Potential targets for selection during the evolution of viviparity in cold-climate reptiles.

Viviparity (live-bearing) has evolved from oviparity (egg-laying) in more than 100 lineages of squamate reptiles (lizards and snakes). This transition generally has occurred in cool climates, where thermal differentials between eggs in the (cool) nest versus the (warm) maternal oviduct influence embryonic development, in ways that may enhance offspring fitness. To identify specific traits potentially under selection, we incubated eggs of a montane scincid lizard at conditions simulating natural nests, maternal body temperatures, and an intermediate stage (2-week uterine retention of eggs prior to laying). Incubation at maternal temperatures throughout incubation affected the hatchling lizard's activity level and boldness, as well as its developmental rate, morphology, and locomotor ability. A treatment that mimicked the initial stages of the transition toward viviparity had a major effect on some hatchling traits (locomotor speeds), a minor effect on others (tail length, total incubation period) and no effect on yet others (offspring behaviors). More generally, different aspects of the phenotype are sensitive to incubation conditions at different stages of development; thus, the evolution of reptilian viviparity may have been driven by a succession of advantages that accrued at different stages of embryogenesis.