Fish allurement that predators don't see. (Secret Signal).

In a rare demonstration of secret messaging in animals, a swordtail fish uses ultraviolet (UV) wavelengths as a private courtship channel, biologists now report.

Males sport bold UV-reflecting horizontal stripes that attract feminine interest, says Molly E. Cummings of the University of Texas at Austin. She and her colleagues also found that the fish's main predator doesn't see this UV finery. Males, therefore, can court conspicuously without increasing the danger of becoming somebody's dinner, Cummings and her colleagues report in an upcoming issue of the Proceedings of the Royal Society of London B.

"We are the first group to provide direct behavioral evidence of a private visual communications channel," Cummings says.

Studies in the 1980s showed that if researchers stocked artificial streams with aggressive predators, populations of guppies there shifted during 14 generations to subdued coloration. In comparable setups with less-threatening predators, however, flashy, golden spots became prominent in guppy populations.

Cummings and her colleagues discovered that male Xiphophorus nigrensis swordtails sport sexy, UV-reflecting stripes. Cummings says this probably explains why some 14 years of earlier experiments by her Austin collaborator Michael J. Ryan and his associates hadn't yielded clear indications of female preferences for swords. The old setups inadvertently blocked UV signals, she says.

In new tests, she and her colleagues placed a female in a tank with a male visible in a compartment at each end. A filter blocked the UV wavelengths from the markings on one of the males. The male revealed in full UV glory was twice as likely to attract the female as the filter-blocked male was, the researchers report.

The researchers used the same setup--and the same males--to test preferences of swordtail-hunting Mexican tetras. These predatory fish, however, didn't pay extra attention to UV-bedecked males. Cummings also reports that the eye lenses of the predators block UV light.

Xiphophorus malinche, the only close relative of X. nigrensis that lives far from Mexican tetras, doesn't seem to bother with UV signaling, Cummings reports.

This hint that UV signaling imposes demands on the fish interests Philip Stoddard of Florida International University in Miami, who studies courtship signals of electric fish and how predators eavesdrop on them (SN: 7/17/99, p. 37). "If UV's so great, why doesn't everybody do it?" he asks. He speculates that maintaining sensitivity to such signals may hamper long-distance vision.

John Endler of the University of California, Santa Barbara, who pioneered the guppy studies, sighs over "this huge brouhaha over UV signals." He laments that people, with their feeble human vision, have been slow to take seriously the UV signals of so many other creatures. Still, he welcomes the new work as "a nice, clean experiment."