Degree Type

Dissertation

Date of Award

2009

Degree Name

Doctor of Philosophy

Department

Ecology, Evolution, and Organismal Biology

Major

Ecology and Evolutionary Biology

First Advisor

Anne M. Bronikowski

Abstract

Life-history theory predicts that optimal life histories are shaped by trade-offs among traits expressed in different evolutionary and ecological contexts. Fast growth and high reproduction are predicted to trade off with lifespan, with the result that "fast-living" organisms will have shorter lives, while "slow-living" organisms will have longer lives. However, the generality of this theory for both determinately and indeterminately growing species, and the physiological mechanisms that underlie trade-offs, are poorly understood. We tested life-history theories of aging in natural populations of fast- and slow-living ecotypes of the indeterminately growing garter snake Thamnophis elegans. Long-term data on age-specific reproduction in both ecotypes revealed that neither showed signs of reproductive senescence even at the latest ages. Instead, both ecotypes continued to increase reproduction with age/size throughout life, with the fast-living ecotype increasing reproduction at a greater rate. These findings suggest that fast-living does not come at a cost to reproductive performance later in life. However, as the fast-living ecotype is known to exhibit shorter adult median lifespan than the slow-living ecotype in the field--a phenomenon proceeding from either extrinsic or intrinsic sources of mortality--we also tested for potential physiological mechanisms for trade-offs. Specifically, we tested for differences in immune defense between the two ecotypes, according to the ecoimmunological hypothesis that suggests fast-living populations should invest more in innate immunity than slow-living populations. As predicted, the fast-living ecotype showed higher levels of constitutive innate immunity than the slow-living ecotype. We also tested for differences in endocrine function between the two ecotypes. We predicted that the fast-living ecotype would exhibit higher levels of plasma insulin-like growth factor-1 (IGF-1), as high IGF-1 signaling is known to stimulate growth and reproduction at a cost to lifespan in model organisms. We found ecotype differences with respect to gravidity, body size, and annual climate, indicating that fast-living snakes may experience cumulatively higher levels of IGF-1 than slow-living snakes over their lifetimes. Thus IGF-1 may be an important mediator of life-history trade-offs in natural populations. Finally, through sequencing IGF-1 mRNA in a variety of reptile species, we found evidence that IGF-1 has been duplicated several times in the reptile lineage, which may have important for the physiology of life-history traits. All of these findings are discussed with reference to their contribution to the study of life-history evolution in general, with emphasis on the unique insights to be gained from the study of indeterminately growing species.

Copyright Owner

Amanda M. Sparkman

Language

en

Date Available

2012-04-28

File Format

application/pdf

File Size

188 pages

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