Hot Temperatures During Development Dull Blue Morpho Butterflies' Iridescent Wings

The iridescent blue color of morpho butterflies does not come from pigments but from tiny ridged nanostructures on their wing scales that reflect light, much like tiles overlapping on a roof. In an experiment described in the journal PNAS Nexus, researchers at the Smithsonian Tropical Research Institute (STRI) documented for the first time that hot temperatures during development can reduce the brightness of this iridescence.

Scientists exposed Morpho helenor pupae, the stage of metamorphosis before adulthood, to three temperature categories: average tropical temperatures in which the butterflies normally live, temperate-zone temperatures, and hot temperatures such as those projected under future climate-warming scenarios. Consistent with other studies, temperate and hot temperatures extended the time butterflies spent as pupae, and pupae died more often at hot temperatures.

Adult morpho butterflies whose pupae were exposed to hot temperatures were duller, and the researchers found that common bird predators and butterflies of the same species could detect the resulting color change. "The scales on these butterflies are like tiles overlapping on a roof," said lead author and STRI fellow Juliette Rubin. "The change in color we found in adults after exposing pupae to hot temperatures can be partly explained by the distance between nanostructures shrinking, which narrows the entire scale. This probably caused less overlapping between the scales."

The effects of this color change on predators or potential mates are still unknown; because iridescence might confuse predators, duller or altered butterflies might be more vulnerable to predation or less attractive to mates. "Under a warming climate, the color of some of our favorite iridescent animals can be altered, and the brightness of their iridescent display can become duller," Rubin said, adding that the study would not have been possible without STRI's facilities located within the butterflies' natural tropical range.