Date of Award

2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Abstract

Stress physiology in bats can be studied through various non-invasive methods, with hair cortisol levels being a particularly valuable measure. Hair cortisol analysis provides insights into the long-term stress levels and physiological states of bats, as it captures cortisol accumulation over time. This method helps researchers understand how different stressors and environmental factors impact bat populations. In mydissertation, I focused on assessing the validity and reliability of hair cortisol measurements in bats and explored the implications of these measurements for understanding bat stress physiology. In the first chapter, I evaluated the use of commercial enzyme immunoassay kits to measure cortisol (CORT) in the hair of big brown bats ( Eptesicus fuscus ). I tested several methodological variables, including extraction efficiency, storage conditions, and differences in CORT content between dorsal and ventral hair samples. My results demonstrated accurate antibody binding across the standard curve range, with consistent CORT measurements achievable using only 3 mg of hair. I found that subsamples extracted once in methanol yielded about 80% of the total CORT. Storage conditions impacted CORT content in samples from bats stored for long periods, but no significant differences were found between dorsal and ventral hair samples. Based on these findings, I recommend shorter-term storage up to one year, limited storage in the pulverized state, and consistent sampling locations on the body. In the second chapter, I investigated hair cortisol levels in three bat species—big brown bats, eastern red bats ( Lasiurus borealis ), and Indiana bats ( Myotis sodalis )—captured over three summers in northeastern Missouri. Using validated immunoassays, I examined species and demographic variations in hair CORT levels, using information-theoretic modeling approach. I found that hair CORT levels were influenced by mass, year, and their interaction across all species. Among reproductive females, species and year explained variation, with lower hair CORT levels for Myotis sodalis and Lasiurus borealis compared to Eptesicus fuscus . Within Eptesicus fuscus , differences in body mass were positively but weakly associated with hair CORT. These data provide baseline values for hair cortisol in temperate-zone bats, although interpretation is complicated by the incomplete understanding of molt timing in these species. In the third chapter, I explored the relationship between short-term and long-term glucocorticoid (GC) measures and immune function in big brown bats ( Eptesicus fuscus ). I examined the neutrophil to lymphocyte ratio (NLR) in relation to hold time before blood sampling and hair cortisol levels. Using an information-theoretic modeling approach, I analyzed data on 243 bats and found that sex predicted NLR, with females having higher NLR than males, but hold time did not. In a subset of the same bats for which I had both GC measures, hair cortisol was a weak and positive predictor of NLR, suggesting that longer-term stress is a predictor of shorter-term stress responses. For young bats, hair cortisol, which reflects both prenatal and postnatal stress exposure , did not predict NLR. Overall results suggest a muted white blood cell response to elevated GCs in both adults and juveniles, with in that the average time in captivity prior to blood sampling (~30 minutes) did not elevate blood cortisol sufficiently to induce a white blood cell response that was reflected in the NLR.

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