Fearful symmetry? Intra-individual comparisons of asymmetry in cryptic vs. signalling colour patterns in butterflies

The signalling role of asymmetry has attracted considerable recent interest among evolutionary biologists. Although it has been studied primarily within the context of sexual selection, symmetry of signals may play a role also in inter-specific communication, such as predator-prey interactions. Both theory and experimental evidence suggest that asymmetry may impair the efficacy of visual warning signals used to deter potential predators, but increase the protective value of non-signalling, cryptic colour patterns used to decrease the risk of detection. Here we tested the prediction from this hypothesis by means of intra-individual comparisons of asymmetry in colour pattern elements in three species of moths (Arctia caja (L.), Noctua orbona (L.), Smerinthus ocellata (L.)) that possess cryptic fore wing patterns and signalling hind wing patterns. Mean asymmetries constituted 4.3% (range 2.1-7.0%) of trait size for colour pattern elements, whereas individual asymmetry levels reached as high as 26%. Asymmetry tended to be somewhat larger in cryptic patterns on fore wings than in signalling patterns on hind wings in five of six comparisons, but in only one case was the difference statistically significant. In addition, pattern elements were somewhat more asymmetric on fore wings also in Saturnia pavonia (L.), which possesses identical signalling eyespots on both fore and hind wings. The relatively low levels of asymmetry also in cryptic patterns imply either that selection does not favour increased asymmetry in cryptic patterns, or that the evolution of pronounced asymmetry is developmentally or genetically constrained.