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.