Seasonal Acclimatization of Lipid Mobilization and Catabolism in House Finches (Carpodacus mexicanus)

Seasonal variation in cold tolerance is a well-documented form of winter acclimatization in birds. The physiological mechanisms of metabolic seasonal acclimatization, however, are not completely understood. This study of house finches (Carpodacus mexicanus) assessed the extent of seasonal changes in various parameters associated with lipid mobilization, including plasma levels of free fatty acids (FFA), albumin, and glucagon. Also, seasonal variation in the catabolic capacity of the pectoralis muscle was examined through the activities of key metabolic enzymes citrate synthase and β-hydroxyacyl-CoA dehydrogenase. Plasma FFA levels were seasonally stable in house finches exposed to moderate temperature. In birds exposed to severe cold stress, however, winter FFA values were significantly greater than those in late spring, suggesting a seasonal difference in lipid mobilization capacity. Mass-specific enzyme activities were seasonally stable, but greater pectoralis mass during winter resulted in increased total catabolic capacities in this muscle. Along with previous findings on body composition, these results provide evidence that adjustments in fat stores, ability to mobilize lipids, and catabolic capacity are all involved in metabolic seasonal acclimatization in house finches.