An electric eel leaps out of the water to deliver the shock

When a threat is partly out of the water, an electric eel rises, presses its chin against it, and uses the lifted body as the live path of the circuit. The current no longer leaks away through the river.

An electric eel at rest is a fish. Annoyed, it is a battery of modified muscle that can throw pulses of several hundred volts. In water the current spreads, and a large animal standing in the same water gets a diluted share. Kenneth Catania showed what happens when part of the target is in the air. The eel rises along it, chin in contact, body clear of the surface, and the circuit runs from chin through the target and back to the tail still in the water. Less current is wasted on the pond. More of it goes through the thing being climbed. High-speed film and voltmeters recorded the leap and the jump in voltage across the target.

The experiment used a conductive arm and a fake predator, not a horse. Humboldt's horses are still an anecdote. What the anecdote got right is the behaviour, which looks like an attack and is a way of concentrating a shock that water would have spoiled. The eel is not aiming in the human sense. It is following a rule that puts more of its own voltage into whatever is touching the surface and moving. The rule is enough to make a fish leave the element it breathes in, on purpose, to finish a circuit.