Sexy stripes. The same male X. nigrensis photographed under ambient light (top) and UV light (middle), and a composite image (bottom).

Swordtails' Sexy Secret

To lure sexual partners, animals often try to be as flashy as possible. But this often has the downside of attracting predators, too. Researchers now find that certain male fish solve the problem by wooing females with secret ultraviolet signals their major predators can't see.

Male northern swordtails attract females by virtue of their size and swordlike tail appendages. Unfortunately for them, their main predator, the Mexican tetra, Astyanax mexicanus, also eats up these traits. For more than 20 years, researchers have suggested that private messages could help animals like the swordtail show off for potential mates while avoiding unwelcome carnivorous attention. Still, virtually no direct evidence has supported this idea.

While northern swordtails tend to lack colors visible to humans, behavioral ecologists Molly Cummings and Michael Ryan of the University of Texas, Austin, and Gil Rosenthal of Boston University Marine Program in Woods Hole, Massachusetts, found that the fish have substantial diversity in ultraviolet (UV) markings. To see if the presence of UV signals mattered to female swordtails or Mexican tetras, the investigators put the fish in aquariums with illumination that mimicked natural underwater light conditions and filters that either blocked or permitted UV light.

Swordtail females of species Xiphophorus nigrensis spent almost twice as much time with males when UV light was let through, the team reports online 20 March in the Proceedings of the Royal Society. But Mexican tetras' attention to the males was the same with or without UV light. X. nigrensis males reflected significantly more UV than females, with their swords reflecting the most--further evidence that such coloration is used for sexual signaling.

On the other hand, males of a related species, X. malinche, had about as much UV ornamentation as females, who were no less attentive to males when UV light was blocked. When the researchers took UV photographs of five swordtail species, they found that UV ornamentation is most prominent in males of species that inhabit regions with lots of Mexican tetras. Rosenthal speculates that fish in risky environments have evolved a secure line of communication for attracting mates, allowing them to "sidestep the tradeoff between being sexy and being dinner."

This is the first direct evidence that private communication makes a difference for both mate choice and predator attacks, says evolutionary biologist John Endler of the University of California, Santa Barbara: "The paper is excellent."

Related sites
Rosenthal's site
Abstract of the paper
Images of northern swordtails