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 indivi dual growth rates and its fitness consequences among several populations of an indeterminate grower, the western terrestrial garter snake Thamnophis e legans. Twenty years of mark-recapture data and six years of laboratory bre eding 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 examine d for their effects on individual growth. Two ecotypes were revealed whose distribution coincided with differences in prey availability. Individuals f rom populations that had continuous access to prey and water across years e xhibited fast growth, early maturation, high fecundity, and low adult survi val. In contrast, individuals from populations that experienced variable pr ey 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 hypothese s. Food availability may be a primary limiting factor to growth and subsequ ent life history traits, which is consistent with the maximization hypothes is. However, negative phenotypic correlations between growth and survival a nd between growth and reproduction may indicate an underlying negative gene tic correlation, consistent with the trade-off hypothesis. Field studies su ch as this one are useful for documenting the patterns of life history vari ation that occur in nature, identifying possible causes of such variation, and generating testable hypotheses for controlled experiments.