Heliconius cydno (top) polarizes reflected light. H.melpomene malleti (bottom) doesn't. Credit: source:

It's not just the colour of butterflies' shimmering wings that attracts mates, suggests a new study. The polarized light that bounces off the wings of some females lures males. This is the first evidence of a land creature reacting to light in this way.

The scales on butterfly wings were known reflect light waves in only one plane - but no one knew why. The latest experiments on the butterfly Heliconius cydno, which is common in tropical rainforests in Central and South America, "put a whole new perspective on butterfly-wing evolution", says team member Alison Sweeney of Duke University in Durham, North Carolina.

One possibility, she argues, is that iridescent wing colours stand out in environments where there is a lot of unpolarized light, such as the jungle. Many shade-dwelling butterflies, like H. cydno, have iridescent colours. Often meadow-dwellers do not.

Tom Cronin, who works on polarizing signalling in cuttlefish and shrimp at University of Maryland, Baltimore County, says that the butterfly finding hints that more animals use polarization for communication than we might think. He points out that several marine species also use polarized light to find partners.

Sweeney's group worked in an enclosure. They pinned female butterfly wings to a board, in front of which they placed neutral or depolarizing filters. They then released five male butterflies nearby and counted how many times these flew towards the female wings.

The males approached more often when the female wings were under the neutral filter, which let their natural effect on light waves shine through. A display viewed through a depolarizing filter seemed to leave the males cold.

Butterflies' ability to detect polarized light could have other functions - they could, for example, use it for migration as some birds do. Sweeney and her colleagues now plan to test their hypothesis by comparing butterflies from different habitats.