Chytridiomycosis and climate change: exposure to Batrachochytrium dendrobatidis and mild winter conditions do not increase mortality in juvenile agile frogs during hibernation

Abstract

Hibernation is often associated with high mortality, especially during early life stages, and losses can be exacerbated by unusual winter conditions or if animals enter hibernation carrying a disease. Here, we examined how overwintering amphibians may be affected by the combined effects of mild winters, which are projected to increase in frequency due to climate change, and of chytridiomycosis, a disease that has contributed to the decline of hundreds of species worldwide. We exposed juvenile agile frogs Rana dalmatina to Batrachochytrium dendrobatidis (Bd), the causative agent of chytridiomycosis, and subsequently subjected them to either a long, cold winter (1.5°C for 91 days) or a short, mild winter (4.5°C for 61 days) under laboratory conditions. Agile frogs proved to be highly resistant to Bd as only 37% of Bd-exposed individuals became infected as determined before hibernation, and prevalence further decreased to 8% by the end of hibernation, with individuals showing very low infection intensity values. We observed lack of mortality in control and Bd-exposed groups also, in both types of winter. The two types of winter we simulated did not result in differing body mass loss either alone or in combination with experimental infection. In the Bd-exposed group, the two types of winter also did not cause differences in prevalence and infection intensity. However, among Bd-exposed frogs, individuals that were Bd negative when entering hibernation lost more body mass than their conspecifics that carried the fungus at the onset of overwintering. Based on our results, warming winter climate conditions, with or without Bd infection, do not decrease body mass and survival rate of hibernating agile frogs, and do not increase susceptibility of individuals to chytridiomycosis. It remains to be seen to what extent the relatively weak effects of milder winters can be generalized to other amphibians of the temperate climate zone.