Standardized Criteria for the Recognition of Embryonic Nutritional Patterns in Squamate Reptiles

In squamates, nutrients for embryonic development can be derived from the egg itself or from the placental membranes. This paper critically evaluates criteria that have been used to distinguish developmental nutritional patterns, with an emphasis on viviparous squamates. Composition analysis of eggs and neonates is the most reliable means of quantifying sources of nutrients for development but tends to underestimate placental nutrient provision. Data on gestational changes in dry mass of the viviparous conceptus indicate only the predominant source of nutrients for development. Specialized allantoplacental morphology offers moderate evidence for placentotrophy, but the functional correlates of several placental features remain unknown. Radiotracer studies reveal the capacity for placental transport but not its importance to embryonic nutrition. Gestational changes in wet mass of the conceptus reveal little about nutrient sources in relatively lecithotrophic forms, as do comparisons of the relative size of the ovum with the fetus or the mother. Future reports should consider the nature and quality of evidence for inferences about developmental nutritional modes.