The effect of fluctuating temperatures on ectotherm life-history traits: comparisons among geographic populations of Wyeomyia smithii

Hypothesis: The effects of temperature variation on life-history traits depend on both mean developmental temperature and geographic population of origin. Organism: Pitcher plant mosquitoes, Wyeomyia smithii. Field sites: We established laboratory colonies from three geographic populations spanning a latitudinal and altitudinal gradient in eastern North America. Sites at lower latitudes or altitudes experience both higher mean temperatures and greater thermal variability during the growing season. Methods: For each of the three sampled populations we analysed the effects of rearing temperature, population, and sex on survival, development time, and mass at pupation for mosquito larvae reared at 16, 20, and 27 C constant temperatures. We also measured the same variables in two fluctuating temperature treatments with means of 20 and 27 C. Using mixed linear models, we tested for the effects of mean temperature, temperature variation, population, and sex including all data except those from 16 C constant. Results: Temperature variation did not have a significant effect on survival to pupation at 20 or 27 C constant. However, low survival at 16 C constant compared with high survival after transient exposure to the same temperature implies that duration of exposure may affect survival to pupation. The effects of temperature variation on both development time and pupal mass depended on mean temperature. Differences between constant and fluctuating temperatures for both these traits were predicted by the non-linear relationship between development rate and temperature (the Kaufmann effect). Moreover, this effect appears to explain geographic variation in the relationship between temperature variation and the measured life-history traits.