Fig. 1

The effect of protein and carbohydrate concentration on female and male body size and sexual size dimorphism (SSD). a, b Surfaces show the fitted relationship between body size (PC1 from a PCA on the size of four body parts), carbohydrate level and protein level in female and male flies, based on the statistical model specified by the equation above each chart. T = body size, P = protein, C = carbohydrate, ε = error, subscripts refer to levels within each factor. c Surface shows the difference in female and male body size (SSD: female size – male size) across the same nutritional landscape, using fitted values from a and b. Points indicate diets tested and dotted lines connect diets with equal protein-to-carbohydrate ratios (1:14.6, 1:7.2, 1:3.5, 1:1.7, 1.3:1, 1.4:1). Asterisk indicates approximate composition of standard cornmeal–molasses medium. Corresponding thin-plate spline plots are shown in Additional file 1: Fig. S1