Temperature, metabolic rate, and constraints on locomotor performance in ectotherm vertebrates

1. Adaptation to cold environments of ectotherm populations is expected to result in increased standard metabolic rates if resources are not limiting. However, it is still unclear how the maintenance of high standard metabolic rates would affect locomotor performance and its impact at the population level. 2. We compared standard metabolic rate and swimming performance of Brown Trout populations inhabiting rivers with contrasting thermal conditions. To avoid environmental confounding effects, we tested offspring raised in controlled conditions. There were significant differences among populations and families for both traits, suggesting that the observed differentiation pattern has a genetic basis. 3. We found a significant negative relationship between standard metabolic rate and swimming performance at the population level. On the other hand, standard metabolic rates tended to be negatively related to the temperature in the rivers of origin, whereas swimming performance was positively related to river temperature, but these trends were not significant. 4. These results suggest that selection for traits positively linked with standard metabolic rates may result in lower relative prolonged swimming performance. The potential evolutionary consequences of the observed negative relationship are discussed.