Meeting Abstract
Phenotypic flexibility allows animals to match phenotypes to prevailing environmental demands and may improve fitness. The climatic variability hypothesis (CVH) posits that phenotypic flexibility and climatic variability should be correlated, with greater flexibility in more heterogenous environments. In north-temperate climates, climatic variability is greater in winter than in summer, so the CVH predicts that phenotypes should be more flexible in winter than in summer. We tested the CVH by acclimating house sparrows in both summer and winter to 25, 5 and -10 C and measuring basal (BMR) and summit (Msum = maximum cold-induced) metabolic rates before and after acclimation treatments. To examine mechanistic bases for metabolic variation, we measured skeletal muscle and heart masses and pectoralis and heart citrate synthase and β-hydroxyacyl coA-dehydrogenase activities. Summer metabolic rates increased with acclimation for 5 C birds, but not for other groups, and were generally higher after acclimation at cold temperatures than at 25 C. Winter BMR was significantly downregulated at 25 C and nearly so at 5 C relative to -10 C, but Msum was not significantly impacted by acclimation. Masses of pectoralis and heart were upregulated under cold temperatures in summer, but not downregulated under warm temperatures in winter. Catabolic enzyme activities were not clearly related to metabolic variation among acclimation groups. In contrast to predictions of the CVH, these data suggest that metabolism-temperature reaction norms of house sparrows are more prominently upregulated in summer than downregulated in winter and that BMR is more flexible than Msum.
