Use of lipid reserves by monarch butterflies overwintering in Mexico: Implications for conservation

During their 5-mo overwintering period in Mexico, tens of millions of monarch butterflies (Danaus plexippus) form dense aggregations in forests dominated by oyamel fir trees (Abies religiosa). These forests provide a cool, moist environment that most monarchs use to maintain a state of reproductive diapause and to remain largely inactive until March, when they migrate back to the southern United States. In 1986, the Mexican government created the Monarch Butterfly Special Biosphere Reserve (MBSBR), but it is under pressure to allow forest extractions from core areas of the reserve. A recent argument to justify logging maintains that tree extraction would benefit monarchs by creating forest openings in which more plants would flower. The increased availability of nectar might mean that fewer monarchs would deplete their lipid contents, and therefore, more monarchs would survive the overwintering period. We investigated this hypothesis by comparing, throughout the overwintering period, lipid utilization and three other physical characteristics of monarch butterflies that were clustered on trees vs. monarchs that were collected while they visited flowers. Comparisons were also made with autumn migrants collected in Texas, successful spring migrants collected in the southern United States, and reproductively active summer generations collected in Wisconsin and Minnesota. We also examined changes in lipid mass during the annual cycle of eastern North American monarch populations. We found that clustered butterflies had significantly higher lipid mass, water content, lean mass, and larger wings than did monarchs collected from flowers. These differences were consistent throughout the overwintering period. Clustered monarchs consumed their lipid reserves passively in relation to the ambient temperature, as would be expected based on their resting metabolic rate. In contrast, a high proportion of flower-visiting monarchs had lipid masses close to zer