Organisms undergo environmental fluctuations and early stages (embryonic life) are especially sensitive to perturbations that can modify developmental trajectories. Embryonic thermal sensitivity is a universal feature in vertebrates and may have been a major selective force in the evolution of parental care (Farmer's hypothesis, 2000). Besides, embryonic development is a complex process and environmental influences can have contrasted effects depending on the timing of exposure. However this variability is insufficiently considered in evolutionary ecology and in studies on parental care. Maternal environment can constitute an interface between the external environment and embryos. Regulation of thermal developmental conditions is widespread in vertebrates. Since ectotherms produce negligible metabolic heal, they have developed a diversity of behavioral and physiological strategies lo buffer the embryos from thermal variations. Prenatal parental care is associated with a shift in maternal behavior with an increase of thermoregulalory activities, as well as strong physiological and energy modifications. We examined the importance of maternal regulation of prenatal conditions in Iwo snakes species with contrasted reproductive modes, one oviparous (Children's python, A. childreni] and the other viviparous (the aspic viper, V. aspis). The originality of this work stems from the explicit consideration of the dynamic nature of embryonic sensitivity and maternal constraints. Our results demonstrated (1) the existence of a stage-dependent thermal sensitivity, with a higher sensitivity lo perturbations early in development; and (2) the importance of accessing preferred température during that period. In parallel, we evidenced a strong maternal investment (behavioural thermoregulation), and associated constraints (risk taking increase, water loss, energetic costs). Our data supported Farmer's theory and the importance of maternal regulation of early stages. This stage-dependent gradient in thermal sensitivity may have been a major selective force in the evolution of increased parental care (transition toward viviparity, eggs care).
