Small changes, big benefits: testing the significance of maternal thermoregulation in a lizard with extended egg retention

Early life stages are particularly vulnerable to environmental perturbations. Embryonic thermal sensitivity might be a driving force in the emergence of prenatal parental care, such as maternal thermoregulation. Viviparity has emerged on repeated occasions among squamate reptiles, and two main evolutionary hypotheses based on maternal thermoregulation have been proposed to explain these transitions, namely the 'cold climate hypothesis' and the 'maternal manipulation hypothesis'. Squamate embryos typically face important daily temperature fluctuations either in the nest or within the maternal body, but most experimental studies on development have relied on constant temperature. Therefore, we may have only limited insight on the effect of maternal thermoregulation on embryo development. We manipulated thermal conditions to compare the influence of a typical maternal temperature cycle (M) or nest thermal conditions (N) both during gravidity and during incubation in the oviparous form of a reproductively bimodal squamate (Zootoca vivipara). Although the two treatments had a similar mean temperature, we found that M treatment accelerated development, notably when applied during gravidity. Only limited effects were found when considering offspring phenotype and performance. Overall, our results suggest that small changes in thermal conditions can have a strong impact on reproductive phenology and might be a proximate target in the emergence of egg retention and, ultimately, of viviparity. Further studies are required to address long-lasting effects of maternal thermoregulation on offspring performance.