Hindered and constrained: limited potential for thermal adaptation in post-metamorphic and adult Rana temporaria along elevational gradients.

Adaptation to warming climates could counteract the effects of global warming. Thus, understanding how species cope with contrasting climates may inform us about their potential for thermal adaptation and which processes may hamper that ability (e.g. evolutionary trade-offs, phenology or behavioural thermoregulation). In addition to temperature, time constraints may also exert important selective pressures. Here, we compare the thermal sensitivity of locomotion of metamorphic and adult European common frogs (Rana temporaria) originating from populations along an elevational gradient. We employed the template mode of variation (TMV) analysis to decompose the thermal sensitivity of locomotion and explore the existence of trade-offs ('hotter is better' and 'specialist-generalist') and the degree of local adaptation. To that end, we studied the relationship between TMV parameters and local environmental conditions. Further, we compared preferred temperatures to assess whether behavioural thermoregulation could dampen the effects of thermal variation, reducing the intensity of selection and limiting thermal adaptation (i.e. 'Bogert effect'). We suggest that behavioural thermoregulation has promoted the conservatism of thermal sensitivity in R. temporaria. Yet, we observed a trend towards narrower thermal niches shifted towards warmer temperature in populations with severe temporal constraints, conforming to the 'generalist-specialist' trade-off. Apparently, this enables time-constrained populations - especially in the case of metamorphs - to effectively exploit resources during the warmest periods. The limited potential of R. temporaria for thermal adaptation suggests that forecasts of global warming should incorporate thermoregulation and explore its potential to buffer species from rising temperatures.