It takes a sparkle in the eye to attract a female mate, butterfly researchers have found.

While some male animals use bright colours or intricate mating dances to seduce females, U.S. biologists found one species of butterfly flaunts the reflective patches on their wings during courtship.

Kendra Robertson paints the eyespot on the wing of a male butterfly, shown in inset. (Courtesy: University of Buffalo)

Biology Prof. Antonia Monteiro of the University at Buffalo investigated the evolutionary role of eyespots in Bicyclus anynana butterflies.

Researchers suspect eyespots distract predators away from a butterfly's vital organs. The insects also have eye patterns on their inner dorsal wings, but their purpose is unknown.

Males flicker their wings during courtship to show off the eyespots, and females make the mating decisions for the species, the authors note in the June 29 online issue of the Proceedings of the Royal Society B: Biological Sciences.

Monteiro's team thought the pupils reflect ultraviolet light to catch a female's eye.

To test their idea, they painted the wings with UV-absorbing paint that did not change the colour of the limbs.

Painting the white pupil on 50 male dorsal wings made them less desirable to females by a ratio of two to one compared to untreated males, the team found.

Changes to the ventral side appeared to make no difference to the females' mating decisions.

"This is one of the first studies to show that such a small pattern element really matters in female choice," Monteiro said in a release.

It's thought that the males flutter their wings to stimulate females visually and to spread pheromones to her antennae. It probably looks like a strobe-like effect to her, said study co-author Kendra Robertson.

Male butterflies may lose reflectivity over time as the scales on their wings fall off. If so, sparkly wings may signal younger, healthier mates, the team said.