Alternate migration strategies of eastern monarch butterflies revealed by stable isotopes

Abstract

Abstract Alternative life history strategies are mechanisms by which organisms are able to maximize fitness across a range of environmental conditions. Fitness is maximized by different strategies depending on context, resulting in trade-offs between life history strategies. Monarch butterflies (Danaus plexippus) employ both migratory and resident life history strategies. Since residents breed throughout the year, but migrants overwinter in reproductive diapause, there are fitness trade-offs between the two strategies. We used stable isotope analysis to evaluate the geographic origins of monarchs in a yearround population in south Florida. Based on stable isotope profiles of hydrogen and carbon (δ2H and δ13C values), we found that 48% (16/33) of monarchs collected in south Florida are migrants that originated from outside the sampling region. Migrants had a larger wing length than residents; thus, switching to a resident strategy could alter their probability of reproductive success. Further work is needed to investigate the mechanism underlying this pattern, but these findings show that alternate life history strategies and sex-specific behaviors are underexplored factors influencing monarch migration and evolution.