Hot temperature during development alters iridescence in Morpho butterflies

Environmental temperature during critical stages of development can define the phenotype of an individual, especially for ectotherms. Color is a particularly biologically important phenotypically plastic trait but has been primarily studied in the context of pigmentary, rather than structural, coloration. In Morpho butterflies (Nymphalidae), structural coloration created by nanostructures on the wing scales leads to their striking blue iridescent effect. We reared Morpho helenor larvae and exposed the pupae to three temperature regimes during development: control tropics, temperate cool, and hot house, with hotter temperatures simulating those predicted under a future climate change scenario. We hypothesized that the hot house treatment would be the most disruptive to iridescence. In support of this hypothesis, hot conditions reduced the brightness of adult butterfly wings and altered the color profile enough that they would look different from control butterflies to bird predators and conspecifics. For certain bird perceivers, hot conditions would obliterate the iridescent effect entirely. The mechanism underlying these shifts appears to be, at least in part, driven by a decrease in the gap between nanostructures on the wing scale and consequent narrowing of the entire scale. Our study highlights the ways in which structural color is alternately vulnerable or robust to environmental temperature during development. From this work, we can glean insight into the environments that would be most supportive of iridescence in tropical butterflies and make predictions about the effects that changing climate regimes will have on the diversity of some of the earth's most dazzling organisms.