Do North American monarch butterflies travel to Cuba? - Stable isotope and chemical tracer techniques

Since the discovery of monarch butterfly (Danaus plexippus) overwintering colonies in Mexico in the 1970s, it was assumed that monarchs from eastern North America migrated only to Mexico. This paper reveals that monarchs from Canada and the east coast of the United States also regularly travel to Cuba during the migration period. The natal grounds of Cuban monarchs were determined through the combined use of stable hydrogen (deltaD) and stable carbon (delta(13)C) isotope measurements and by cardenolide fingerprint analysis using thin-layer chromatography (TLC). The TLC data revealed that there was an influx of migrants in November to Cuba, and the stable isotope data revealed that migrant Cuban monarchs originated from southeastern Canada and the eastern United States. Our findings suggest that North American migrant monarchs that move to Cuba hybridize with resident populations there and do not return to the continent. The differences in the natal grounds, migratory route, and reproductive stages between monarchs wintering in Mexico and Cuba suggest that there are at least two subpopulations of eastern North American monarchs. The extent to which Cuba may act as a bridge for monarch movement to the Yucatan and other Caribbean islands and the genetic impact of this newly revealed flux in monarch movements remain to be determined.