A full-factorial study of the effects of rates of temperature change and start temperatures was undertaken for both upper and lower critical thermal limits (CTLs) using the tsetse fly,Glossina pallidipes. Results show that rates of temperature change and start temperatures have highly significant effects on CTLs, although the duration of the experiment also has a major effect. Contrary to a widely held expectation, slower rates of temperature change (i.e. longer experimental duration) resulted in poorer thermal tolerance at both high and low temperatures. Thus, across treatments, a negative relationship existed between duration and upper CTL while a positive relationship existed between duration and lower CTL. Most importantly, for predicting tsetse distribution,G. pallidipessuffer loss of function at less severe temperatures under the most ecologically relevant experimental conditions for upper (0.06°C min⁻¹; 35°C start temperature) and lower CTL (0.06°C min⁻¹; 24°C start temperature). This suggests that the functional thermal range ofG. pallidipesin the wild may be much narrower than previously suspected, approximately 20–40°C, and highlights their sensitivity to even moderate temperature variation. These effects are explained by limited plasticity of CTLs in this species over short time scales. The results of the present study have broad implications for understanding temperature tolerance in these and other terrestrial arthropods.

The evolution of metabolic rate-temperature (MR-T) reaction norms is of fundamental importance to physiological ecology. Metabolic cold adaptation (MCA) predicts that populations or species from cooler environments will have either a higher metabolic rate at a common temperature or steeper MR-T relationships, indicating greater sensitivity of respiratory metabolism to temperature. Support for MCA has been found in some insect species by comparing species or populations differing in latitude. However, the generality of these findings are contentious, with most studies either unable to account for phenotypic plasticity or the evolutionary relatedness of species or populations. Hence, the importance of MCA is vigorously debated from both evolutionary and ecological perspectives. Furthermore, few species, particularly from tropical environments, have been shown to differ in MR-T sensitivity along altitudinal temperature gradients. Here, using four populations of tsetse flies (Glossina pallidipes, Diptera: Glossinidae) from thermally distinct geographic regions, we test the hypothesis that there is evolved variation in MR-T relationships to cold climates. We found that a high-altitude equatorial population from a cool habitat has a steeper MR-T reaction norm. By contrast, other populations from warmer environments in East Africa do not differ with respect to their MR-T reaction norms. Squared-change parsimony analyses, based on the combined mitochondrial 16S rDNA ribosomal subunit and cytochrome c oxidase subunit I (COI), support the hypothesis of adaptive differentiation of MR-T reaction norms in the cool-climate population. Seasonal adjustments or laboratory-temperature-induced phenotypic plasticity changed the intercept of the reaction norm rather than the slope, and thus the observed intraspecific variation in slopes of MR-T reaction norms could not be accounted for by phenotypic plasticity. These results therefore suggest evolutionary adaptation of MR-T reaction norms to cool climates (<22 degrees C) in tsetse and provide novel support for MCA within an insect species.

Global change alters the environment, including increases in the frequency of (un)favorable events and shifts in environmental noise color. However, how these changes impact the dynamics of populations, and whether these can be predicted accurately has been largely unexamined. Here we combine recently developed population modeling approaches and theory in stochastic demography to explore how life history, morphology, and average fitness respond to changes in the frequency of favorable environmental conditions and in the color of environmental noise in a model organism (an acarid mite). We predict that different life-history variables respond correlatively to changes in the environment, and we identify different life-history variables, including lifetime reproductive success, as indicators of average fitness and life-history speed across stochastic environments. Depending on the shape of adult survival rate, generation time can be used as an indicator of the response of populations to stochastic change, as in the deterministic case. This work is a useful step toward understanding population dynamics in stochastic environments, including how stochastic change may shape the evolution of life histories.