Date of Award
Doctor of Philosophy
Ecology, Evolution, and Organismal Biology
Ecology and Evolutionary Biology
Anne M. Bronikowski
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.
Amanda M. Sparkman
Sparkman, Amanda M., "The evolution of life histories in garter snakes: Reproduction, aging, and the physiology of trade-offs" (2009). Graduate Theses and Dissertations. 10490.