Drought‐driven lipid deficits in overwintering monarch butterflies revealed by δ <scp> ² H </scp> and fatty acid analysis

Abstract 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 shows 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. Abdominal mass, lipid mass fraction, total lipid mass, and fatty acid composition were compared between both years. The δ 2 H values of nonessential palmitic and oleic fatty acids were evaluated as source indicators of dietary energy. In both years, nonessential fatty acids served as metabolic fuel, but all lipid metrics were significantly reduced in the drought year. Lower lipid mass fractions were found in monarchs that had more 2 H‐depleted isotopic values (palmitic acid = 0.35; oleic acid = 0.28; p &lt; 0.05), suggesting altered nectaring behavior and limited lipid accumulation on 2 H‐depleted nectar at high‐altitude sites near the overwintering colonies during extreme drought. Fatty acid δ 2 H values became increasingly negative while lipid mass fractions converged between years by the end of overwintering, possibly due to in situ lipid synthesis or selective mortality. These findings demonstrate that drought‐related nectar scarcity likely constrains monarch lipid accumulation, warranting future quantification of nectar quality and floral abundance in north‐central Mexico.