Electric eels

On a field trip to the Amazon in 1807, 19th-century explorer Alexander von Humboldt witnessed a group of horses led through a muddy pool filled with electric eels, which he described as dramatically leaping up to attack the intruders. But scientists have doubted the story. An illustration of Alexander von Humboldt's story of the battle between the horses and electric eels. "The first time I read von Humboldt's tale, I thought it was completely bizarre," Catania says. "Why would the eels attack the horses instead of swimming away?" But then he observed the same behaviour by accident as he transferred the eels in his lab from one tank to another using a metal-rimmed net. Instead of swimming away, larger eels attacked the net by leaping out of the water. Catania tracked the strength of the eels' electric shock by attaching a voltmeter to an aluminium plate, or conductive metal strips to "predator" objects such as a crocodile head replica. The zap a submerged eel distributes through the water is relatively weak when it reaches the target.