To take the zap out of a school of electric eels, fishermen in 17th century South America sent teams of horses into the water as bait, scooping up the eels after they had exhausted themselves in the attack. According to famed naturalist Alexander von Humboldt, the eels would leap out of the water to shock the frightened—but mostly unharmed—horses. Until now, no one else had recorded evidence of such behavior, and many scientists were skeptical of Von Humboldt’s account. A new study shows that not only do the animals leap from the water to attack their prey, but they also increase their voltage as they leap (see video). The jumping behavior was first observed by accident: As scientists were studying the eels in an unrelated experiment, they noticed that the animals would sometimes jump to attack the rim of the nets used to capture them. In a follow-up study, reported today in the Proceedings of the National Academy of Sciences, the researchers found that the attacks became markedly more powerful the higher the eel rose out of the water, in one case increasing from 10 to 300 volts. As the animal leaves the water, electrical resistance through its body increases, forcing the charge into a less resistant target—its prey. The farther the eels can get out of the water, the more current they can deliver—a shocking bit of evidence to support Von Humboldt’s electric account.