Two neonate Enhydris enhydris emerging from the birth canal at the same time.
Most homalapsid snakes are tropical yet viviparous.
The usual answer to the question of which came first the egg or the neonate in lizards and snakes is usually answered as the egg. Squamates reproduce either by laying eggs (oviparity) or by giving birth to live young (viviparity). Most squamates (about 80–85%) are oviparous, and the reproductive mode is generally phylogenetically constrained [e.g. all homalopsids are considered to be viviparous and all anoles are oviparous. Nonetheless, some families (e.g. Elapidae, Natricidae and Viperidae) and genera (e.g. Eryx, Liolaemus and Pseudechis) contain both viviparous and oviparous species. Transitions between oviparity and viviparity are even present in different populations of the same species (e.g. Zootoca vivipara, Saiphos equalis and Helicops angulatus) although such variability is rare. Oviparity is traditionally considered to be the ancestral mode of squamate reproduction viviparity is thought to have evolved independently in at least 30 lineages of snakes and in more than 100 lineages of squamates, but recently it was suggested that viviparity may have evolved first.
In a forthcoming paper in Global Ecology and Biogeography, Feldman et al. (2015) test two prevailing hypotheses regarding the biogeography of reptile reproductive modes to evaluate the selective forces driving the evolution of viviparity in snakes. The cold climate hypothesis posits that viviparity is selected for in cold climates, whereas the climatic predictability hypothesis predicts that viviparity is advantageous in seasonal climates. They collated detailed distribution maps and reproductive mode data for 2663 species of the world’s terrestrial alethinophidian snakes; studied the relationship between snake reproductive mode and environmental predictors; applied an ecological and an evolutionary approach to study snake reproductive mode by performing the analyses at the assemblage level and species level
Respectively; and analyzed our data at the global and continental scales to learn whether tendencies to viviparity are similar world-wide.
The authors found strong support for the cold climate hypothesis and the assumption that viviparity is an adaptation to cold environments. There was little support for the climatic predictability hypothesis. Nonetheless, viviparous species are not restricted to cold environments. They conclude that viviparity is adaptive in cold climates, but not necessarily in unpredictable/ seasonal climates. Current distributions may not reflect the climate at the time and place of speciation. The authors suggest many viviparous snakes inhabiting warm climates are members of lineages that originated in colder regions, and their occurrence in maladaptive environments is a result of phylogenetic conservatism.
Feldman, A., Bauer, A. M., Castro‐Herrera, F., Chirio, L., Das, I., Doan, T. M., ... & Meiri, S. (2015). The geography of snake reproductive mode: a global analysis of the evolution of snake viviparity. Global Ecology and Biogeography.