Introduction

Although oviparity characterises most living vertebrate species, including all birds and most reptiles, viviparity has evolved many times in animal phylogeny. Approximately 100 independent evolutionary origins of viviparity (herein defined to mean production of fully formed young rather than laying of eggs) have been identified within squamate reptiles (Blackburn, 1982; Shine, 1985). These multiple evolutionary origins provide excellent opportunities for comparative analyses of the implications of this change in incubation conditions, from the external nest to the female's oviduct, for the physiology of developing embryos. Similarly, studies of embryonic physiology, and especially of the effects of incubation conditions on the fitness of hatchlings, may clarify evolutionary models on the selective forces important in the shift from oviparity to viviparity.

In this chapter, I consider the relationship between physical influences on embryonic development on the one hand, and the evolution of viviparity on the other. Each of these factors could affect the other, so two main questions arise:

1) Have direct effects of incubation conditions on viability of hatchlings played a significant role in promoting or constraining the acquisition of viviparity in various vertebrate lineages, or the probability of subsequent re-evolution of oviparity? This question is the main focus of the present chapter.

2) What adaptations of embryonic and maternal physiology have resulted from changes in incubation conditions brought about by the evolution of prolonged uterine retention of eggs, ovoviviparity, and placentotrophy?