The relationship between climatic variables and ventral-dorsal differences
Here, we tested whether climate predicts dorsal-ventral reflectance contrast in butterflies. Ventral reflectance showed higher reflectance than dorsal reflectance for all three body regions in most of the species (all t342 > 25, P< 0.001, Fig. S5). Phylogenetic signal was present in these differences (λ = 0.66). In the best model, only PC1 (F6,334 = 17.01, P < 0.001) was a significant predictor (other terms removed with P> 0.2) with species from colder environments showing a greater difference between dorsal and ventral reflectance.Post-hoc PGLS models revealed that this relationship was present for both the thorax (coef = 1.47, t = 2.63,Padj =0.01) and basal wing regions (coef = 1.14, t = 4.92, Padj < 0.001), but not in entire wing region (coef = 0.00, t = -1.42,Padj = 0.16). This indicates that butterflies in warmer environments evolved higher contrast between ventral and dorsal reflectance in body regions that are important for thermoregulation.