Nutrient balancing in grasshoppers: behavioural and physiological correlates of dietary breadth

SUMMARY We examined correlates of nutrient balancing with dietary range by comparing diet selection and ingestive, post-ingestive and performance-related responses to macronutrient imbalance in two species of grasshopper. One of the two species, Locusta migratoria (the African migratory locust), is a specialist grass-feeder, while the other, Schistocerca gregaria (the desert locust), is a generalist herbivore that includes both grasses and forbs in its diet. In ad libitum conditions, both species composed a balanced intake of the two macronutrients protein and carbohydrate from nutritionally complementary synthetic foods, but the composition of the selected diet differed, with the generalist selecting more protein, but not carbohydrate, than the grass-specialist. The grass-specialist, by contrast, retained ingested nitrogen more efficiently on the ad libitum diets. When confined to nutritionally imbalanced foods, both species regulated ingestion in such a way as to mitigate excesses as well as deficits of the two nutrients. The responses were, however, distinct in the two species, with the generalist feeder ingesting greater excesses of protein than the specialist. The species also differed in their post-ingestive responses to ingested excesses of nutrient, with the generalist but not the specialist using protein-derived carbon as an energy source when fed carbohydrate-deficient foods. The generalist also retained a higher level of body protein when confined to protein-deficient diets. The data suggested one functional reason why the generalist species selected a diet with higher protein content in the ad libitum treatment because, when confined to the nutritionally imbalanced foods, development rate peaked on higher protein foods for the generalist compared with the specialist. Many aspects of these data agree with the prediction that generalist-feeding animals should show greater behavioural and physiological flexibility in their responses to nutrient imbalance than do specialists.