Thermoregulation and Flight in Colias Butterflies: Elevational Patterns and Mechanistic Limitations

Abstract

This study identifies the physical determinants of thermoregulation in Colias, develops and tests models that predict the patterns of body temperature and flight activity along an elevational gradient (1.7-3.9 km), and explores the relationship of the thermoregulatory characteristics to these patterns among Colias populations. The limitations on adaptation provided by environmental vari- ability and the adaptive mechanisms involved are emphasized. Colias behaviorally thermoregulate by body orientation to solar radiation to achieve the body temperatures (30'-40'C) needed for flight. There are differences among Colias populations in solar absorptivity of the ventral hind wing and fur thickness on the ventral thorax which correlate with elevation. Energy balance and behavioral models based on meteorological inputs and the thermoreg- ulatory characteristics of the butterfly are developed and tested. These accurately predict body tem- peratures and flight activity of the population throughout the day at each site. High levels of flight activity occur whenever body temperatures are 30'-40', particularly at higher elevations. Short-term meteorological variation, such as intermittent cloudy periods, can rapidly reduce body temperature excess and flight activity to zero. The time available for flight activity per day is greater in low- elevation than in higher-elevation populations, even when differences in cloudiness are taken into account. Climate space analysis reveals that the overlap in flight space for populations at low and high elevations is -80% for sunny conditions. The minimum possible overlap in flight space between Colias butterflies is -40%, due to limitations imposed by the mechanisms of thermoregulation and wing color determination. These limitations are inadequate to explain the observed differences in flight activity with elevation. The microevolution of thermoregulation in Colias is discussed in terms of mechanistic limitations to adaptation, population tracking of the thermal environment, and the role of environmental variability.