DOUGLAS, J.M.*; DOMINY, N.J.; University of Chicago; University of Chicago: Light Habitats and the Functional Ecology of Polarized Iridescence in Neotropical Butterflies

Sweeney et al. (2003; Nature 423:31-32) describe the use of polarized light as a mating signal in Heliconius cydno butterflies. In addition, they hypothesized that exploitation of polarized light to detect mates may have adaptive value in forest habitats, where illumination varies greatly in spectrum and intensity. Here we investigate the extent to which neotropical lepidoptera exhibit polarized iridescence, and evaluate the types of habitats in which the trait is found. We examined species belonging to the families Nymphalidae, Papilionidae, and Pieridae found within Costa Rica. Polarization was determined from museum specimens with a polarizing filter and a light source that simulates the spectrum of ambient sunlight. Wing surfaces were positioned with an angle of incidence of approximately 90-degrees. Of the 178 species examined, 68 species exhibited prominent or modest polarization. Adult flight habits and natural environments were obtained from the literature (DeVries 1987; The Butterflies of Costa Rica and Their Natural History, Princeton University Press). Taxa associated with open, canopy, disturbed, and/or secondary growth areas were categorized as open habitat species. Taxa associated with forest and riparian edges, light gaps, and trails were categorized as forest species. Forest species were significantly more likely to exhibit polarized iridescence than open habitat species (chi-square = 32.4; d.f. = 1; P < 0.001). This result is consistent with the hypothesis that the utilization and detection of polarized light has adaptive ecological value in forest habitats having complex light conditions. Further analysis of the degree of polarization, spectra of polarization, and the importance of differing spectral environments on wing color will be discussed.