Roughly 30 horses and mules were rounded up into an eel-filled pool, trapped inside by a barrier of humans. Below, eels emerged from the muck and leapt from the water, pushing themselves onto the horses and shocking them. While a couple of the horses died and others were injured, the task was physically exhausting for the eels. Five of the sleek creatures were captured for 19th-century explorer Alexander von Humboldt’s experiments.

In the ensuing years, the veracity of Humboldt’s eel-catching story from the Amazon was questioned.

Now, over 200 years later, a researcher with Vanderbilt University’s Department of Biological Sciences has confirmed electric eels do engage in such behavior as a defense mechanism.

“This shocking behavior likely allows electric eels to defend themselves during the Amazonian dry season, when they may be found in small pools and in danger of predation,” researcher Kenneth Catania wrote in his study published in Proceedings of the National Academy of Sciences. Not only does the discovery support Humboldt’s observations, it also “highlights sophisticated behaviors that have evolved in concert with the eel’s powerful electrical organs.”

Catania made the discovery by chance. Studying eel predatory behavior and sensory abilities, Catania transferred eels from their home cage into an experimental chamber. When a net with a metallic rim and handle came into the environment, the eels usually were coerced into predatory behavior, chasing the net and following to its point of exit from the water. Leaping out of the water, they kept their chin in contact with the apparatus “while discharging high-voltage volleys,” according to Catania.

“Under no other circumstances were eels observed to leap out of the water,” Catania noted.

In images published with the study, an eel is shown attacking a predator prop whose snout is dipped in the water. The eel lifts from the water, placing its body on top of the predators head. A conductive carbon strip taped to the front of the plastic predator head allowed the eel’s shock to power LEDs positioned on the prop’s head.

“Electric eels most likely use an aggressive attack to defend themselves because they cannot retreat,” Catania wrote.

Flooded areas in the Amazon usually provide various fish with habitats. However, these areas are also known to rapidly drain, leaving the fishes stranded in small pools. It’s likely the experimental chamber simulated these confined pool environments.

“In light of these findings, it seem reasonable to suggest that Humboldt observed a similar eel behavior on March 19, 1800,” according to Catania. “Importantly, the events took place toward the end of the dry season, and the eels were trapped in a muddy basin, which was all that was left from a drying stream.”