The evolutionary ecology of life history variation in the garter snake Thamnophis elegans

The purpose of this study was to document the extent of variation in individual growth rates and its fitness consequences among several populations of an indeterminate grower, the western terrestrial garter snake Thamnophis elegans. Twenty years of mark-recapture data and six years of laboratory breeding data provided evidence of large differences among six populations in individual growth rates and subsequent reproductive maturation, fecundity, and survival. Weather, diet composition, and prey availability were examined for their effects on individual growth. Two ecotypes were revealed whose distribution coincided with differences in prey availability. Individuals from populations that had continuous access to prey and water across years exhibited fast growth, early maturation, high fecundity, and low adult survival. In contrast, individuals from populations that experienced variable prey availability exhibited slow growth, late maturation, low fecundity, and high adult survival. This growth rate variation was examined in the context of two competing explanations: the maximization and optimization hypotheses. Food availability may be a primary limiting factor to growth and subsequent life history traits, which is consistent with the maximization hypothesis. However, negative phenotypic correlations between growth and survival and between growth and reproduction may indicate an underlying negative genetic correlation, consistent with the trade-off hypothesis. Field studies such as this one are useful for documenting the patterns of life history variation that occur in nature, identifying possible causes of such variation, and generating testable hypotheses for controlled experiments.