Early biologists, including Charles Darwin, were puzzled by what function the faint electric fields could serve, noted Peter Moller, an electric-fish expert at the City University of New York.
Now scientists know the fields are part of an "electric sense" used for both navigation and communication.
The unusual sensory skill helps the fish thrive in murky streams of west Africa.
"These fish are active at night, it's pitch black, but they behave as if the lights were on," Moller said. "It's like they have electric eyes."
External objects—including prey and other fish—cause distortions that are sensed by electrical receptors in the fish's skin, Moller explained.
Previous work has shown that each electric fish species has a unique form of electrical discharge, like a fingerprint.
(Related: "Sharks' Electro-Sensing Organs Linked to Human Features?" [February 14, 2006].)
In the new study, the researchers found that certain discharge patterns were associated with certain types of behavior.
Pulse sequences varied between the sexes and between breeding and nonbreeding periods, they found.
Males produced more "rasp" and "medium burst" patterns when they were near females and during courtship, suggesting that the signals "advertise" the males' quality as mates.
Females were found to create a "creak" pattern only during spawning, possibly to facilitate or signal the release of eggs.
After extended courtship interactions, the two sexes sometimes engaged in electrical "duets" of alternating rasps and bursts.
"Since both male and female electric fish generate signals, it is a natural for 'duetting,'" Hopkins said.
(Hear an amplified recording of the electric fishes' "duet".)
The new study appears in a recent edition of the Journal of Experimental Biology.
Truth in Advertising
By studying the buzzes of electric fish, researchers hope to gain new insights into the evolution of animals' breeding behavior and communication.
The new study suggests that electrical signaling may have been shaped in part by sexual selection, the evolutionary process responsible for many animals' elaborate decorative features and courtship behaviors.
Bruce Carlson of the University of Virginia was not part of the new study but has collaborated with Hopkins in the past.
"While visual and auditory [signals] have been well studied in the context of mating and courtship, the electrical [ones] have not," Carlson said.
In many species, he noted, males and females "read" each other's signals to evaluate the suitability of a potential mate or the fighting ability of a potential competitor.
Stronger male birds may sing louder and longer than others, for example, or show greater stamina in courtship displays.
"If a signal is costly to produce, then only high-quality, healthy individuals can afford to produce it," Carlson said.
In the electric fish, he continued, "the energetic cost of signaling is probably a critical factor."
"Only individuals that can afford to burn a lot of energy can continuously produce these burst displays, and the displays could thereby function as honest indicators of quality."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES