High thermal plasticity, low heritability and constant directional selection on breeding phenology in a viviparous ectotherm

Shifting breeding phenology is a widespread response to global warming in ectothermic species such as reptiles, with considerable impacts on reproductive success and population dynamics. Thus, understanding causes and consequences of plasticity and microevolution in breeding phenology is important for determining vulnerability to climate change. Here, we study environmental and individual determinants of breeding phenology, its heritability and the temporal variation of selection on parturition date using a 31-year monitoring of a wild, viviparous population of common lizards (Zootoca vivipara). Warmer daily maximum temperatures during the post-winter activity advanced parturition dates and larger females gave birth earlier, but warmer temperatures also reduced size-related phenological variability. Parturition dates were repeatable but weakly heritable, suggesting that inter-individual consistency of parturition dates may be related to non-heritable factors such as home range quality, nutritional condition or behaviour. In addition, we found evidence for a directional selection towards earlier parturition without fluctuation in time. This study highlights the strong inter-annual thermal plasticity coupled with a limited adaptive potential of breeding phenology in the common lizard. In future, warmer conditions, the interplay between such plasticity and a selection apparently independent from environmental fluctuations would thus essentially depend on the yet unknown costs of thermal plasticity.