASER. Following the net incident, he devised a series of experiments to observe their defensive mechanisms, and found that eels tend to ignore most objects that are not conductors. In addition, he found that the voltage and amperage of the shock increased the higher up the eel leaped on its target.

When an eel is completely submerged, the power of its electrical pulses is distributed throughout the water, but when its body extends out of the water, the current travels directly from its chin to its target. The shock travels through the recipient, back into the water and finally back to the tail of the eel, thus completing the circuit.

In a statement, Catania explained that this allows them to “deliver shocks with a maximum amount of power to partially submerged land animals that invade their territory” and “electrify a much larger portion of the invader’s body.”

The pulses they produce while fully submerged may not be powerful enough to keep a hungry land-based predator at bay if the predator keeps its body out of the water, he explained. By leaping out of the water, however, the eel can defend itself far more effectively, and this behavior was likely what von Humboldt witnessed.

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Image credit: Thinkstock

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