A major goal in ecology is to understand the determinants of species distribution. Species distributions result from a dynamic response to biotic and abiotic factors. It is of crucial importance to idenfity limiting factors and their relevance, to identify proximate factors shaping species distributions. In ectotherms, ambient temperature directly influences body temperature and therefore affects physiology, activity budget, and most life history traits. Consequently, abiotic factors (climate) should have a prevalent influence on the distribution of ectotherms. Few species are able to exploit cold climates, raising the problem of their physiological specificities and responses to global change. We studied the ecophysiology and distribution of a boreal species, the adder (Vipera berus). This geographically widespread species (Eurosiberian) meets its southern limit in France (Massif Central). We compared V. berus with the asp viper (Vipera aspis), a sister species with a parapatric distribution limited to southwestern Europe. Our work on thermoregulation, metabolism, hydric sensitivity, and reproductive phenology reveals specific cold adaptations in V. berus that make this species vulnerable to high temperatures, especially during summer. Correlative modeling indicated that climate largely explains the species' distribution. In parallel, mecanistic niche modeling demonstrated the importance of thermal dependence of maintenance costs and reproductive phenology. For instance, southern distribution limits in both species are linked to energetic and hydric constraints while northern limits in V aspis likely involve altered reproductive performance. Past climatic variations (glacial/interglacial periods) have strongly affected the genetic structure of adder populations. Scenarios of future climate change are likely to impact the persistence of this species by 2100 in heathland and peat bogs in the Loire upstream zone.
