Macronutrient Balance Modulates the Temperature-Size Rule in an Ectotherm.

Most ectotherms mature at a larger body size in colder conditions, a phenomenon known as the temperature-size rule. While a number of hypotheses have been proposed to explain this rule, little work has been done to understand it from a nutritional perspective. We have used the final-instar caterpillars of Spodoptera litura to investigate how dietary protein∶carbohydrate (P∶C) balance influences the relationship between temperature and body size. The strength and direction of the thermal reaction norm for body size were significantly altered by dietary P∶C balance. The slope of the reaction norm was nearly flat for caterpillars raised on a balanced food ([Formula: see text]) but was significantly negative for those on nutritionally imbalanced foods (1∶5 or 5∶1), especially when carbohydrates were in considerable excess. These nutrient-dependent effects of temperature on body size were caused mainly by corresponding changes in body lipid storage. When allowed to choose between imbalanced diets, caterpillars increased their preference for carbohydrates to meet high energy demands at higher temperatures. The slope of the thermal reaction norm for body size was substantially reduced by such a temperature-driven shift in nutrient preference, indicating that the impact of high temperature on body size was buffered by altered food selection. This study highlights the importance of macronutrient balance as a key factor modulating the relationship between temperature and body size in ectotherms and provides a novel approach for understanding the temperature-size rule.