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.

One of the greatest current threats to biodiversity is climate change. However, understanding of organismal responses to fluctuations in temperature and water availability is currently lacking, especially during fundamental life-history stages such as reproduction. To further explore how temperature and water availability impact maternal physiology and reproductive output, we used the viviparous form of the European common lizard (Zootoca vivipara) in a two-by-two factorial design manipulating both hydric and thermal conditions, for the first time. We collected blood samples and morphological measurements during early pregnancy and post-parturition to investigate how water availability, temperature and a combination of the two influence maternal phenology, morphology, physiology and reproductive output. We observed that dehydration during gestation negatively affects maternal physiological condition (lower mass gain, higher tail reserve mobilization) but has little effect on reproductive output. These effects are mainly additive to temperature regimes, with a proportional increase in maternal costs in warmer environments. Our study demonstrates the importance of considering combined effects of water and temperature when investigating organismal responses to climate changes, especially during periods crucial for species survival such as reproduction.

Thermo-hydroregulation strategies involve concurrent changes in functional traits related to energy, water balance and thermoregulation and play a key role in determining life-history traits and population demography of terrestrial ectotherms. Local thermal and hydric conditions should be important drivers of the geographical variation of thermo-hydroregulation strategies, but we lack studies that examine these changes across climatic gradients in different habitat types. Here, we investigated intraspecific variation of morphology and thermo-hydroregulation traits in the widespread European common lizard (Zootoca vivipara louislantzi) across a multidimensional environmental gradient involving independent variation in air temperature and rainfall and differences in habitat features (access to free-standing water and forest cover). We sampled adult males for morphology, resting metabolic rate, total and cutaneous evaporative water loss and thermal preferences in 15 populations from the rear to the leading edge of the distribution across an elevational gradient ranging from sea level to 1750 m. Besides a decrease in adult body size with increasing environmental temperatures, we found little effect of thermal conditions on thermo-hydroregulation strategies. In particular, relict lowland populations from the warm rear edge showed no specific ecophysiological adaptations. Instead, body mass, body condition and resting metabolic rate were positively associated with a rainfall gradient, while forest cover and water access in the habitat throughout the season also influenced cutaneous evaporative water loss. Our study emphasizes the importance of rainfall and habitat features rather than thermal conditions for geographical variation in lizard morphology and physiology.