Before he was shocked by a leaping electric eel, Kenneth Catania wasn’t sure what it would be like. In fact, no one was really sure. So, in the name of finding out exactly how much of a jolt the creatures could deliver, Catania, a professor at Vanderbilt University, volunteered his own arm. The eel jumped out of the water and into the air to deliver its shock. It looked like a nuzzle, but it packed the wallop of an electric fence.

It looked like a nuzzle, but it packed the wallop of an electric fence

Catania’s report appears today in the journal Current Biology. While underwater jolts from eels are well-known, above-water shocks aren’t. In fact, until now, the behavior had been reported only once before — over 200 years ago by German naturalist Alexander von Humboldt. The explorer described watching electric eels leap out of a shallow pool to electrify wild horses that had been herded into the water as eel bait.

Catania observed something similar a few years ago in his own lab: when he reached into their tanks with a net, the eels would first try to dodge the mesh. Then, they’d leap at the net’s handle — unleashing crackling volleys of electricity. That made him suspicious that the shocking leaps were a way for the eels to defend themselves against land predators.

After spending years studying the creatures, volunteering to be shocked by them “almost seemed like destiny, in a weird way,” Catania says.

Leaping shocks — in the air — are more powerful than the shocks eels can deliver in the water. When an eel presses its chin against its victim to deliver a shock, electricity flows through the eel to the target. But water carries electricity, so the if the eel’s still submerged when it delivers the shock, the charge dissipates. When the eel’s airborne, more of the electrical current flows through its victim.

Catania could measure most parts of the system, but he didn’t know how much current ultimately flowed through the limb of the eel’s victim. “I really wanted to know,” he says. “It just felt like a puzzle to which I had all of the pieces except one.”

He was spurred to his self-experimentation by a video: a fisherman shocked by a leaping eel. “I’d been using these prop arms, and put a fake wedding ring on them to make it look like my arm just for fun,” Catania says. “There was a little foreshadowing there.” He stuck his hand in a tank with an electric eel, and measured how much current flowed through his arm when it zapped him.

It took about 10 attempts to get enough good readings for the paper, Catania says. Each time, he found himself involuntarily jerking his hand out of the water. Eventually he was able to determine that about 40 to 50 milliamperes of electricity coursed through his arm. That’s about four to five times what it takes to make a person jerk away from a painful shock, Catania writes.

It was a clever experimental set up, says Bruce Carlson, a sensory and evolutionary neuroscientist at Washington University in St. Louis, in an email to The Verge. “It seems that what makes the eel’s leaping behavior effective is that it induces intense pain that is sufficient to frighten off a potential predator.” But, he adds, it’s important to keep in mind that this is just one eel, and just one subject. “These measurements should be considered ballpark estimates of the effect of this behavior on potential predators,” he says.

In fact, the size of the zap depends on the size of the eel and where on the body it shocks you. That’s why Catania picked a small, young eel to shock him on his arm. He calculated, however, that if person who was standing in water were to be shocked square in the torso by a full-sized eel, they’d experience a jolt “an order of magnitude greater” than the shocks delivered by a taser.

The paper is a start to closing the circuit on this shocking electric eel behavior. But if Catania needs to do a repeat study, he now has plenty of new volunteers to be the eels’ victims — not that he’d ever take anyone up on the offer. “I’ve been telling some people about it and they all pretty much say I would love to do that,” he says. “That really surprises me.”