Some male animals resort to bright colors or impressive physical displays to win over the ladies, but a new study shows that a simple sparkle can do the trick. Male Bicyclus anynana butterflies exhibit small reflective patches on their wings that females find irresistible.

The wings of Bicyclus anynana butterflies contain spots that resemble eyes, each with a central white "pupil." Previous studies have suggested that the eyespots confuse predators, diverting attention away from the butterfly's vital organs. However, the insects also display the eyes on their inner dorsal wings, which are less visible to predators. "We were always puzzled, wondering what ... are they there for?" says Ant?ia Monteiro, a biologist at the University of New York at Buffalo.

To investigate the function of the spots, Monteiro's team, led by graduate student Kendra Robertson, collected more than 1700 butterflies from the lab's colony. The researchers found that females tended to flock to males who had white pupils within their dorsal eyespots, which are exposed by wing flickering during courtship. "I was so shocked that they were selectively interested in this one tiny pupil when there are so many other intricate designs," says Robertson.

Because the pupils are composed of a thin scaly layer of cells that reflect ultraviolet (UV) light, the team wondered if the UV sparkle lured the females. To test this, the researchers dabbed the pupils of over 50 males with a UV-absorbing pigment that did not change their wing coloration. When introduced to treated and untreated males, female butterflies were twice as likely to mate with males that reflected regular levels of UV.

Males with more UV reflectivity probably look younger and healthier and are thus a bigger draw for females, says Robertson. This may be because butterflies lose their reflectivity over time as the scales on the wings get chipped away. The team will report its findings online 6 July in Proceedings of the Royal Society B: Biological Sciences.

"The results are convincing, and the outcome is amazing," says Ronald Rutowski, a biologist at Arizona State University in Tempe. But one wrinkle, he points out, is that the amount of UV light available in the environment can change from one habitat to the next. "It would be interesting to see how quality of light affects these experiments," he says.

--CATHY TRAN

Related sites
Monteiro Lab site, including more pictures of Bicyclus
More about Butterfly Wing Patterns