Defense Date

6-26-2015

Graduation Date

2015

Availability

Immediate Access

Submission Type

dissertation

Degree Name

PhD

Department

Biological Sciences

School

Bayer School of Natural and Environmental Sciences

Committee Chair

Sarah Woodley

Committee Member

Nancy Trun

Committee Member

Kyle Selcer

Committee Member

Matthew Venesky

Keywords

Amphibian, Batrachochytrium dendrobatidis, Corticosterone, Kairomone, Stress, Temperature

Abstract

Amphibians are exposed to numerous environmental stressors, which may contribute to population declines being experienced by amphibians worldwide. Upon exposure to stressors, animals will release the glucocorticoid hormone, corticosterone (CORT), which typically mediates physiological and behavioral responses for resisting or coping with the stressor. Additionally, amphibians are at risk from a chytrid fungal pathogen, Batrachochytrium dendrobatidis (Bd). After Bd infection, amphibians may develop the potentially fatal disease chytridiomycosis. To better understand how CORT mediates responses to different stressors, and possible immunosuppressive effects of CORT on Bd susceptibility, I performed several experiments. First, I tested the predation stress hypothesis in terrestrial salamanders by exposing animals to acute and prolonged predator kairomones, and measuring anti-predator behaviors and plasma CORT. The predation stress hypothesis predicts that, upon predator cue exposure, both anti-predator behaviors and plasma CORT will increase. Despite expression of anti-predator behaviors from an acute exposure, there was no CORT response after acute or prolonged exposure to predator kairomones. Secondly, I tested the effects of prolonged thermal elevation on body mass, locomotory activity, plasma CORT, and immunity, in order to examine behavioral and physiological consequences of prolonged temperature shifts for amphibians. After 3 weeks at 24°C, male and female salamanders lost more body mass compared to animals housed at 17°C, a preferred temperature. There were also differences in immunity, with relatively more monocytes and fewer neutrophils and lymphocytes in animals held at 24°C. However, prolonged thermal elevation did not affect activity or plasma CORT when measured after 3 weeks at 24°C. Finally, I examined whether repeated plasma CORT elevations affected Bd susceptibility and chytridiomycosis development. Chronic CORT treatment simulates chronic exposure to stressors, which may reduce Bd resistance. I found that prior, repeated CORT elevations increased Bd infection abundance after Bd exposure, with no discernable effect on expression of chytridiomycosis symptoms. Additionally, I observed that prior CORT elevations increased incidence of skin sloughing after Bd exposure. Collectively, these studies show that CORT responses to putative stressors may be context-dependent. Furthermore, chronic CORT elevations may contribute to pathogen susceptibility in amphibians, and may partially explain the variable Bd resistance among different species.

Format

PDF

Language

English

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