The naturalist and explorer Alexander von Humboldt arrived at the headwaters of the Amazon River in March of 1800. There, Humboldt, who is regarded as one of the first European scientists to study South American rainforests, came across a very odd fishing practice. The locals had devised a way of catching electric eels using horses as bait (sort of). The horses were made to charge into a pond laden with resting eels, which caused a disturbance that resulted in the cranky eels lunging out of the shallow water and shocking the horses. Eventually, and at the cost of a few drowned horses, the eels would run out of juice. Dinner.
Humboldt's tale is a somewhat popular legend, but the odd eel behavior at the center of it apparently hasn't been investigated or replicated until now. In a paper published Monday in the Proceedings of the National Academy of Sciences, Kenneth Catania, a biologist from Vanderbilt University, describes experiments in which electric eels were made to attack terrifying simulated land-predators. As in the legend, they lunged out of the water, delivering sharp jolts to the fake horses (that look like dinosaurs).
Image: Ken Catania
"[Humbolt's] famous story has been illustrated and recounted many times," Catania writes. "However, some have doubted its accuracy. Sachs suggested the story was 'poetically transfigured,' Coates flatly considered it 'tommyrot,' and Moller gently suggested Humboldt's accounts were 'tales.' The aggressive behavior of the eels, taking the offensive against horses, seems the most fantastic and questionable part of the story. Why would electric eels do this? No similar behavior has been reported since Humboldt's publication."
In his experiments, Catania was able to induce electric eels to attack large, moving, and partially submerged predators by pressing themselves against the perceived threat and discharging bursts of high-voltage.
"Measurement of the voltage and current delivered to stimuli during this behavior suggest it is a formidable defensive strategy," Catania notes. "It allows eels to deliver much of their prodigious electrical power, normally distributed throughout the surrounding water, directly to a threat."
Fortunately for us Catania documented his efforts in video form.
As the paper explains, electric eel evolution has been driven toward ever-increasing voltage output, as we'd expect. The ability to lunge out of the water and deliver intense shocks to a mammal as large as a horse represents an impressive adaptive outgrowth of this ability.
Why would an eel ever need to attack a horse or other terrestrial mammal in the real world? It seems likely that this is an adaption to yearly rainy season/dry season cycles in the Amazon. In the dry season, water sources dry up, often leaving small ponds isolated from larger bodies of water. Electric eels naturally become trapped in this stagnant ponds, something that land-based predators (including humans) take advantage of.
Discharging voltage into the surrounding water works pretty well against other sea creatures but it doesn't make for much of a defense against large land-roaming animals. Hence, electric eels developed this ability to lunge and deliver shocks directly—a much more powerful deterrent. Way to evolve, eels. I'll leave you alone if you promise not to leap out of the water and shock the daylights out of me.
Finally, Catania offers a brief and amusing anecdote about how he even got the idea for these experiments: "The behavior described in this investigation was serendipitously discovered during research into electric eel predatory behavior and sensory abilities."
"For these previous investigations," he writes, "eels were transferred from a home cage to an experimental chamber with a net that had a metallic rim and handle. From the outset, eels regularly transitioned from a retreat to an explosive attack when the net approached. They swam rapidly toward the net, followed the metal rim to the point of exit from the water, and leaped upward along the rim and handle, keeping their chin in contact while discharging high-voltage volleys. This behavior was both literally and figuratively shocking."