Reptilian viviparity: past research, future directions, and appropriate models.

Squamate reptiles represent an ideal group for studies of viviparity, because they have evolved this reproductive pattern frequently, relatively recently, and at low taxonomic levels. A phylogenetic approach shows particular promise in helping us interpret anatomical, physiological, and ecological diversity. This review summarizes four major categories of active investigation: (1) reproductive anatomy and physiology; (2) placental structure and function; (3) reproductive endocrinology; and (4) reproductive and physiological ecology. Evolutionary reconstructions suggest that on many occasions viviparity has evolved concomitantly with functional placentation, through reduction of the shell membrane and hormonal modifications that prolong gestation. Studies of placentotrophic clades as well as reproductively bimodal species offer great potential for explaining the evolution of viviparity and placentation. However, live-bearing squamates are reproductively diverse, and appear to have solved physiological problems associated with viviparity by a variety of mechanisms. Consequently, studies on one or a few squamate species appear increasingly unlikely to yield all-inclusive explanations. Future studies and analyses should abandon assumptions of universal physiological mechanisms and a single historical sequence, in favor of the documentation of diversity in phylogenetic and quantitative terms.