Reproducing in a changing world: combined effects of thermal conditions by day and night and of water constraints during pregnancy in a cold-adapted ectotherm

Climate change will continue to increase mean global temperatures with daily minima increasing more than daily maxima temperatures. Altered rainfall patterns due to climate change will also disrupt water availability for terrestrial organisms already facing climatic warming. To explore how organisms may adjust to changes in multiple, concurrent climate-related environmental conditions, we manipulated day and night temperatures as well as water availability during gestation in female common lizards Zootoca vivipara, a cold and wet adapted species facing climatic changes notably in populations located on the warm margin. We jointly manipulated temperature (hot or cold) independently during the daytime and nighttime as well as water availability (+/- ad libitum access to water) throughout pregnancy and quantified the effects on maternal traits (morphology, physiology and phenology) and reproductive output. Overall, we found that higher day or night temperatures decreased gestation length and increased energetic demands. Higher temperatures during the day, coupled with water restrictions, increased dehydration and water restrictions affected ability to allocate resources but had no impact on reproductive output. While high temperatures during the day were beneficial to current reproductive output and success, high temperatures during the night had the opposite effect. Our results suggest that high nighttime temperatures can dramatically increase the burden on pregnant mothers already constrained by heavy resource and water investment. This could provide a mechanistic explanation for the long-term declines of warm-margin populations in this species.