Oviparity, or egg-laying, is the dominant mode of reproduction among vertebrates. Nevertheless, viviparity, the retention of the egg within the reproductive tract until embryonic development is complete, characterizes almost all mammals; it has also had at least 150 independent origins within the fishes, amphibians, and reptiles (Shine 1985, Blackburn 1992, Wourms and Lombardi 1992). These multiple origins suggest pervasive benefits to viviparity across a wide range of taxa, life histories, and habitats. In the squamate reptiles (lizards and snakes), for example, viviparity is the most common reproductive mode in cold climates, and recent origins of viviparity in this group are also associated with cold climates (Shine 1985). Gravid females in cold climates can thermoregulate to keep embryos warmer than they would be in a nest, thus enhancing development. Thermo-regulation by the female may thus ensure that birth occurs at the appropriate season or even that reproduction is successful at all. Viviparity is also advantageous in very wet or dry habitats, for example, because it obviates the need for females to find suitable sites in which to lay their eggs.
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Andrews et al. (2000) studied this question.