Guys roll eyes: fish show some eyeball to their rivals.

Male fish in the Colorado River roll their eyes to flash a novel "Back off, punk" signal at other males, researchers say.

The razorback suckers' gesture--dipping the eyeball to expose its upper third--ranks as the first documented eye roll among territorial signals, says vision specialist Inigo Novales Flamarique of Simon Fraser University in Burnaby, British Columbia. What makes the communication even more unusual, he says, is that it gets much of its punch from ultraviolet (UV) light.

People don't see UV wavelengths, but biologists have in recent years found that certain fish, birds, and other animals do.

Flamarique had puzzled over the UV vision in a razorback relative, a white sucker that spends most of its life in water too deep for the UV wavelengths of sunlight to reach. Then Flamarique got an e-mail from Gordon Mueller of the U.S. Geological Survey in Denver that described another deep-dwelling fish, the razorback sucker (Xyrauchen texanus), that swims to the shallows to breed. Mueller had noticed bright flashes from the eyes of males staking out breeding territories.

Flamarique says, "All of a sudden, I connected the dots" He hypothesized that those flashes included a UV signal that fish detected in shallow waters.

In lab tests, Flamarique and his colleagues confirmed that razorbacks have UV receptors. They're located where the retina receives light from below and would pick up a signal as one fish swam above another.

During razorback's breeding season, the researchers visited one of the few remaining populations in the Colorado River in Arizona. Flamarique and a colleague anesthetized seven razorbacks, moved their eyeballs to the rolled position, and dipped them in the Colorado River.

The rolled eyeballs reflect sunlight brilliantly, Flamarique reports. Compared with the rest of the reflection's spectrum, the UV part offers the strongest contrast to surrounding light and so would make a dramatic signal.

To see how fish react to glinting eyeballs, the researchers set a fake fish with LED eyes on each side of a big tank and loosed razorbacks in the middle. Male fish shied away from whichever model flashed its eyes.

Because female fish didn't noticeably react, eye flashes probably warn off intruding males, Flamarique, Mueller, and their colleagues say in an article now online for the Proceedings of the Royal Society B.

Yellow-bellied slider turtles also roll their eyes, but as a different kind of communication, says Jeffrey E. Lovich of the U.S. Geological Survey in Flagstaff, Ariz. He's seen eye rolling only in willing females being courted.

For creatures in water shallow enough for UV light to penetrate, those wavelengths seem useful as intimate signals, notes Molly Cummings of the University of Texas at Austin. She has documented UV-reflecting decorations that boost the sex appeal of fish. Because those decorations and the eyeball rolls aren't visible far away underwater, they can signal a nearby fish without alerting more-distant predators.