How electric eels leap to attack land animals

Slimy eels leaping from muddy water to electrocute an unsuspecting victim is something you might expect to see in a B-grade horror flick.

Turns out, eels do in fact leap and attack predators – and it’s all because of physics, a US scientist has discovered.

Electric eels are born to shock. Thanks to cells called electrocytes which, stacked like batteries, make up 80% of their bodies, these cunning South American hunters can deliver debilitating blows of up to 600 volts to their prey.

But they’ve harnessed their electricity to pack even bigger punches.

Last year, Vanderbilt University biologist Kenneth Catania revealed electric eels bring their positively charged head and negatively charged tail closer together to generate a more powerful current as they wrap around tough prey (see How electric eels double their zapping power).

Now, Catania reveals electric eels can leap out of the water to deliver a bigger shock to predators, as he sheds light on a 200-year-old tale.

On a field trip to the Amazon in 1807, 19th-century explorer Alexander von Humboldt witnessed a group of horses lead 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 von 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 using 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 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.

The video above shows an electric eel leaping to shock a split aluminium plate while the voltage was measured. The behaviour was filmed at 1,000 frames per second. The red frames were coloured to indicate the timing of each high-voltage pulse during the eel’s volley.

The shock of a leaping eel would be painful, Catania says, and just like an electric fence would make a predator retreat.

In Humboldt’s account, consequences are even worse. Two unsuspecting horses he saw step into the eel-laden waters were stunned and drowned within five minutes.