Thermal tolerance and acclimation capacity in the European common frog (Rana temporaria) change throughout ontogeny

Phenotypic plasticity may allow ectotherms with complex life histories such as amphibians to cope with climate‐driven changes in their environment. Plasticity in thermal tolerance (i.e., shifts of thermal limits via acclimation to higher temperatures) has been proposed as a mechanism to cope with warming and extreme thermal events. However, thermal tolerance and, hence, acclimation capacity, is known to vary with life stage. Using the common frog (Rana temporaria) as a model species, we measured the capacity to adjust lower (CTmin) and upper (CTmax) critical thermal limits at different acclimation temperatures. We calculated the acclimation response ratio as a metric to assess the stage‐specific acclimation capacity at each of seven consecutive ontogenetic stages and tested whether acclimation capacity was influenced by body mass and/or age. We further examined how acclimation temperature, body mass, age, and ontogenetic stage influenced CTmin and CTmax. In the temperate population of R. temporaria that we studied, thermal tolerance and acclimation capacity were affected by the ontogenetic stage. However, acclimation capacity at both thermal limits was well below 100% at all life stages tested. The lowest and highest acclimation capacity in thermal limits was observed in young and late larvae, respectively. The relatively low acclimation capacity of young larvae highlights a clear risk of amphibian populations to ongoing climate change. Ignoring stage‐specific differences in thermal physiology may drastically underestimate the climate vulnerability of species, which will hamper successful conservation actions. RESEARCH HIGHLIGHTS Thermal tolerance and acclimation capacity changed with the ontogenetic stage. Acclimation capacity was well below 100%. Acclimation capacity was lowest in young larvae. Young larvae pose a thermal bottleneck during ontogeny. In this study, we tested whether thermal tolerance and acclimation capacity change throughout ontogeny in the European common frog (Rana temporaria). We aimed to identify the most susceptible ontogenetic stage to thermal extremes in the life cycle of R. temporaria. Acclimation capacity was lowest in young larvae. Hence, young larvae are the most temperature‐sensitive and thus the most vulnerable group to thermal extremes associated with climate change. Thermal acclimation might be an important component among R. temporaria's repertoire of buffering mechanisms against global warming.