Partially coupled reaction norms of pupal colour and spot size in a butterfly

Abstract

Many species cope with heterogeneous environments through adaptive phenotypic plasticity, and plasticity of body colour is a widely used means of minimizing predation risk. Pupal colour in many butterflies is a plastic trait sensitive to pupation substrate, with green and brown pupae primarily formed on leaf and off-leaf substrates respectively. This is thought to be an example of adaptive phenotypic plasticity because the green and brown pupae are better camouflaged against their respective backgrounds. We tested whether pupal ‘spots’—paired circular markings on the anterior abdominal pupal segments of many butterflies—are also phenotypically plastic. Using experiments on the tropical satyrine butterfly Mycalesis mineus, we confirm that pupal spot size is plastic. Because it is known that pupation substrate influences pupal colour in this species, we tested whether substrate also influences pupal spot size. Further, we explored whether there was a correlation between pupal colour and spot size. We tested the effects of pupation substrate and pupal colour on both absolute spot size and spot size relative to body size. Pupal spot size (absolute and relative) was correlated with both pupal colour and pupation substrate. Brown pupae had larger spots, and green pupae formed off-leaf had intermediate sized spots compared to green pupae formed on leaves and brown pupae, the latter of which always formed off-leaf. Thus, the reaction norm of pupal spot size is partially coupled with both pupal colour and pupation substrate in this species. Since pupation substrate in turn influences pupal colour, the correlated traits of pupal colour and pupal spot could comprise a single functional suite, suggesting a developmental link. We speculate that pupal spots may function against predation, but further studies are needed to understand their possible adaptive significance.