An animal's relative body mass (i.e., mass adjusted for length or some other size dimension) is a potential indicator of its body condition, which in turn can influence demographic traits. In this study, I revisited data collected in 1969–1970 on garter snakes, Thamnophis sirtalis parietalis, at a communal hibernaculum in Manitoba, to test hypotheses generated by more recent literature. First, I compared fall-through-spring variation in relative mass to a similar study of another species of Thamnophis in a milder, Mediterranean climate and found a similar temporal pattern, suggesting common features across a wide latitudinal range. Female snakes had a higher relative mass than males and experienced lower decline in relative mass overwinter and through the spring. This suggests different hibernation strategies of the 2 sexes, possibly involving different temperatures or differences in metabolism, and also reflects the preoccupation of males upon spring emergence with searching for mates, at energetic cost while not feeding. Comparison of my data with a second study of the same species at another den in Manitoba indicated nearly identical patterns of loss of mass, based on recaptures of males throughout the spring, and also corroborated that study's finding that initially relatively heavy males remained at the den longer. Thus, males that have accumulated more capital the previous year presumably have more opportunities to mate. Although recapture intervals of most males were only a few days long, similar to the previous study, some males remained in the vicinity of the den for up to 5 weeks. These findings underscore the fundamental differences between males and females in the temporal distribution of their costs of reproduction.
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