METABOLIC ASPECTS OF SHIVERING THERMOGENESIS IN PASSERINES DURING WINTER

Various passerines enhance their cold resistance in winter through a form of metabolic acclimatization. This involves an improved ability to sustain elevated rates of regulatory thermogenesis, primarily through shivering. This muscular activity depends principally on catabolism of fatty acids. Significantly, establishment and daily replenishment of substantial fat reserves are associated with winter acclimatization in many passerines. This acclimatization also appears linked with winter increases in the activities of certain catabolic enzymes in skeletal muscle, notably beta-hydroxyacyl Co-A dehydrogenase and phosphofructokinase. The factors that initiate and regulate winter acclimatization in passerines are poorly understood. The metabolic character of this form of adjustment, which certainly involves processes affecting deposition, mobilization, and use of energy substrates assures that an array of hormones will be involved. We review several possible candidates, emphasizing glucagon, corticosterone, and the thyroid hormones. Recent studies indicate seasonal variation in levels of thyroid hormones, but not of corticosterone in free-living individuals of the American Goldfinch Carduelis tristis, a prominent species in studies of passerine winter acclimatization.