Let’s start this summary of a new paper (by Kenneth Catnia in Proc. Nat. Acad. Sci. USA; reference and free download below) with a historical incident that helped motivate the work. This is how Catania’s paper begins:
In 1807, Alexander von Humboldt published his account of a battle between electric eels and horses. The stage for this event was set when Humboldt hired local fishermen to supply him with eels for research. Their method was to “fish with horses”. About 30 horses and mules were herded into a pool containing eels, which (according to Humboldt) emerged from the mud, swam to the surface, and attacked by pressing themselves against the horses while discharging. The fishermen kept the horses from escaping by surrounding the pool and climbing nearby trees with overhanging branches while crying out and waving reeds. Two horses drowned, and others stumbled from the pool and collapsed. Humboldt thought more horses would be killed, but the eels were exhausted before this happened. Five eels were then captured and Humboldt was able to conduct his experiments.
Here’s an illustration of that epic battle in which two horribly abused horses died so that Humboldt could satisfy his curiosity about electric eels. Ignore for a moment the photographs below the drawing:
The article continues:
This famous story has been illustrated and recounted many times (Fig. 1A). However, some have doubted its accuracy. Sachs suggested the story was “poetically transfigured,” Coates flatly considered it “tommyrot,” and Moller [and Catania] 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. Here I report that electric eels attack large, moving, partially submerged conductors by leaping from the water while pressing themselves against the threat and discharging high-voltage volleys (Fig. 1B). This behavior appears to be ubiquitous for comparatively large eels (over 60 cm).
But the story was not tommyrot, as Catania discovered as a byproduct of his other studies on predatory behavior of electric eels (Electrophorus electricus; note that electric eels aren’t eels, but bony fish). While watching them, he discovered that they can indeed jump out of the water if they’re disturbed by an intrusive object, and give a powerful shock to those objects. The full-text version of the paper includes several movies showing this shocking behavior, but none is more compelling that this one (tweeted by science presenter Ziya Tong), in which a fake alligator head, containing LEDs to show any current, is lit up by a jumping eel:
In all of these videos, the eel leaps out of the water with its head and belly contacting the hapless intruder, and delivers a powerful shock. This shock is more localized than if the eel stayed in the water where the electric impulse it emits is diluted throughout the water. When in contact with a large animal, the entire animal absorbs the shock, as shown in the diagram below (vertical arrow):
It’s thus far more efficient for the eel to leave the water and deliver a direct shock. How strong is the shock? Very strong: here’s a diagram showing that it can exceed 200 volts (go here to read how it makes the shocks—a marvel of evolution):
Some eels can deliver nearly 900 volts, and that’s enough to kill a person by stopping the heart or causing him to drown, though I haven’t found reports of any human being killed by an eel. But caiman have been killed when trying to nom an electric eel–you can see the video here.
I’ve already alluded to why the eels leave the water: they can deliver more voltage than if they remained in the water.The jumping behavior is almost certainly evolved rather than learned; it’s likely that eels that already could deliver shocks to stun their prey would be more likely to survive if they could jump out of the water to hurt potential predators.
Why did this behavior evolve? According to Catania, these eels often live in shallow water or even isolated pools that form during the dry season (electric eels are air-breathers and can survive in such situations). That makes them susceptible to predation by or injury from larger animals, so the shock, evolved to stun prey (which are swallowed whole), can be coopted—and intensified by selection—to protect the animal from being eaten itself.
The life of a naturalist interested in evolution is an exciting one, for fantastic stuff like this appears all the time. As the saying goes, “Evolution is cleverer than you are.” Who could have predicted that after all this time, Humboldt’s observation, dismissed as poppycock, would be substantiated?
You might have asked yourself why aren’t the eels stunned by their own shocks, especially in the water. As far as I know, this remains a mystery.
Catania, K. C. 2016. Leaping eels electrify threats, supporting Humboldt’s account of a battle with horses. Proc. Nat. Acad. Sci. USA. P