Sexually mature female red-sided garter snakes (Thamnophis sirtalis) were collected from Manitoba, Canada, in the springs of 1983, 1984, 1985, and 1987. Females were brought into the laboratory to examine the relationship among body mass at emergence from hibernation, ovarian development, and spring mating. A large difference in the incidence of ovarian development was observed among the four years; vitellogenesis was absent in >75% of adult females in 1983. A multivariate analysis of covariance of body mass was performed on data from all years with snout-vent length (SVL) as the covariate; body mass (BM) was then expressed as adjusted BM to control for effects of SVL on BM. The analysis tested within and among year differences in the adjusted BM on emergence of vitellogenic and nonvitellogenic females. Within each year, mean adjusted BM at emergence of females that became vitellogenic was significantly greater than that of females that did not become vitellogenic. However, differences in the incidence of vitellogenesis among years could not be attributed to differences in the adjusted BM at emergence. There was no interaction between mating and the adjusted BM on vitellogenesis. Finally, we examined the role of climate in regulating reproduction and the adjusted BM of this species. There were no consistent climatic trends that could explain the differences in the adjusted BM or vitellogenesis observed among years; however, a cooler summer in 1982, the summer preceding the year of low reproductive output, was observed. These findings suggest that in some years females may be able to overcome deficits in BM and reproduce.
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