Partial thermoregulatory compensation by a rapidly evolving invasive species along a latitudinal cline

In fewer than two decades after invading the Americas, the European fly Drosophila subobscura evolved latitudinal clines in several traits. Moreover, its chromosomal inversion frequencies at given localities have shifted with climate warming. Temperature may have driven the evolution of both geographic clines and within-site shifts. Nevertheless, whether body temperature (Tb) of active flies actually varies geographically and temporally is unknown: if these flies are effective behavioral thermoregulators, they might maintain relatively constant Tb when active, independent of season and latitude. To evaluate these possibilities, we monitored activity and estimated Tb of active flies in all seasons and at five sites (37-49 degrees N) in western North America. Latitudinal and seasonal shifts in activity are conspicuous. Flies have longer activity seasons (and are much more active) at higher latitudes. Flies are generally active only at midday in cool seasons, and only early and late in the day (if active at all) in warm seasons. Despite these behavioral shifts active flies have much lower Tb in cooler seasons and at higher latitudes. The observed pattern is consistent with the hypothesis that geographic shifts in Tb may be an evolutionary driver of latitudinal clines in this invading species.