BROWER, L.P.*; FINK, L.S.; RAMIREZ, I.R.; WEISS, S.B.; SLAYBACK, D.; PERAULT, D.R.; Sweet Briar College; Sweet Briar College; Universidad Nacional Autonoma de Mexico; Creekside Center for Earth Observations; NASA Goddard Space Flight Center ; Lynchburg College: The physiological ecology of overwintering Monarch butterflies

Monarch butterflies (Danaus plexippus L.) are extreme specialists in their selection of overwintering sites in Mexico. Up to a billion butterflies, produced in a breeding range covering 87 million hectares in eastern North America, converge into massive aggregations on a handful of hectares high in Mexico�s transvolcanic range. Energetics and thermal constraints interact to restrict the butterflies� winter habitat selection. Although in the tropics, monarch colonies form at elevations of 3000m and higher, where nectar availability is greatly limited and where they are subject to winter storms from the north. The butterflies, therefore, must rely on lipid reserves accumulated during their migration for their metabolic requirements, and they must avoid freezing or chill injury during and after storms. We are combining ecophysiology, climatology, GIS and remote sensing to increase our understanding of the monarch butterfly�s overwintering requirements and the characteristics of the habitats they select. Potential habitat varies in suitability: sites that are too warm will cause the butterflies to burn their lipid reserves, and other sites will increase risks of freezing. We are comparing the microclimate in sites where butterflies do and do not form colonies, and comparing rates of lipid depletion and risks of freezing in colony sites with different microclimates. We are using GIS models to integrate these data. Both illegal logging and climate change are diminishing the amount of suitable wintering habitat for the monarch butterfly. The spectacular migration is at great risk.