Triangulating from the cairns…
As a case study of adaptive strategies in temporally varying environments, thermoregulation in three populations of Colias butterflies along an elevation gradient in Colorado is studied in relation to the fluctuating meteorological environment. Emphasis is placed on short time scale (15-300 s) variation in air temperature and wind speed and its role in determining elevational patterns of body temperature, flight activity, and thermal stress due to overheating. A stochastic, linear filter model of an organism in a variable environment is used which views the adaptive process as the adjustment of the organism's filter. The relation-ship between this filter model and a transient energy balance model of the butterfly is examined to show how the thermoregulatory mechanisms of adaptation determine the filtering properties of the organism. Heat shocks at 45⚬ C significantly decrease survivorship and fecundity in Colias. Time series analysis indicates that short-term variation in wind speed and air temperature under sunny, midday conditions is significantly greater at higher elevation sites. Negative cross-correlations between wind speed and air temperature at certain time scales amplify the probability of overheating in Colias. Simulation and field results show that Colias from higher elevation populations are more sensitive, in terms of body temperature response, to a given level of wind speed variability, because of their higher wing solar absorptivities. As a result of these factors, variation in body temperature under sunny, midday conditions is significantly greater for butterflies in higher elevation Colias populations, regardless of behavioral thermoregulation. While mean body temperatures under these conditions are 2⚬ C-3⚬ C higher for low elevation than for high elevation Colias, the maximum body temperatures experienced in these populations are similar. These results are consistent with the hypothesis that microevolution of thermoregulatory characteristics in Colias is constrained by the need to avoid high body temperatures. The differences in mean body temperatures which follow from this constraint and the elevational differences in meteorological variation may be a major factor in the elevational patterns of daily flight activity time observed in previous studies. By documenting the biological importance of thermal stress and flight activity for Colias, we can develop optimality models for thermoregulatory strategy. To our knowledge, this is the first comprehensive demonstration of quantitative differences in environmental variability and their consequences for differences in adaptive characteristics among animal populations. Results are discussed in relation to strategies of insect thermoregulation, the structure of the micromete-orological environment, and general principles of adaptive design in variable environments.
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