Patterns of Maternal Provision and Embryonic Mobilization of Calcium in Oviparous and Viviparous Squamate Reptiles

Embryos of oviparous squamate reptiles obtain all organic and most inorganic nutrients from yolk; yolk provides 19-86% of hatchling calcium content. The remaining calcium is extracted from the eggshell. Yolk calcium provision to viviparous embryos also is variable and includes three patterns. The contribution of yolk to embryonic development for most viviparous squamates is similar to oviparous species, but the attenuated eggshell of viviparous species is a poor source of calcium because it lacks an outer layer of calcium carbonate, and embryos supplement yolk calcium via placental transfer. In a second pattern, yolk provides all organic nutrients and calcium. The final pattern occurs in viviparous species that are substantially placentotrophic and placental transfer accounts for most organic and inorganic nutrients, including calcium. The many independent evolutionary transitions to viviparity among squamates have inspired interest in a possible link to patterns of embryonic calcium nutrition. A prominent model predicts that the pattern of maternal provision and embryonic uptake of calcium unique to squamates facilitates the evolution of viviparity. A primary assumption of the model is that the evolution of viviparity precedes the evolution of calcium placentotrophy. An alternative model predicts that viviparity and placentotrophy evolve concurrently because mechanisms for nutrient provision and mobilization are not dependent on reproductive mode. These hypotheses have not been tested directly but review of the literature indicates that neither fully explains the diversity of squamate embryonic calcium nutrition. Viviparous species differ from oviparous species primarily in the timing of uterine calcium secretion and structure of eggshell calcium. Future studies should focus on the mechanisms that promote these differences