Researcher Molly Cummings and her coworkers at the University of Texas at Austin placed a female swordtail in a tank with two males visible at either end. One of the males, however, was surrounded by filtering glass that blocks UV light. They found that the female was twice as likely to demonstrate interest in the male whose UV flashers were visible to her. Twice as likely! That’s a big finding. Put a Mexican tetra in the middle tank–that’s a predator who snacks on swordtails–and you get a different result. The tetra shows equal interest in getting at both swordtails. Like humans, tetras are unable to register UV light. Interpretation? Male swordtails have developed a subtle way of advertising sexual readiness to females of their own species… while avoiding courting disaster.