Multiple paternity, male reproductive success, hormonal regulation of fertility, and the evolution of life-histories

The life-history strategy that an individual exhibits is shaped by trade-offs between maximizing the number of viable offspring it produces and retaining enough resources to ensure its own survival. The balance between these two aspects is strongly influenced by the environmental conditions the individual experiences in its lifetime and can be affected by the effort an individual puts into each reproductive cycle. In this thesis, some of these reproductive trade-offs are studied in discrete populations of the western terrestrial garter snake, Thamnophis elegans. Populations of this species in the vicinity of Eagle Lake, California, have diverged along the two ends of the life-history continuum. Populations at the slow life-history end of the continuum can be found in high elevation meadow habitats, where they experience lower temperatures, lower predation, a high occurrence of parasitic infections, and much variability in food supply. In these populations, individuals mature and grow more slowly, have few offspring per litter, and reproduce infrequently, often skipping years in reproduction. On the other end of the continuum, populations of the fast life-history strategy are found at lower elevations along the lakeshore, and experience higher temperatures, higher rates of predation, and an abundant food supply. Individuals in these populations mature quickly and grow larger overall, have many offspring per litter, and reproduce annually. In this study, we qualified levels of multiple paternity and reproductive success between these two different life-history strategies as a measure of comparing possible mating strategies, and also tested for differences in the plasma concentrations of testosterone, a hormone that regulates fertility and mating behaviors. Multiple paternity, a situation in which multiple fathers can contribute to a single litter of offspring, was consistent between the two life-history strategies. Although individuals of the two different life-history strategies are achieving a similar number of fathers contributing to each litter, they are doing so by different means. In the slow life-history strategy individuals, it appears that females obtain more diverse litters through bet-hedging, or mating with multiple males in order to increase the genetic diversity of their offspring. In fast life-history populations, which have more offspring per individual than in the other population, the similar number of fathers contributing to a litter suggests that females are being more selective in which males they mate with. If they were bet-hedging, we would have expected to see more fathers in one litter of offspring than in the slow life-history individuals. We also found that reproductive skew is higher in the fast life-history litters, indicating that relatively few males are responsible for fathering a majority of the offspring within those populations. It is likely that these findings are related to the differences in circulating plasma testosterone, between the two populations. This higher concentration of testosterone could mediate the reproductive behaviors and processes that account for these significant differences in reproductive success and multiple paternity observed between these two divergent populations of garter snake. These finding are discussed in terms of life-history evolution, as is mediated by environmental constraints and physiological processes