Abstract
Most reptiles lay eggs, but many lizards and snakes give birth to live young by retaining the eggs within the oviducts until birth. The origin of live-bearing in reptiles is investigated here by posing a theoretical model and testing the model with published data. Predictions are (1) that live-bearing should evolve in cold environments and (2) that maternal care of the eggs facilitates the evolution of live-bearing. Analyses of data from over 1,000 species of lizards and snakes reveal that live-bearing has evolved recently at least 38 times, and an additional 60 species show the intermediate evolutionary stages (egg retention). Tests of these data support the predictions: Species which have recently evolved live-bearing or show the intermediate stages are much more likely to be found in cold climates or have maternal care of eggs than are squamates in general. The presentation in this paper differs from those of most earlier workers in distinguishing the cases in which live-bearing arises from the cases in which a live-bearing species radiates and invades new habitats.
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