Natural History of Reptilian Development: Constraints on the Evolution of Viviparity

OViParity, or egg-laying, is the dominant mode 01 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. In reptiles, viviparity is associated with a plethora of integrated morphological and physiological features that are not present in oviparous reptiles; these features are presumed necessary for successful embryonic develop­ ment in the oviduct (Packard et al. 1977, Guillette 1993). Early insights into the evolution of these reproductive fea­ tures were based on comparisons between typical oviparous and viviparous species. Some of the distin­ guishing features of the viviparous species examined were the major reduction or absence of an eggshell and the presence of some form of placentation (Weekes 1935 and included references). However, because the species used in these comparisons represented the extremes of a putative evolutionary sequence, their use as a model for elucidating the actual sequence or timing of the morphological and physiological changes attending the evolution of vivipari­ ty is limited. In fact, these observations are consistent with both a saltation model that posits that the characteristic features of viviparity arise suddenly and simultaneously, and a gradualist model that posits incremental evolution from one reproductive mode to the other (Blackburn 1992,1995).