Size-dependent effects of temperature and food stress on energy reserves and starvation resistance in yellow dung flies

Competing hypotheses: (1) Large body size confers more efficient energy use (relative efficiency hypothesis). (2) Large body size requires more energy to be sustained, a disadvantage when food is limited (absolute energy demand hypothesis). Organism: Yellow dung flies, Scathophaga stercoraria (Diptera: Scathophagidae), artificially selected for large and small body size in the laboratory for 11 (24) generations. Methods: Flies were reared in limited and unlimited larval food conditions and low and stressfully high temperatures, and after adult emergence they were held on water only to study the relationship between energy content (lipids, glucose, glycogen) and (physiological) adult life span under complete starvation (starvation resistance) in the laboratory. Conclusions: Limited larval food and high temperature decreased life span. Life span increased markedly with body size due to the greater energy content of larger flies. Small selection line flies had relatively more energy and longer life spans under complete starvation, indicating a compensatory correlated genetic response to body size selection. The larger males suffered most under multiple stresses. Our results support the relative efficiency hypothesis, which more than compensates for the greater absolute energy demand of larger individuals.