Flesh flies regulate the consumption of 3 macronutrients to maximize lifespan and egg production

Dietary macronutrient balance has been shown to have large effects on the growth, reproduction, and survival of a wide range of animals. Yet, most studies of dietary nutrient balance are limited to pairs of nutrients even though food items can differ in their concentrations of all 3 macronutrients. We used a right-angled mixture triangle design to quantify the self-selected nutrient intake of flesh flies, Sarcophaga crassipalpis, and to test the separate and interactive effects of all 3 macronutrients on lifespan and lifetime egg production. Flesh flies presented with 3 food dishes, each biased in 1 macronutrient, self-selected a nutrient intake that was highly carbohydrate-biased. When maintained on 1 of 7 diets with fixed macronutrient mixtures, flies maximized both lifespan and lifetime egg production on carbohydrate-biased diets. Despite overwhelming effects of carbohydrates on fitness, there were subtle but significant interactions between carbohydrate and lipid for lifespan and carbohydrate and protein for egg production. These results demonstrate that flesh flies are able to balance their intake of all 3 macronutrients simultaneously to reach an intake at which they achieve high fitness. Furthermore, the effects of carbohydrate and protein on the fitness of flesh flies were similar to that of fruit flies (Bactrocera tryoni) and vinegar flies (Drosophila melanogaster). This suggests that regardless of whether they feed on plant tissue or animal flesh as larvae, flies converge on similar dietary requirements as adults.