EVOLUTION OF VIVIPARITY - VARIATION BETWEEN 2 SCELOPORINE LIZARDS IN THE ABILITY TO EXTEND EGG RETENTION

The evolutionary transition between oviparity and viviparity in squama te reptiles presumably occurs via a gradual increase in the duration o f egg retention, the production of thinner eggshells, and increases in the vascularity of maternal and embryonic tissues. The 'ease' of this transition may differ among taxa. For example, in the genus Sceloporu s, the scalaris species group contains both oviparous and viviparous s pecies, and female Sceloporus scalaris can extend egg retention facult atively in response to the absence of a suitable site far oviposition without impairing embryonic development. In contrast, the undulatus sp ecies group contains only oviparous species, and, while female Scelopo rus virgatus can extend egg retention, doing so retards embryonic deve lopment. I tested several hypotheses that would explain the greater ab ility of S. scalaris than S. virgatus to extend egg retention. In this study, female S. scalaris retained eggs for 19 d without affecting th e mortality of embryos, total developmental time, or dry mass of hatch lings. In contrast, when female S. virgatus retained eggs for 18 d, em bryos had very high mortality and eggs took significantly longer to ha tch than control (non-retained) eggs, although the dry mass of hatchli ngs was not affected. The ability of S. scalaris females to retain egg s with little negative effect on embryonic development was associated with relatively large chorioallantois, relatively thin eggshells, and relatively small clutch masses. These observations suggest that phylog enetic differences in the ability to extend egg retention may facilita te or constrain the evolution of viviparity in some lineages.