1. Large body size is typically strongly favoured by sexual selection in males and by fecundity selection in females, while disadvantages of large size are poorly documented. Two contrasting physiological hypotheses have been suggested to affect selection on body size. According to the relative efficiency hypothesis, large body size confers more efficient energy use; according to the absolute energy demand hypothesis, large body size requires more energy to be sustained, a disadvantage particularly when food is limited. 2. We investigated the effect of food availability on glycogen and lipid allocation to reproduction in Yellow Dung Flies (Scathophaga stercoraria; Diptera: Scathophagidae) of varying body sizes. Male and female flies stemmed from selection lines artificially selected for large and small body size for 11 generations. Flies were reared at unlimited and limited larval food (dung), and the emerging adults were further held at low and high adult food (prey).
3. Adult food conditions strongly influenced reproductive maturation and energy reserves in both sexes, while larval food conditions played a minor role primarily in males. Unlimited adult food led to earlier maturation and larger clutches and egg volumes, and female investment of glycogen and lipids into the clutch was proportionally greater, supporting the relative efficiency hypothesis.
4. In contrast, at limited adult food females selected for small size matured earlier and had relatively larger clutches than large line females (when correcting for body size), in correspondence with greater glycogen investment in reproduction. Small line males at limited prey also tended to reproduce sooner. These correlated genetic responses to artificial selection support the absolute energy demand hypothesis. How important these admittedly small effects are in nature remains to be investigated.