Essential and nonessential fatty acid composition and use in overwintering monarch butterflies.

Monarch butterflies (Danaus plexippus) undertake one of the most remarkable long-distance insect migrations, travelling thousands of kilometres to overwinter in the central trans-volcanic belt of Mexico. This study explored how monarch butterflies use essential fatty acids (EFA) and nonessential fatty acids (NFA) during overwintering. We collected 150 (male/female) butterflies from the Sierra Chincua wintering colony from the time of arrival (December 2022) to before departure (February 2023) and analysed their lipid content. Our findings revealed that although females have a higher mass fraction of lipids, male and female monarch butterflies depleted their lipids similarly over time, resulting in low abdominal lipid mass fractions by late February. NFA, including oleic and palmitic acid, were predominantly used for energy during overwintering by male and female butterflies. In contrast, the EFA alpha-linolenic and linoleic acids, critical for reproductive success and cellular functions, were conserved in both sexes. Males began the overwintering period with a higher mass fraction of EFA in the polar components of the head and thorax, which may impact the degree of cold acclimation of these tissues during this period. Strategic lipid utilisation, prioritising the preservation of EFA over NFA and optimizing overwintering survival probably enhance readiness for spring remigration and reproduction. This differential fatty acid use underscores the delicate balance monarch butterflies maintain to survive overwintering and highlights the potential impacts of environmental changes on their lipid dynamics and survival.