Roles of corticosterone in the development and physiological ecology of <italic>Rana pipiens</italic> tadpoles and the disruption of this endocrine system by organochlorine contamination.

Global contamination with organochlorine compounds (OCs) has posed developmental and reproductive problems in wildlife. However, little is known about the impact of OCs or other pollutants on amphibians, despite mounting concerns about amphibian conservation. Organochlorine compounds have disrupted the adrenal (or interrenal) endocrine systems of many taxa, including amphibians. However, the consequences of such disruption for amphibian fitness are unknown, as the role of corticosterone in amphibian physiological ecology has been largely unexplored. I studied the population- and individual-level effects of exposure to polychlorinated biphenyls (PCBs) on amphibians. I conducted surveys of anuran populations in PCB-contaminated wetlands in southwestern Michigan and exposed tadpoles of the northern leopard frog (Rana pipiens) to a PCB congener, 77-TCB, in the laboratory. While the relative densities of field populations showed no relationship to sediment PCB levels, laboratory exposures significantly affected growth, survivorship, behavior, competitive performance, and whole-body corticosterone content of R. pipiens tadpoles. I explored the potential importance of alterations in corticosterone content and found that small changes in corticosterone content affected growth, development, morphology, and the response to adrenocorticotropic hormone (ACTH) in R. pipiens. I then identified a role for corticosterone in mediating the negative growth response of R. pipiens tadpoles to increased density. Both high tadpole density and limited resources were associated with elevated corticosterone content, and both conditions negatively affected growth and development rate. However, only the growth response to elevated density was apparently mediated by corticosterone, as this response was reversed by treatment with metyrapone, an inhibitor of glucocorticoid synthesis. Corticosterone responsiveness of R. pipiens was further explored by determining the sensitivity of the hypothalamo-pituitary-interrenal (HPI) axis to a generalized stressor and to ACTH injection at different developmental stages. Responsiveness differed over development, suggesting that the response of tadpoles to natural stressors also may vary with developmental stage. In summary, disruption of the amphibian BPI axis by OC compounds or other pollutants is likely to have significant consequences for amphibian populations, as corticosterone may be an important mediator of the behavioral, morphological, growth, or developmental responses of tadpoles to their environments.Ph.D.Animal PhysiologyBiological SciencesEcologyEnvironmental scienceHealth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132793/2/9990899.pd