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Enforced winter inactivity affects life histories of cool-climate squamates in two ways: (1) a trade-off between oviparity and viviparity favoring the predominance of viviparity in cool climates; (2) reduction in frequency of reproduction in females. In this study we determined annual variation in reproductive traits in a cool-climate population of a viviparous lizard (Elgaria coerulea), as well as year-to-year patterns of breeding (versus skipping reproduction) of individual adult females. We collected reproductive data during a mark–recapture study in southeastern British Columbia, Canada. Gravid females were captured in the field and subsequently gave birth under seminatural conditions in the laboratory, whereupon the resulting newborns were measured. There were no differences in snout–vent length (SVL), relative tail length, or age of gravid females among years. Only head width, mass, and condition of newborns varied among years. Most litters consisted of >80% live young, and the proportion of live young in a litter increased with newborn condition and SVL. Litter size increased with maternal SVL, and mean newborn SVL was highest with medium-sized maternal relative tail length. Most females (87%) did not reproduce annually but had one or more years between reproductive events. There was no effect of year on whether a given female was reproductive, but larger females and females with intact tails were more likely to be pregnant in any year. The possible consequences of skipping reproduction in a given year include trading off immediate investment in offspring against increased growth in the current year and higher potential future reproductive output. Testing such hypotheses will be challenging but would contribute significantly to our understanding of the evolution of reproductive effort in both squamates in particular, and animals in general. [ABSTRACT FROM AUTHOR]