Overwintering clusters of the monarch butterfly coincide with the least hazardous vertical temperatures in the oyamel forest

Abstract

During winter, monarch butterflies form dense colonies in oyamel fir forests on high mountains in central Mexico, where the forest canopy serves both as a blanket, moderating temperature, and an umbrella, shielding the butterflies from rain. In this study we investigated the vertical dimension of the butterflies' use of the oyamel forest: we predicted that clusters form at the heights above ground that provide the greatest protection from freezing. By suspending temperature recorders at eight heights, from ground level up to 22m, we established two vertical transects in the forest. We set one transect in a densely forested area and the other in a thinned area, and we recorded hourly temperatures from Jan 13 through Feb 5,2006. Intermediate heights in the forest, from 10 to 15m above ground, remained the warmest during the cold night and early morning. We also determined that this temperature distribution matched available records of the vertical distribution of butterflies roosting in branch and trunk clusters. The vertical temperature profiles became uniform during mid-day. Temperature extremes were moderated more during clear than during cloudy periods and more in the denser than in the thinned forest. Our results illustrate how the monarch butterfly is behaviorally adapted to the three-dimensional complexity of microclimate in the oyamel forest. A denser forest provides better protection for the overwintering survival of monarch butterflies than a thinned forest. This research bolsters our other microclimatic studies, all of which point to the urgency of prohibiting logging within the oyamel forest used as wintering habitat by monarch butterflies.