THE LOCATION OF MONARCH BUTTERFLY DANAUS-PLEXIPPUS L. OVERWINTERING COLONIES IN MEXICO IN RELATION TO TOPOGRAPHY AND CLIMATE

Each year monarch butterflies migrate from breeding grounds in the United States and Canada to the Transvolcanic Belt of central Mexico. Here, within the montane fir forests, they initially aggregate in small groups of loose clusters scattered along high ridge crests. During November and December the numerous small groups consolidate into a few large compact aggregations and move downward into more protected positions closer to water. Butterfly activity increases in the last half of February due to seasonal warming. The consolidation and compaction processes that marked the beginning of the season reverse, and the colonies spread out and often split into two or more parts. After mid-March, colony size decreases as the butterflies begin to remigrate northward. Several characteristics of the climate and physiography of the Transvolcanic Belt, including moisture, altitude, and slope exposure and inclination, are important to the overwintering biology of the monarch butterfly. The forests of the zone playa major role in satisfying the overwintering monarchs' microclimatic requirements by moderating temperature extremes and conserving moisture. By colonizing this high altitude area in the tropics, the butterflies appear to satisfy microclimatic requirements that include temperatures low enough to keep activity, metabolism, and lipid expenditure to a minimum, but not so cold as to cause freezing; sufficient solar input to allow thermoregulatory basking and consequent flight; and sources of moisture and nectar. Each autumn, millions of monarch butterflies (Danaus plexippus L.) migrate southwest or south (Urquhart & Urquhart 1978, Schmidt-Koenig 1979) from breeding grounds in eastern and central United States and southern Canada to overwintering sites in Mexico. Funneling through Texas, they cross into Mexico and encounter the southern extension of the Rocky Mountains, the Sierra Madre Oriental. Here they change their southwesterly course and follow the ranges to the southeast, eventually cross them, and continue to the Transvolcanic Belt, the volcanic mountains that extend across the southern end of Mexico's Central Plateau (Altiplanicie Mexicana) between 19° and 200N latitude. At a few isolated places within the high altitude coniferous forests, which are scattered through this belt of mountains (Fig. 1), monarchs spend the winter in aggregations estimated to be in the tens of millions (Brower et al. 1977, Calvert, in prep.). Monarchs migrate south in the fall to avoid winter cold and survive in cool, moist places where they can conserve fuel reserves in a state of reproductive inactivity until making the return trip north in the spring. Yet weather in the overwintering areas does not ideally meet monarch requirements. Not only do temperatures occasionally fall into the lethal range (Calvert et al. 1983), but also intense insolation on VOLUME 40, NUMBER 3 165 clear and partly cloudy days stimulates butterfly activity to an extent that appears to contradict their need to conserve fuel. In an attempt to resolve these apparent contradictions, and to understand better why the monarchs choose these particular areas in Mexico, we here describe characteristics of the annual overwintering cycle and ecological features of several overwintering areas that we studied for nine seasons (December 1976 through spring 1985). PHYSIOGRAPHIC FEATURES, CLIMATE AND VEGETATION Volcanic cones and ranges dominate the terrain of the Transvolcanic Belt, which has an area of 60,000 km2, and measures approximately 640 km across by 95 km wide (Moore 1945). To the north it is bounded by the high Mexican plateau, and on the south by the large Balsas River drainage (Rzedowski 1978). Its eastern portion averages 2200 m elevation with numerous peaks rising above 3600 m, including the highest mountains in North America south of Alaska (Goldman 1951). The western portion contains fewer high peaks, and declines in elevation towards the Pacific. The central area where the monarch colonies are located (Fig. 1) is drained to the north and east by the Rio Lerma and to the south and west by the Balsas-Mezcala river system (Arbingast et al. 1975). Classic wet-dry season weather patterns prevail through most of the Transvolcanic Belt. Precipitation and heavy clouding is frequent from May until October, especially in the mountains, but winters are dry, and arid conditions prevail on the interior plains (Goldman & Moore 1945). However, winter and early spring storms occasionally occur in the area, and the higher elevations are subjected to high winds, heavy rains, snow, and ice storms (Mosina-Aleman & Garcia 1974). While potentially lethal to the overwintering butterflies (Calvert et al. 1983), these storms are also beneficial because they reduce the severity of the winter drought in the high-elevation overwintering areas. Because of the wide range of altitudes and climatic conditions, vegetation within the Transvolcanic biotic province is extremely varied. High interior plains and valleys consist largely of grasslands intermixed with patches of small trees, shrubs, yuccas, agaves and cacti. On mountainous slopes, forests dominated by oaks and pines give way to firs at about 2750 m (Goldman 1951), but in more humid areas, the firs commence as low as 2400 m (Rzedowski 1978). On the highest peaks, firs give way to alders and other species of pine and eventually to grassland and tundra (Goldman 1951, Goldman & Moore 1945). As is true of the lower limits, vegetational transitions depend on moisture and exposure, and the altitudinal limits of the fir zone may be influ-