Temporal Thermal Heterogeneity as a Potential Evolutionary Driver of Thermoregulation in the Generalist Grasshopper Melanoplus differentialis from Five Sites in the Midwestern and Southern United States

Environmental drivers of the evolution of thermoregulation, especially thermal fluctuation, have been incompletely investigated. I used a combination of field and laboratory studies to determine if thermal heterogeneity selected for greater thermal tolerance or ability to thermoregulate in a generalist grasshopper by testing upper and lower thermal limits, behavioral thermoregulation, and acclimation. I measured these from individuals originating from sites that had varying degrees of thermal fluctuation over time. I found partial support for thermal heterogeneity as an evolutionary driver of thermal tolerance, behavioral thermoregulation, and acclimation. However, whether or not differences in these metrics conformed to my hypothesis varied by sex; males more often followed my hypothesized pattern than did females. This may be because females, which are larger than males, are over a critical thermal limit for mass, which affects thermal properties. I also tested gene expression from different sites in males, and found that gene expression differed significantly, but not in a manner that suggested thermal heterogeneity was the cause. This could have been due to population-specific evolutionary influences not related to temperature. These studies highlight the complex interactions among thermal tolerance, thermoregulation, plasticity, and thermal properties of the environment