Extended egg retention and its influence on embryonic development and egg water balance: Implications for the evolution of viviparity

Viviparity in squamate reptiles presumably evolves via gradual increases in the time that eggs are retained within the oviducts. We evaluated the extent and consequences of phenotypic plasticity in egg retention for the oviparous lizard Scelopours scalaris. This species not only exhibits facultative egg retention, but has close relatives that are viviparous. Thus, S. scalaris may possess reproductive features that are transitional between oviparity and viviparity. We tested the hypothesis that oviposition normally occurs when further retention would impair embryonic development. To do so, we determined the effects of extended egg retention on embryonic development and on egg water balance for a population in which females normally retain eggs to Dufaure and Hubert stages 31.0-33.5. Under substrate conditions that inhibited oviposition, females retained eggs to stage 39.5, or 0.5 stage units short of hatching. Extending egg retention did not retard the development of embryos relative to that of embryos in control eggs. Water did not accumulate in the extraembryonic compartments of retained eggs as it did in control eggs; all water uptake was associated with the embryo. The pattern of embryonic development within the retained eggs does not support the hypotheses (1) that oviposition occurs when gas exchange in utero is no longer sufficient to support the needs of the embryos, or (2) that increases in the duration of egg retention and decreases in eggshell thickness evolve concurrently. Our observations on water uptake additionally suggest that viviparity may evolve more easily in taxa that are able to preclude storage of excess water in the extraembryonic compartment of the egg while allowing retained embryos access to sufficient water for normal embryonic development.