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 zero
very few had medium or high lipid levels. This suggests that flower-visiting monarchs either maintained their lipid reserves at low levels by visiting flowers, but were unable to reach levels found in clustered monarchs, or that, as the flower-visiting monarchs died, clustered monarchs with low lipid reserves departed from their roosting trees to visit flowers. Migrating monarchs collected in April in the southern United Stales had significantly higher Lipid masses than did flower-visiting monarchs collected in March at the overwintering site. Furthermore, migrating monarchs arrived in the southern United States with <50% of the lipid mass found in clustered, overwintering monarchs in March. Due to this apparent cost of migration, we hypothesize that the cohorts of monarchs visiting flowers at the overwintering site may not be able to migrate successfully to breeding areas in the southern United States. Our data do not support the hypothesis that there is a need to create open areas in the core zones of the MBSBR to promote flower production for monarchs overwintering in Mexico. To the contrary, the pattern of lipid loss observed in monarchs clustered in shaded areas is consistent with the hypothesis that intact, closed forest is necessary for successful overwintering because it permits monarchs to conserve their lipid reserves for the spring migration.
