Morph-specific differences in disease prevalence and pathogen-induced mortality in a terrestrial polymorphic salamander

Abstract

Polymorphic species provide an excellent system to study population divergence because different phenotypes often face diverse selection pressures within their shared environment. Previous studies have demonstrated that different color morphs of Plethodon cinereus (striped and unstriped), which vary in their degree of melanin-based coloration, vary temporally in their seasonal activity and differ in metrics associated with stress physiology. Despite the known association between melanin-based coloration and disease resistance among vertebrates, few studies have examined this relationship in amphibians. In a laboratory experiment, we exposed juvenile and adult P. cinereus to the pathogenic chytrid fungus [Batrachochytrium dendrobatidis (Bd)] and tested the hypothesis that the different color morphs would differ in their response to Bd. If the degree of melanization is positively associated with immune traits in this species, we predicted that unstriped salamanders would be more resistant to Bd and would exhibit higher survival rates compared to striped salamanders. Our results suggest that unstriped salamanders have a higher prevalence of infection when they do not behaviorally avoid Bd. Unstriped salamanders also exhibited higher rates of Bd-induced mortality compared to striped salamanders. These results conflict with our initial predictions and with general findings suggesting that individuals with higher levels of melanin production are more resistant to disease. Behavioral traits of the unstriped morph, including responses to predators and interactions with territorial opponents, may contribute to increased levels of stress and reduced resistance to infection. Future studies that directly manipulate glucocorticoid levels and/or temperature are needed to better understand the differences in Bd resistance in this species.