Whispering sweet nothings to your loved one is not half so much fun if you think that someone else might be listening. In the same way, the threat of eavesdropping predators is thought to act as a brake on the further elaboration of the often complex signals that animals use to woo their mates. In the 15 July issue of Nature, however, Philip K. Stoddard of Florida International University, Miami, Florida describes an example of the converse - of signals that have become more complex, not less, under threat of predation.
Stoddard has been looking at life, death and sex among the so-called gymnotiform ?knife?-fishes of tropical South and Central America. These fishes - of which there are more than 100 species - are nocturnal, and tend to favour rather murky water. Lack of visibility is no barrier to communication, because these fishes can send and receive weak electrical signals: far from dim and dark, the world of the knife-fish is a Las Vegas of coruscating electrical fields. Happy-go-lucky knife-fish swains chirp out simple-but-amorous, low-frequency pulses of electricity. Lady knife-fishes find these pulses irresistible, but so too does the ravenous electric eel (Electrophorus electricus), not to mention a range of large, hungry catfishes. These fishy predators - close relatives of knife-fishes - can produce and detect electrical pulses in the same low-frequency range used by knife-fishes in search of a mate.
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