The topics of “lies” and “size” and “lying about your size” sound like a recap of the latest presidential debate, but they actually come from new research on species of South American fish that send out deceiving electrical impulses to trick predators into thinking they’re much smaller than they really are. Does it have anything to do with the size of their hands fins?

Philip Stoddard is a biologist at Florida International University who does research on electric fish – fish that generate electrical fields using a special internal organ that discharges signals of varying strengths with some strong enough to kill other creatures. These are different than electroreceptive fish, which are able to detect electric fields (sharks, for example). Both play a role in Stoddard’s research on the South American freshwater fish known as the feathertail knife fish (Brachyhypopomus pinnicaudatus).

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Feathertail knife fish close-up

Raised in tanks on the roof of Biological Sciences building to protect them from foxes, the small (8 in/20 cm) nocturnal fish generate weak pulses of electricity that Stoddard broadcasts on loudspeakers – they sound like a motorcycle engine. He expected larger fish to have louder signals and vice versa. What he heard was astonishing.

After fattening up one test group on worms while restricting the diet of another, he discovered that some fat fish sent weaker signals while some thin fish sent louder impulses. At an upcoming conference, Stoddard will present his conclusion that the fatter fish are trying to fool predators by saying “not much to eat here” while the skinny fish are saying “I’m bigger than you are.” This deception is also used to look more attractive to mates (keep that in mind if a fish contacts you on

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Handy tool for female feathertail knife fish

Stoddard found that the deception works with predators but not so well with female feathertail knife fish (who can also hear the signals, meaning the species is both electric and electroreceptive). In tests in a darkened aquarium, he found that females didn’t always believe the false signals from the skinny male fish. How they make this determination is part of Stoddard’s ongoing research.

Perhaps the best way to watch a presidential debate is with the help of a tank full of female feathertail knife fish.

Paul Seaburn

Paul Seaburn is the editor at Mysterious Universe and its most prolific writer. He’s written for TV shows such as "The Tonight Show", "Politically Incorrect" and an award-winning children’s program. He's been published in “The New York Times" and "Huffington Post” and has co-authored numerous collections of trivia, puzzles and humor. His “What in the World!” podcast is a fun look at the latest weird and paranormal news, strange sports stories and odd trivia. Paul likes to add a bit of humor to each MU post he crafts. After all, the mysterious doesn't always have to be serious.

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