Butterflies 'Flash' for Winged Dates

Eyes on the surprise: Sweeney with Heliconius Jim Wallace.

Researchers have discovered that some iridescent butterflies use the polarization of light refracted from their intricate, prismatic scales to lure mates--the first time that light polarization has been identified as a mating signal for any terrestrial animal species. The discovery was reported in an article in Nature by Duke biologists Alison Sweeney and Sonke Johnson and Christopher Jiggins of the University of Edinburgh in Scotland.

The initial mystery that attracted her, says graduate student Sweeney, was why butterflies possessed such intricate iridescent structures on their wings. "When you see a butterfly collection, you're struck by how they can make every color imaginable by having pigments in the cells of their wings," she says. "But if you can make any color you want, why would you want to build these elaborate intricate light-refracting scales, and why would they be important in an evolutionary context?"

The butterfly's visual system also contributed to the mystery, she says. "Butterflies are known to have a very sophisticated color vision system, which also includes an ability that vertebrates don't have--to see the direction in which the electric field of a beam of light is oscillating." This direction of oscillation is called the polarization of light.

" One of the interesting things I noticed when I was studying this question is that all the iridescent forms seemed to be deep-forest species. And so I thought, Aha! Maybe there is something about being in a deep forest where being iridescent is going to be important," Sweeney says.

Sweeney's theory is that deep-forest species might find polarization a useful signal in an environment where shifting light and shadow might make pattern recognition more difficult. Jiggins, working at the Smithsonian Tropical Research Institute in Panama, had already demonstrated that male butterflies recognize color patterns in seeking mates. So Sweeney decided to explore whether polarization of light reflected from the wings might also provide a mating signal.

In her experiments, she presented male butterflies of the iridescent genus Heliconius with female wings of the same species, but covered by one of two kinds of filters. One filter depolarized light reflected from the wings, and the other did not. She found that males approached the females whose wings displayed polarized light significantly more often than those whose wings did not.

According to Sweeney, while her experiments demonstrate that polarization is important in many iridescent species, not all iridescent species produce polarized reflected light. Also, she says, the butterfly is not the only animal species known to produce polarized light. Cuttlefish also emit such light patterns from their arms, although researchers have yet to puzzle out the role of polarization in the complex communication among those animals.

More broadly, says Sweeney, the discovery of polarization as a mating signal represents another clue to why tropical butterflies are able to maintain such a vast diversity of species.