Drought-driven lipid deficits in overwintering monarch butterflies revealed by δ²H and fatty acid analysis

Eastern populations of monarch butterflies (Danaus plexippus) overwinter in the central trans-volcanic belt of Mexico, relying primarily on lipids accumulated during migration to sustain this several-month-long overwintering period. This study examined how drought conditions in north-central Mexico influenced lipid dynamics in monarchs overwintering in Sierra Chincua (Michoacán, Mexico) across two consecutive seasons. Butterflies were collected in 2022-2023 (n = 150), following mildly dry conditions, and in 2023-2024 (n = 180), after an extreme drought affecting the entire Mexican migratory corridor. We measured abdominal mass, lipid mass fraction, lipid mass, fatty acid composition, and the δ2H values of non-essential palmitic and oleic acids as source indicators of dietary energy. In both years, non-essential fatty acids served as metabolic fuel, but all lipid metrics were significantly reduced in the drought year. Correlations between fatty acid δ2H values and content (palmitic = 0.35; oleic = 0.28; p < 0.05) suggested altered nectaring behavior and limited lipid accumulation at high-altitude sites near the overwintering colonies during extreme drought. Fatty acid δ2H values became increasingly negative while lipid levels converged between years by the end of overwintering, possibly due to in situ lipid synthesis or selective mortality. These results highlight the vital role of nectar resources along the Mexican migratory corridor and near overwintering sites in supporting lipid reserves crucial for the survival of overwintering monarchs. While our findings strongly indicate a connection between drought and reduced monarch lipid reserves, we recognize that additional efforts are needed to evaluate nectar quality and floral abundances more effectively to establish causality.