Group living influences larval melanism in gregarious caterpillars

Group living in gregarious caterpillars provides several benefits, including enhanced anti-predator defense and improved thermoregulation. Melanism affects both thermoregulation, with melanic forms warming up faster, and anti-predator protection, through its link to aposematic coloration and cuticle thickening. Due to the macroevolutionary association between larval gregariousness and aposematic coloration in Lepidoptera, it has been hypothesized that gregariousness could have evolved to maximize the warning coloration signal. We evaluated melanism patterns in caterpillars from the gregarious swallowtail Battus polydamas archidamas reared singly and in groups of ten individuals. We compared melanism in caterpillars from two populations differing in climate and predation risk, testing for an association between melanism and thermoregulation and/or anti-predator protection. We expected to find darker caterpillars in the colder population, which also showed a higher predation risk from ants. Considering that the group of caterpillars may maximize the warning signal, we hypothesized that solitary caterpillars would be darker (more aposematic). The degree of larval melanism was similar in both populations. Caterpillars in groups showed decreased levels of melanism compared to solitary caterpillars in both populations. Concerning performance costs of melanism, darker caterpillars were slightly—yet not significantly—smaller. Results suggest that the patterns of larval melanism in solitary vs gregarious caterpillars might be rather fixed, less related to current ecological conditions than to the evolutionary importance of aposematic coloration. If the general pattern for gregarious caterpillars results to be that larval melanism has negligible costs, then decreased melanism could be considered another benefit of gregariousness in Lepidoptera.