Date of Award
Doctor of Philosophy
Stephan J Schoech
David A Freeman
Matthew J Parris
Michael H Ferkin
Analysis of twenty years of historical reproductive data from our study population of Florida Scrub-Jays reveals a quadratic relationship between breeder age and number of fledglings produced, with the youngest and oldest birds having the lowest reproductive success. Such a decrease in reproductive performance in older birds, despite the likely benefits of increased experience, is often attributed to senescence. My dissertation research considered reproductive endocrinology, stress physiology, immunology, egg hatchability, life history trade-offs, and parental behavior. The oldest and youngest females have greater rates of hatching failure than middle-aged females. Results also indicate that circulating levels of reproductive hormones decrease with age in male Florida Scrub-Jays, but no such pattern was seen in females. Further, young and middle-aged males responded to a gonadotropin-releasing hormone challenge of their hypothalamo-pituitary-gonadal axis with a significant response of testosterone production, whereas old males did not. There were no significant differences among different aged birds in baseline stress hormone levels (corticosterone), however, the youngest and the oldest birds had a significantly greater magnitude of stress response than middle aged birds. There were also age-related differences in paternal care, as older male breeders consistently deliver more food to nestlings; however, there were no apparent physiological costs to do so (assessed with measures of corticosterone levels, innate immunocompetence, and body condition). The importance of biparental care was evinced as female breeders with mates who delivered more food strayed from the nest less often, and for shorter periods of time and female breeders that spent more time away from the nest were less likely to fledge young. Finally, a disease epidemic killed approximately 40% of the jays in our study population in 2008. Interestingly, older birds did not suffer greater mortality than any other age group, although surviving individuals all showed greater innate immune capabilities before the epidemic, providing a rare opportunity to show natural selection in a population of free-living animals. Combined, the results from the experiments and analyses of long-term demographic data show a comprehensive picture of age-related reproductive success, behavior, and underlying physiology in the Florida Scrub-Jay.
Dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.
Wilcoxen, Travis Eli, "Reproductive senescence in the Florida Scrub-Jay (Aphelocoma coerulescens)" (2010). Electronic Theses and Dissertations. 59.