Facultative Placentotrophy and the Evolution of Squamate Placentation: Quality of Eggs and Neonates in Virginia striatula

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Abstract
Newborn size in the viviparous snake Virginia striatula is determined by female size, egg size, and placental nourishment and is correlated inversely with current fecundity. Of the two modes of embryonic nutrition used by this species, lecithotrophy and placentotrophy, an increase in vitellogenic nourishment results in a reduction in current fecundity, whereas placental nourishment does not affect current fecundity. Newborn composition is determined by two mechanisms: one is correlated with female size, and the other is regulated by the developing embryo. Placental uptake of water, potassium, total dry mass, and nitrogen correlates positively with post-parturient female mass. Post-vitellogenic gains in calcium and sodium, and egg losses of magnesium and phosphorus, are independent of female size and probably are influenced by embryonic metabolism. Placental transport of calcium, and perhaps also of nitrogen, in V. striatula is facultative and provides a post-vitellogenic supplement to embryonic nourishment. The significant components of this nutritional strategy, which may be characteristic of many viviparous squamates, are (1) supplementation of a primary source of embryonic nutrition (vitellogenesis) with a secondary non-compulsory placental source, and (2) no reduction in current fecundity as a result of the placental nutritional provision. Moreover, this combination of reproductive mode and embryonic nutritional pattern results in a functional-structural complex on which selection can act, should suitable conditions arise, to produce obligate placentotrophy and yolk reduction.