Friday, June 10, 2016

[Ichthyology / Behaviour • 2016] Leaping Eels Electrify Threats, Supporting Humboldt’s Account of A Battle with Horses

Fig. 1. Fishing with horses. (A) This illustration depicts the battle between eels and horses observed by Alexander von Humboldt in March 1800. It was published in 1843 as the frontispiece for The Naturalist Library, Ichthyology, Volume V, Part II, the Fishes of Guiana, authored by Robert H. Schomburgk, a friend and protégé of Humboldt’s (3).
(B) Example of an eel leaping from the water to shock a simulated predator. LEDs are powered by the eel through a conductive carbon strip taped to the front of the plastic prop.

Electric eels are shown to leap from the water to directly electrify threats. This shocking behavior likely allows electric eels to defend themselves during the Amazonian dry season, when they may be found in small pools and in danger of predation. The results support Alexander von Humboldt’s story of electric eels attacking horses that had been herded into a muddy pool during the dry season in 1800. The finding highlights sophisticated behaviors that have evolved in concert with the eel’s powerful electrical organs.

In March 1800, Alexander von Humboldt observed the extraordinary spectacle of native fisherman collecting electric eels (Electrophorus electricus) by “fishing with horses” [von Humboldt A (1807) Ann Phys 25:34–43]. The strategy was to herd horses into a pool containing electric eels, provoking the eels to attack by pressing themselves against the horses while discharging. Once the eels were exhausted, they could be safely collected. This legendary tale of South American adventures helped propel Humboldt to fame and has been recounted and illustrated in many publications, but subsequent investigators have been skeptical, and no similar eel behavior has been reported in more than 200 years. Here I report a defensive eel behavior that supports Humboldt’s account. The behavior consists of an approach and leap out of the water during which the eel presses its chin against a threatening conductor while discharging high-voltage volleys. The effect is to short-circuit the electric organ through the threat, with increasing power diverted to the threat as the eel attains greater height during the leap. Measurement of voltages and current during the behavior, and assessment of the equivalent circuit, reveal the effectiveness of the behavior and the basis for its natural selection.

Keywords: evolution; behavior; Humboldt; electroreception; neuroethology

Kenneth C. Catania. 2016. Leaping Eels Electrify Threats, Supporting Humboldt’s Account of A Battle with Horses. PNAS. DOI: 10.1073/pnas.1604009113

Jumping electric eels pack more zap via @YouTube
Electric eels make leaping attacks: Research confirms 200-year-old story by Alexander von Humboldt