Migration of Eastern North American monarch butterflies via the South-east and the Atlantic: evidence from stable isotopes, thin layer chromatography, DNA and phenotype.

This article investigates the existence of two lesser-known migratory routes of the eastern North American monarch butterfly (Danaus plexippus plexippus), termed the south-eastern and Atlantic routes, which diverge from the well-documented migration to Mexico. Using multiple methods—including stable hydrogen isotope (δ²H) analysis, thin layer chromatography (TLC) of larval host plant chemical fingerprints, and DNA sequencing—the study traces monarch natal origins and migratory pathways through Florida, Cuba, the Yucatán Peninsula, Guatemala, and northern South America. Results indicate that monarchs following these routes originate primarily from the south-eastern and Atlantic regions of the USA, feed on distinct Asclepias (milkweed) species different from those used by Mexican migrants, and exhibit unique phenotypic and genetic signatures. The findings suggest these routes are established migratory pathways with specific timing, destinations, and behaviors, rather than aberrant or dispersal movements, and that some monarchs return to North America via Florida, highlighting a more complex migration system than previously recognized.