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Fitness consequences of nutrient balancing in a caterpillar
Foraging is fundamental to animal survival and reproduction, and animals often require more than one nutrient to maximize their evolutionary fitness. Here, we test whether caterpillars of Spodoptera exigua (Hübner) balance the intake of multiple nutrients to meet their nutrient requirement. In the choice test, final-instar larvae were offered a choice of two nutritionally complementary diets [1) p42:c0 vs. p0:c42, 2) p42:c0 vs. p7:c35, 3) p35:c7 vs. p0:c42, 4) p35:c7 vs. p7:c35, 5) p35:c7 vs. p5.6:c28, 6) p28:c5.6 vs. p7:c35 and 7) p28:c5.6 vs. p5.6:c28]. Caterpillars tightly regulated their intake of protein (P) and carbohydrate (C) to defend a specific nutrient composition, which was slightly carbohydrate-biased (P:C=1:1.2). In the no-choice test, larvae were restricted to feed on one of 42 diets that varied both in P:C mixtures (1:0, 5:1, 2:1, 1:1, 1:2 and 1:5) and in total nutrient concentration(P+C=67.2%, 58.8%, 50.4%, 42%, 33.6%, 25.2% and 16.8%). Fitness landscapes fitted for key larval fitness variables (e.g., growth rate) over these range of diets revealed that the larval performance was optimized at the regulated position of nutrient intake in this caterpillar.
