Pupal colour–pupation substrate correlation in butterflies

Abstract

The pupal colour of many butterflies is plastic, being green or brown depending on the environment in which they develop. In most species with pupal colour plasticity (PCP), larvae that pupate on leaves tend to develop into green pupae whereas those that pupate off‐leaf (e.g., on the stem or soil) tend to become brown. This correlation between background and pupal colour has been hypothesized to protect the butterflies against predation by allowing them to match their background, although experimental evidence is scarce. Furthermore, not all butterflies exhibit PCP, and the extent to which substrate choice influences PCP remains unclear. We use a comparative approach involving multiple, closely related species varying in the degree of PCP to test predictions related to the correlation between pupal colour and pupation substrate. We studied PCP in response to larval density in five common satyrine species (Nymphalidae), two dimorphic and three monomorphic. As predicted, species with monomorphic green pupae preferred to pupate on leaves whereas those with monomorphic brown pupae preferred off‐leaf substrates. In species with dimorphic pupae, pupal colour correlated with pupation substrate. We also predicted that higher larval density induces a greater proportion of off‐leaf pupation in the dimorphic species, but not in the monomorphic species. However, this prediction was not supported. Overall, we found a strong across‐species correlation between pupation substrate choice and pupal colour plasticity, and our results support the idea that pupal colour plasticity can be an adaptation against predation.