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 fore sts dominated by oyamel fir trees (Abies religiosa). These forests pro vide a cool, moist environment that most monarchs use to maintain a st ate of reproductive diapause and to remain largely inactive until Marc h, when they migrate back to the southern United States. In 1986, the Mexican government created the Monarch Butterfly Special Biosphere Res erve (MBSBR), but it is under pressure to allow forest extractions fro m core areas of the reserve. A recent argument to justify logging main tains that tree extraction would benefit monarchs by creating forest o penings in which more plants would flower. The increased availability of nectar might mean that fewer monarchs would deplete their lipid con tents, and therefore, more monarchs would survive the overwintering pe riod. We investigated this hypothesis by comparing, throughout the ove rwintering period, lipid utilization and three other physical characte ristics of monarch butterflies that were clustered on trees vs. monarc hs that were collected while they visited flowers. Comparisons were al so made with autumn migrants collected in Texas, successful spring mig rants collected in the southern United States, and reproductively acti ve summer generations collected in Wisconsin and Minnesota. We also ex amined 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 co nsumed their lipid reserves passively in relation to the ambient tempe rature, as would be expected based on their resting metabolic rate. In contrast, a high proportion of flower-visiting monarchs had lipid mas ses close to zero; very few had medium or high lipid levels. This sugg ests that flower-visiting monarchs either maintained their lipid reser ves 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 t han did flower-visiting monarchs collected in March at the overwinteri ng site. Furthermore, migrating monarchs arrived in the southern Unite d States with <50% of the lipid mass found in clustered, overwintering monarchs in March. Due to this apparent cost of migration, we hypothe size that the cohorts of monarchs visiting flowers at the overwinterin g site may not be able to migrate successfully to breeding areas in th e 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 th e contrary, the pattern of lipid loss observed in monarchs clustered i n shaded areas is consistent with the hypothesis that intact, closed f orest is necessary for successful overwintering because it permits mon archs to conserve their lipid reserves for the spring migration.