On a field trip to the Amazon in 1807, 19th-century explorer Alexander Humboldt witnessed a group of horses leads through a muddy pool filled with electric eels, which he described as dramatically leaping up to attack the intruders. But scientists have doubted the story.

“The first time I read Humboldt's tale, I thought it was completely bizarre,” Catania says. Why would the eels attack the horses instead of swimming away? But then he observed the same behaviour by accident as he transferred the eels in his lab from one tank to another use a metal-rimmed net. Instead of swimming away, larger eels attacked the net by leaping out of the water.

Catania tracked the strength of the eels’ electric shock by attaching a voltmeter to an aluminium plate or 47 conductive metal strips to “predator” objects such as a crocodile head replica.

The zap a submerged eel distributes through the water is relatively weak when it reaches the target. But when an eel touches it with its electricity-generating chin, the current travels directly to the target and has to travel through its body before it gets back to the water, Catania reported in Proceedings of the National Academy of Sciences.

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

Catania found the eels leapt to attack, rather than receded, more often when the water in the aquarium was lower. He argues the attack lets electric eels better defend themselves during the Amazonian dry season when they’re cornered in small pools and make easy prey.