The naturalist and explorer Alexander von Humboldt saw a lot of strange things in his day including claiming to have seen Amazon dwellers in Venezuela using horses and mules as bait for the trapping of electric eels. While many disputed these claims made over 200 years ago, a new study is showing that Alexander von Humboldt very likely did see this practice in action.
1800 observations validated by new study on electric eels
In March of 1800, Humboldt has his colleague and fellow naturalist Aime Bonpland were looking to experiment with electric eels and deemed live specimens to be what they required. The electrical eel can reach lengths of up to eight feet, weight nearly 45 pounds, bring their prodigious teeth to the attack as well as deliver up o 600 volts of electricity so simply wading into the shallow waters of the Amazon and trying to grab one was something that neither of them much fancied.
As a result, the pair enlisted the villagers of Rastro de Abaso in Venezuela to help. While Humboldt knew that others trapped the slime burrowing eels after using Barbasco root in the water which left them dazed and confused, this practice ran contrary to the pairs experimentation plans and this practice was not employed. Instead, villagers suggested “horse fishing” which the two had certainly never heard of prior to the suggestion.
The natives then proceeded to gather up a group wild equines and forced them to the water’s edge which woke up the eels and sent them into a frenzy which saw them leap from the water and attack the horses with their electric charges.
When the horses tried to flea, the villagers forced them back to the water. Following the repeated electric attacks, the eels grew tired and were easily snared by the villagers.
Kenneth Catania, a biologist and neuroscientist at Vanderbilt University, quite frankly didn’t believe the stories but in his lab he noticed when his eels were approached with a net that he used to transfer the eels from their enclosure to an experiment chamber the eels actually leaped after the net and discharged their electricity.
“I certainly thought it was a crazy story when I did consider it,” Dr. Catania recently told the Christian Science Monitor before adding, “but I don’t anymore. At this point I don’t think he exaggerated one bit.”
Catania then conducted additional experiments and saw the same behavior from the eels in different situations.
“I had been studying electric eels for a while,” says Catania. “During the course of these studies, I was moving the eels with a metal-rimmed net… and the eels periodically turned around and attacked the net.”
Writing in the Proceedings of the National Academy of Sciences (PNAS), the doctor determined that when dry season affects the eels habitat after the water recedes, a condition he replicated in his aquariums, the eels aren’t afraid to attack almost desperately and quite aggressively. Catania also cited Michael Faraday’s 1838 experiments with electric eels that showed that an electric attack in the water only produces a mild shock to the eels’ natural predator and an aggressive attack was necessary to dissuade .“A motivated terrestrial predator, harrying a trapped eel from above, may not be deterred before an eel is exhausted,” Catania wrote in the new study.
Eels also bear their young in the dry season, so eels would be more likely to launch this full-out-leaping attack during this time of the year.
As Catania was working with his own eels, their behavior brought Humboldt’s often-doubted story to mind.
“I had been studying electric eels for a while,” says Catania. “During the course of these studies, I was moving the eels with a metal-rimmed net… and the eels periodically turned around and attacked the net.”
Coupled with Humboldt’s experience – and, in particular, an illustration of the event, which appeared to show the eels leaping to attack the horses, something Humboldt himself never explicitly described – the biology professor decided to investigate.
He designed an experiment that measured the voltage and amperage produced by the eels’ shocking behavior, finding that both values increased dramatically the higher the creatures leapt. To illustrate the point, he covered a plastic alligator hand-puppet with LEDs, which lit up when they received an eel’s charge, giving a visual representation of the pain the shock would produce.
In terms of efficiently applying an electric shock, this behavior makes sense: when an eel is fully submerged, the power it emits is dissipated in the water, meaning that any adversary would not receive the full effect.
But by removing itself somewhat from the water and making direct contact, the eel intensifies the sensation, and the more it rises from the water, the stronger the shock.
“Recognizing that the voltage imparted increases with the height of the eel coming out of the water explains how this could have evolved,” Catania tells the Monitor. “It all fits so nicely with the successive stages required for selection.”
But why would an eel want to attack something as big as a horse in the first place? Why not just swim away?
It turns out that the ecology of eels has a neat explanation for this, too.
The Amazon basin, where these creatures live, is a place dominated by two distinct seasons: one rainy, and one dry. During the former, waterways swell, banks burst, and floodwaters coat much of the land, giving the eels a huge range to play with. But as the dry season encroaches and the waters recede, the fish can often find themselves stranded in oxbow lakes or ponds.
Under such circumstances, if potential predators wander too close, the eels have nowhere to retreat to; their only choice is to attack. Additionally, some of these eels bear their young during the dry season, so having an effective defense would be even more crucial.
Catania’s experiments lent further credence to this hypothesis, too, by demonstrating that when the water in the aquaria was lower, the eels were more aggressive, striking more quickly.
“This is a species people have studied for hundreds of years,” he says. “Darwin worked with eels, and Faraday did, too. But mostly it’s been about anatomy and physiology, less about behavior. It turns out their behavior is equally fascinating.
