The evolution of viviparity in squamates has involved intermediate stages of egg retention. Reduction in the thickness of the eggshell, in relative clutch mass (RCM), and in clutch frequency would have facilitated the transition from oviparity to viviparity, while low temperatures are likely the ultimate selective force that promoted this evolutionary shift. We tested these ideas using the Sceloporus spinosus group. Because it is the sister clade of the viviparous Sceloporus formosus group, we predicted that members of the S. spinosus group would exhibit extended egg retention and other features associated with the evolution of viviparity. To test this idea, we examined the ability to retain eggs past the time of normal oviposition in the Sceloporus spinosus group and evaluated the association between egg retention and physiological and environmental factors in a historical context. Gravid females were collected from seven localities at a wide range of altitudes. We estimated the normal stage of embryos at oviposition and the stage at oviposition when females were induced to retain eggs under captive conditions. Stages of embryos varied within clutches; less developed embryos were usually dead and the most advanced embryos were usually alive. The maximum stage observed was therefore used as an index of egg retention for each clutch. The maximum embryonic stage at oviposition was contrasted with RCM, egg membrane thickness, and several climatic variables (temperature and precipitation) in a phylogenetic framework. Females exhibited the ability to retain eggs as predicted. Maximum stage at oviposition varied within same clutch, same locality, and among localities. Variation observed in the maximum stage at oviposition was not related to egg membrane thickness, RCM, or clutch frequency or to environmental temperature and precipitation. Instead, mapping the maximum stage at oviposition on a phylogeny of the S. spinosus group suggested that the invasion of high elevations was associated with an enhanced potential for longer periods of egg retention.
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