Variation in thermally induced melanism in monarch butterflies (Lepidoptera:Nymphalidae) from three North American populations

1 As ectotherms, insects often experience varying temperatures throughout their life cycle, and some respond by becoming more or less melanistic (dark coloring) during development to increase or decrease thermal energy absorption as larvae or adults. 2. Monarch butterflies (Danaus plexippus) breed in temperate and tropical environments worldwide and are exposed to different average and extreme temperatures in different parts of their geographic range. In this study, we compared variation in thermally induced melanism among monarch butterflies from eastern and western North America and from South Florida. 3. We raised the progeny of wild-captured adult butterflies from these populations in a common garden experiment, rearing individuals in cold (19 degrees C), moderate (26 degrees C), and hot (32 degrees C) temperatures to examine population variation in larval and adult pigmentation. 4. Across all populations, monarch larvae developed the darkest coloration in the cold treatment and were lightest when reared in hot temperatures. Similar results were observed for measures of adult wing melanism, with the exception of adult females, which developed darker colored wings in warmer temperatures. 5. Significant population-level differences in average measures of melanism among larvae and adult butterflies were observed. Larvae from the eastern population became substantially darker in colder temperatures than S. Florida or western larvae. Western larvae were lightest overall, which might be adaptive to high temperatures experienced throughout portions of their summer breeding range. S. Florida larvae showed a lower response to cold temperatures relative to monarchs from either migratory population. 6. Population level differences were also observed for thermal responses in wing melanism, particularly among adult females. Moreover, we found significant family level effects for each measure of larval and adult melanism, pointing to a genetic basis or strong maternal effects influencing these traits in monarch butterflies. (c) 2005 Elsevier Ltd. All rights reserved.