Developmental plasticity and thermal acclimation can contribute to adaptive responses to climate change by altering functional traits related to energy and water balance regulation. How plasticity interacts with physiological syndromes through lifetime in long-lived species is currently unknown. Here, we examined the impacts of long term thermal acclimation in a long-lived temperate ectotherm (Vipera aspis) and its potential flexibility at adulthood for two related functional traits: standard metabolic rate (SMR) and total evaporative water loss (TEWL). We used climatic chambers to simulate three contrasted daily thermal cycles (warm, medium and cold) differing in mean temperatures (28, 24, and 20°C respectively) and amplitudes (5, 10 and 13°C respectively) during immature life (0 to 4 years of age). Individuals were then maintained under common garden conditions (medium cycle) for an additional 3-years period (4 to 7 years of age). SMR and TEWL were repeatedly measured in the same individuals throughout life during and after the climate manipulation. Individuals reduced their SMR (negative compensation) when experiencing the warm cycle but flexibly adjusted their SMR to common garden conditions at adulthood. In addition, thermal conditions during the juvenile life stage led to changes in TEWL persisting until adulthood. We further found consistent intra-individual variation for SMR and TEWL and a positive intra-individual and inter-individual covariation between them throughout life. Thus, plastic responses were combined with a physiological syndrome linking SMR and TEWL. Our study demonstrates the capacity of long-lived organisms to flexibly shift their SMR to reduce daily maintenance costs in warmer and less variable thermal environments, which might be beneficial for low energy specialist organisms such as vipers. It further suggests that thermal conditions provide cues for developmental changes in TEWL. Beside plasticity, contrasted individual physiological syndromes could be selected for and contribute to the response to climate change.