Browsing by Author "Schoenle, Laura A."

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    Coping with Chronic Infection: The Role of Glucocorticoid Hormones in Mediating Resistance and Tolerance to Parasites
    Schoenle, Laura A. (Virginia Tech, 2017-07-10)
    Parasitic infections are ubiquitous, but the consequences to hosts can vary substantially. Variation in the consequences of infection can be related to individual differences in the use of two parasite defense strategies, resistance and tolerance. Resistance entails reducing parasite burden by removing parasites or restricting parasite reproduction. Tolerance involves minimizing the costs associated with a given parasite burden. Genetic variation, environmental conditions, and life history stage can contribute to variation in resistance and tolerance, but the physiological mechanisms that underlie investment in each strategy are not well understood. I proposed that glucocorticoid hormones, which mediate responses to challenges in the physical and social environment in vertebrates, might alter host investment in resistance and tolerance (Chapter I). Glucocorticoids influence a suite of physiological processes including immune function, resource allocation, and tissue growth, all which could alter resistance and tolerance. Using a combination of observational and experimental studies, I test the hypothesis that glucocorticoids mediate resistance and tolerance to infection in red-winged blackbirds (Agelaius phoeniceus) infected with Haemosporidians, including malaria (Plasmodium) and malaria-like (Haemoproteus and Leucocytozoon) parasites. I performed a medication experiment (Chapter II) to identify the physiological consequences of Haemosporidian infection and explored the relationships between glucocorticoids and parasite resistance and tolerance in both an observational field study and a hormone manipulation experiment (Chapters III and IV). Medication treatment effectively reduced Plasmodium burden, increased hematocrit and hemoglobin, and reduced the rate of red blood cell production (Chapter II). In an observational field study (Chapter III), red-winged blackbirds with higher plasma glucocorticoid concentrations maintained higher hematocrit than expected for their parasite burdens, suggesting a positive association between glucocorticoids and tolerance. In this study, I found no support for a relationship between glucocorticoids and resistance. However, experimental elevation of glucocorticoids (Chapter IV) yielded nearly opposite results: the higher of two doses of glucocorticoids increased Plasmodium burdens and caused a decrease in body mass with increasing parasite burden, indicative of a decrease in tolerance. I discuss possible causes of the differences in our observational and experimental studies and the implications of my work for future studies of individual variation in parasite tolerance (Chapter V).
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    Experimental Food Restriction Reveals Individual Differences in Corticosterone Reaction Norms with No Oxidative Costs
    Lendvai, Adam Z.; Ouyang, Jenny Q.; Schoenle, Laura A.; Fasanello, Vincent; Haussmann, Mark F.; Bonier, Frances; Moore, Ignacio T. (PLOS, 2014-11-11)
    Highly plastic endocrine traits are thought to play a central role in allowing organisms to respond rapidly to environmental change. Yet, not all individuals display the same degree of plasticity in these traits, and the costs of this individual variation in plasticity are unknown. We studied individual differences in corticosterone levels under varying conditions to test whether there are consistent individual differences in (1) baseline corticosterone levels; (2) plasticity in the hormonal response to an ecologically relevant stressor (food restriction); and (3) whether individual differences in plasticity are related to fitness costs, as estimated by oxidative stress levels. We took 25 wild-caught house sparrows into captivity and assigned them to repeated food restricted and control treatments (60% and 110% of their daily food intake), such that each individual experienced both food restricted and control diets twice. We found significant individual variation in baseline corticosterone levels and stress responsiveness, even after controlling for changes in body mass. However, these individual differences in hormonal responsiveness were not related to measures of oxidative stress. These results have implications for how corticosterone levels may evolve in natural populations and raise questions about what we can conclude from phenotypic correlations between hormone levels and fitness measures.
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    Understanding metrics of stress in the context of invasion history: the case of the brown treesnake (Boiga irregularis)
    Claunch, Natalie; Moore, Ignacio T.; Waye, Heather; Schoenle, Laura A.; Oakey, Samantha J.; Reed, Robert N.; Romagosa, Christina (2021-02-09)
    Invasive species can exert rapid depletion of resources after introduction and, in turn, affect their own population density. Additionally, management actions can have direct and indirect effects on demography. Physiological variables can predict demographic change but are often restricted to snapshots-in-time and delayed confirmation of changes in population density reduces their utility. To evaluate the relationships between physiology and demography, we assessed metrics of individual and demographic stress (baseline and 1-h corticosterone (CORT), body condition and bacterial killing ability) in the invasive snake Boiga irregularis on Guam collected in intervals of 10-15 years. We also assessed potential discrepancies between different methods of measuring hormones [radioimmunoassay (RIA) versus enzyme immunoassay (EIA)]. The magnitude of difference between RIA and EIA was negligible and did not change gross interpretation of our results. We found that body condition was higher in recent samples (2003 and 2018) versus older (1992-93) samples. We found corresponding differences in baseline CORT, with higher baseline CORT in older, poorer body condition samples. Hormonal response to acute stress was higher in 2018 relative to 2003. We also found a weak relationship between circulating CORT and bacterial killing ability among 2018 samples, but the biological significance of the relationship is not clear. In an effort to develop hypotheses for future investigation of the links between physiology and demography in this and other systems, we discuss how the changes in CORT and body condition may reflect changes in population dynamics, resource availability or management pressure. Ultimately, we advocate for the synchronization of physiology and management studies to advance the field of applied conservation physiology.