Life-history theory is based on the idea that trade-offs exist in allocation of resources. The same energy cannot simultaneously fuel growth and reproduction or simultaneously increase size and number of offspring. Many genetic, physiological, and environmental variables influence how trade-offs manifest and whether they are detectable through phenotypic correlations. In most squamates, the absence of parental care enables estimation of reproductive investment using mass of offspring. This, as well as their diverse ecologies, makes squamates an attractive system for empirical tests of life history theory. Approximately 20% of squamates are viviparous, and long gestation periods limit most to one litter per year. Carrying offspring until an advanced developmental stage may increase the burden of the litter, and female abdominal volume may constrain litter size. These factors should intensify selection on female tactics of life history and enhance detectability of trade-offs. We compare life history of females in two species within the Sceloporus grammicus species complex, viviparous Mexican lizards from varied habitats. Each inhabits one of the primary ecosystems used by this taxon (chaparral and forest). Litter sizes were lower in the chaparral population, when adjusted for body size of the mother. A trade-off of size versus number of offspring was detected in the forest population but not in the chaparral population. The chaparral population varied more in average neonate mass between years, which may relate to local extinctions, likely linked to climate change, in Mexican montane lizards. Regardless of whether these differences represent adaptation or plasticity, our findings emphasize the importance of environmental influence on trade-offs in life history.