Does Selection for Stress Resistance Lower Metabolic Rate?

Environmental stress reduces the fitness of organisms, and in this manner may shape species distribution and evolutionary change. Ecologists have long been interested in physiological mechanisms that reduce the effects of stress and result in improved fitness. It has previously been suggested (1) that selection for stress resistance will bring about the evolution of lower standard metabolic rates and (2) that organisms with increased stress resistance will have a lower metabolic rate during stressful conditions, compared to stress-sensitive organisms. We have tested these hypotheses using replicate populations of Drosophila melanogaster. Desiccation and starvation-selected populations and their controls, as well as populations selected for postponed senescence and their short-generation controls were examined. The metabolic rate of each selected and control population was measured under three different environmental conditions: (1) desiccation (no food or water), (2) starvation (water present, no food), and (3) food condition (food and water present). The carbohydrate and lipid content of each population were also measured. All selected and control populations lowered their metabolic rates when measured under starving conditions compared to food conditions. The metabolic rates of flies under desiccating conditions did not differ from food conditions regardless of selection treatment. When comparing the mass-specific metabolic rates of stress-selected populations to their controls, stress-selected populations had a significantly lower mass-specific metabolic rate in all cases, regardless of the environmental conditions under which the measurements were made. However, once the mass of nonmetabolizing lipid and carbohydrate was accounted for and metabolic rate was expressed in carbohydrate–lipid free mass, selected populations did not differ significantly in metabolic rate from control populations, regardless of the environmental condition under which the measurements were made. We conclude that in the case of D. melanogaster, resistance to stress does not require the evolution of a lower metabolic rate. Instead, the accumulation of metabolic resources such as carbohydrate and lipid may be important for resistance to specific forms of stress.