Reproductive ecology of female garter snakes (Thamnophis) in Southeastern British Columbia

Most current theories of life history evolution are based on the presumed existence of tradeoffs between reproduction and other activities, whereby increased investment in current reproduction results in a decrease in future contributions to fitness. Often these tradeoffs are framed in terms of costs. However, directly measuring the costs of reproduction for animals in the wild is difficult. An alternative approach is to compare the ecology of reproductive and nonreproductive animals in the same population. Because these animals differ only in their reproductive state, differences in their ecology can be used to identify potential costs and suggest which selective pressures are important in determining their pattern of life history. I compared the ecology of gravid and nongravid female garter snakes (Thamnophis sirtalis and T. elegans) from 1988-1990 at the Creston Valley Wildlife Management Area in Creston, B.C. In particular, I focussed on three main factors that were expected to vary between females in different reproductive conditions: (1) movements, (2) habitat use, and (3) thermoregulation. Rates of movement of gravid females were low during gestation, but increased following parturition until they were similar to those of nongravid females. This pattern is presumed to reflect the fact that gravid females are burdened by the developing embryos and have impaired locomotor ability. A consequence of impaired locomotion may be an increased risk of predation, which might explain the low movement rates of gravid females. However, gravid females were found to thermoregulate with higher mean body temperatures [special characters omitted] and lower variances than nongravid females and it is also possible that their movements are limited by the need to stay near suitable sites for thermoregulation. There were significant differences in the habitats used by gravid and nongravid females. Although both groups used areas characterized by high levels of cover (vegetation, rocks, or trees), there were marked differences in the habitat features of the sites selected. Gravid females remained primarily in rocky areas that were relatively rare on the study site. In contrast, nongravid females used a variety of habitats ranging from grasslands to forests. Predator avoidance may be a primary feature of habitat choice for both groups. However, gravid females appear to have an additional requirement for careful thermoregulation, and may be selecting sites that balance both needs.