A Moment of Science

Ultraviolet Fish

Have swordtail fish mastered the act of dating? Find out on this Moment of Science.

So you want to attract a mate. After all, passing on your genes is what evolution is all about. To do that you need offspring, and to get a lot of those you need to be attractive. The flashier the better. Hey, look over here! Sexy me.

Ah, but there’s the problem. The flashier you are, the more likely you are to attract predators. Hey, look over there–that guy really stands out. I’ll eat him.

This is a pickle. How can you be flashy in the eyes of mates but subdued in the eyes of predators?

New studies show that swordtail fish have evolved a clever solution: use ultraviolet light. These innocuous little critters actually shine brightly in stripes that run along their flanks… but only if you look in the UV wavelength.

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.

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