The discovery was reported in an article in the May 1 Nature by Duke University biologists Alison Sweeney and Sönke Johnson, and Christopher Jiggins of the University of Edinburgh in Scotland. The research was sponsored by the National Science Foundation.
The initial mystery that attracted her, said 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 said. "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 offered another clue to the mystery, said Sweeney. "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." Perhaps, theorized Sweeney, such deep-forest species might find polarization a useful signal in an environment where confusing 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, Sweeney 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 wings that displayed polarized light significantly more than those wings that could 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 said, the butterfly is not the only animal species known to produce polarized light. Cuttlefish also emit polarized light patterns from their arms, although researchers have not yet puzzled out the role of polarization in the complex communication among those animals.
More broadly, said Sweeney, the discovery of polarization as a mating signal represents another piece of the puzzle of why tropical butterflies are able to maintain such a vast diversity of species.
"For example, there are many species within the genus Heliconius, and one reason they are able to maintain such diversity is that there are many signals -- now we know including patterns of polarization of light -- that help each species distinguish its own in mating," said Sweeney.