Subtle variation in size and shape of the whole forewing and the red band among co-mimics revealed by geometric morphometric analysis in Heliconius butterflies

Heliconius are unpalatable butterflies that exhibit remarkable intra- and interspecific variation in wing color pattern, specifically warning coloration. Species that have converged on the same pattern are often clustered in Mullerian mimicry rings. Overall, wing color patterns are nearly identical among co-mimics. However, fine-scale differences exist, indicating that factors in addition to natural selection may underlie wing phenotype. Here, we investigate differences in shape and size of the forewing and the red band in the Heliconius postman mimicry ring (H.erato phyllis and the co-mimics H.besckei, H.melpomene burchelli, and H.melpomene nanna) using a landmark-based approach. If phenotypic evolution is driven entirely by predation pressure, we expect nonsignificant differences among co-mimics in terms of wing shape. Also, a reinforcement of wing pattern (i.e., greater similarity) could occur when co-mimics are in sympatry. We also examined variation in the red forewing band because this trait is critical for both mimicry and sexual communication. Morphometric results revealed significant but small differences among species, particularly in the shape of the forewing of co-mimics. Although we did not observe greater similarity when co-mimics were in sympatry, nearly identical patterns provided evidence of convergence for mimicry. In contrast, mimetic pairs could be distinguished based on the shape (but not the size) of the red band, suggesting an advergence process. In addition, sexual dimorphism in the red band shape (but not size) was found for all lineages. Thus, we infer that natural selection due to predation by birds might not be the only mechanism responsible for variation in color patterns, and sexual selection could be an important driver of wing phenotypic evolution in this mimicry ring.