Brent Horton

Date of Award

2007

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Zoology

Advisor

Rebecca Holberton

Second Committee Member

William Glanz

Third Committee Member

William Halteman

Abstract

Corticosterone is a multifaceted hormone that can influence physiological and behavioral adjustments to changes in energy demand. Across and within species, breeding birds show great variation in the secretion of corticosterone, the primary avian energy-regulating steroid. Two factors are hypothesized to influence variation in corticosoterone secretion in birds: 1) the unpredictability of the breeding environment and 2) the degree of parental care. To examine how corticosterone secretion varies as a function of parental care, while controlling for the breeding environment, I measured baseline corticosterone and the adrenocortical response to capture stress in the polymorphic white-throated sparrow. Within each sex, this species exhibits morphspecific variation in reproductive behavior, including differences in parental care, in the same breeding environment. I found that baseline corticosterone as well as the adrenocortical response to capture stress varied with morph, sex, and stage of breeding, and that variation in corticosterone secretion correlated with variation in reproductive behavior. I observed a particularly interesting pattern in these sparrows. Within males, the two morphs differed in their nestling provisioning behavior, and their baseline corticosterone levels differed when they were provisioning nestlings. To test the hypothesis that variation in baseline corticosterone influences morph-specific differences in male provisioning behavior, I manipulated baseline corticosterone levels in free-living males during the nestling stage and measured the effects of corticosterone treatments on male provisioning rates. To do so, I first developed a technique for creating biologically relevant manipulations of baseline corticosterone in these birds during an experiment on captive white-throated sparrows. In the technical study, I showed that administration of exogenous corticosterone and RU486, a glucocorticoid receptor antagonist, via intraperitoneal osmotic pumps was an effective hormone delivery method for use in these sparrows. Using this technique on free-living white-throated sparrow males, I found that experimental increases in baseline corticosterone reduced male provisioning behavior, while blocking the effects of corticosterone with RU486 increased male provisioning behavior. The combination of these correlative and experimental results support the hypothesis that variation in baseline corticosterone directly or indirectly influences morph-specific differences in the parental behavior of male white-throated sparrows. Specifically, high baseline corticosterone levels can inhibit male provisioning behavior, while blocking corticosterone's effects can enhance male provisioning behavior. The contributions of this research to understanding a potential role for baseline corticosterone in mediating variation in parental care in this and other species is discussed.

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