Death risk drives shocking love songs.

The risk of getting killed often has a dampening effect on the silly excesses of courtship, but the opposite may have been true in electric fish.

The need to evade predators could have driven the electric serenades of knife fish to evolve from a simple zap ... zap ... zap to a variety of zippity doo-dahs, according to an analysis by Philip K. Stoddard of Florida International University in Miami. In the July 15 NATURE, he reports that a predator can't detect the complex signals as easily as the simple ones.

"Predators really have been a creative force," he says.

Knife fish mostly cruise murky water at night and emit weak signals to communicate and scan for obstructions. More than 100 species, which include predatory electric eels, roam the fresh waters of Central and South America. Another group, in Africa, evolved electrical signals independently.

Stoddard argues that ancestors of New World knife fish made a one-phase signal. He draws it as a single peak. Many modern fish have gone multiphasic, adding foothills and sunken gorges before and after that main peak.

At first, Stoddard suspected that female fancy had inspired complex signals, just as it sent male peacocks to fashion excess in tail feathers. Males and females typically differ in such exaggerated traits, Stoddard says, yet he saw fewer sex differences in fish than expected.

Electric eels and catfish home in on electric signals when they hunt small knife fish. To assess a typical predator's capabilities, Stoddard trained an eel named Sparky to swim toward electric signals. Sparky responded to one-blip signals about twice as often as he did to multiphasic signals. Predators don't seem daunted by electrified mouthfuls. As Stoddard points out, "We eat wasabi on sushi."

Some modern knife fish species still send risky one-blip signals. The electric eels do, but have little to fear. They kick out about 100 volts per foot of body length. One species of small knife fish seems to have lost its signal complexity, but it lives far from eels and catfish. A third one-blip species may be masquerading as an eel.

Prevailing wisdom decrees that predators limit flashy, sexy traits in their prey, comments Marlene Zuk of the University of California, Riverside. Guppies in waters beset with predators, for example, sport colors less conspicuous than neon orange. However, after writing a review article exploring the dampening effect of predators, Zuk says, "There's very little evidence for it." Stoddard's claim that predators had the opposite effect does not surprise her.

Her own work focuses on the serenades of male crickets in places where flies track the songs, land on the singer, and give birth to larvae that burrow into the cricket's body. Crickets in fly-infested areas sound different from crickets in fly-free zones, but Zuk hesitates to rank one's sound as more complex.

Sounds and colors have gotten most of the attention, according to knife fish specialist Carl D. Hopkins of Cornell University. He looks forward to seeing if theories hold up in the electric realm. "The electric fish [research] has a lot to offer because it's so new," he says.