GEOGRAPHIC-VARIATION IN REPRODUCTIVE CHARACTERISTICS AMONG CANADIAN POPULATIONS OF THE COMMON GARTER SNAKE (THAMNOPHIS-SIRTALIS)

We studied geographic variation in reproductive characteristics, especially litter size and neonate size, among several populations across Canada of the wide-ranging garter snake, Thamnophis sirialis. Gravid females differed significantly in body size among sites. However, even after we corrected for intersite differences in maternal body size, there were highly significant differences among populations in litter size and neonate size. Populations with large litters tended to have small progeny, but we found only weak evidence of a ''tradeoff'' between neonate size and litter size within populations. There was a conspicuous east-west difference in reproductive characteristics of snakes: snakes from eastern Canada were relatively small at maturity and produced large litters of very small young, whereas those from western Canada generally were large and produced smaller litters (for a given body size) of larger young. Although litter size and neonate size are both phenotypically plastic traits, the differences observed among populations in this study were often much larger than those expected from simple environmental influences. We, therefore, hypothesize that reproductive traits differ genetically among populations of T. sirtalis, at least between eastern and western Canada.