When chytrid doesn't kill : how it spread in túngara frogs and how females might avoid it
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My dissertation aims to examine how pathogen-induced stress might affect reproductive behaviors such as sexual communication, mate choice, and reproductive success. To pursue this topic I studied the interaction between an emergent infectious disease, chytridiomycosis, caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd), and the tropical tungara frog as its host. The first goal of the dissertation was to understand the basic epidemiology of chytridiomycosis in this wide-spread tropical lowland anuran. From 2010 to 2015, I sampled annually for the presence of B. dendrobatidis in populations of tungara frog along an approximately 750 km transect, ranging from the mountains of western Panama to inside the Darien Gap. Highland populations in western Panama were already infected with B. dendrobatidis at the start of the study. In central Panama, I collected the first positive samples in 2010, and by 2014, I detected B. dendrobatidis in samples from remote sites in eastern Panama (Darien National Park) where B. dendrobatidis had not been documented before. I discuss the importance of studying B. dendrobatidis in lowland species, which may serve as potential reservoirs and agents of dispersal of B. dendrobatidis to highland species that are more susceptible to chytridiomycosis. The second goal of my thesis was to understand how B. dendrobatidis might influence frog reproductive behavior. Some anuran species, including the tungara frog, seem to be tolerant to chytridiomycosis, but for others it is lethal. Tolerant species carry the pathogen, but do not exhibit symptoms of chytridiomycosis and their populations are not declining. Although chytridiomycosis might not be lethal for such tolerant species, it might nonetheless have other long-term effects. Such sub-lethal effects of chytridiomycosis have received little research attention. I examined how the potential pathogen-stress effects induced by B. dendrobatidis influence reproductive behavior such as sexual communication, mate choice, vigor, and reproductive success in the tungara frog. I tested the hypothesis that B. dendrobatidis influences the male mating call, and that females can use mating call cues to assess B. dendrobatidis infection. I performed female phonotaxis experiments to determine if males infection with B. dendrobatidis influences female mate choice, and I determined if there is a cost of the response to the infection in offspring number and development. Overall, the research presented here improves our understanding of the physiological and behavioral trade-offs confronted by a species during response to a pathogen and shows that B. dendrobatidis can have long-term population-level effects in tolerant species that are not severely affected by the disease. In addition to frogs and salamanders, emerging infectious diseases affect a number of other important lineages including honeybees, bats, birds, and humans. Study of the effects of non-lethal infections might therefore have more general application towards our understanding of the interactions between devastating pathogens and their wildlife hosts.