Reptile viviparity is strongly associated with cool climates in interspecific comparisons, but intraspecific comparisons provide an even stronger opportunity to identify causal pathways and distinguish facultative (plastic) effects from canalized adaptive responses. I documented reproductive traits in the Australian three-lined skink (Bassiana duperreyi), an oviparous scincid lizard from alpine south-eastern Australia, and manipulated thermal regimes of captive gravid females and of their eggs to identify proximate causes of life-history variation. Embryonic developmental stages at oviposition were higher in eggs laid by females from high-elevation (1615-1670 m asl) populations than in eggs produced by females from lower elevation (1240 m asl). Furthermore, in the laboratory, experimentally-imposed low maternal body temperatures delayed oviposition and resulted in more advanced embryonic development at oviposition. Warm conditions both in utero and in the nest increased hatching success and offspring body size. In total, my results suggest much of the elevation-related variation in life-history traits among populations of B. duperreyi reflects plastic responses to different thermal environments, rather than canalized genetic differences.