Mechanism of winter acclimatization: Seasonal and geographic variation in lipid metabolism of house finches.

Abstract

The primary means by which birds deal with winter conditions is through metabolic adjustments. However, the physiological and biochemical mechanisms of these adjustments are incompletely understood. Seasonal changes in (1) stores of energy substrates, (2) ability to mobilize substrates, or (3) capacity to catabolize them may be involved in winter acclimatization. Seasonal changes in body composition, indicators of lipid mobilization, and activities of key metabolic enzymes of house finches (Carpodacus mexicanus) were examined to evaluate the possibilities above. All three adjustments were found to be associated with seasonal acclimatization of house finches in Michigan. Originally a western North American species, house finches were released in New York in 1940. Introduced house finches have expanded their range dramatically in the past 25 years and currently maintain an expansive eastern distribution. The released individuals appear to have been originally from southern California stock, and therefore had experienced relatively mild climatic conditions prior to their release. In contrast, these released birds and their descendents in the eastern United States have since faced considerably harsher winter conditions than their ancestors encountered. I compared seasonal changes in lipid metabolism among three house finch populations--Colorado and southern California house finches from the western distribution, and Michigan birds from the introduced eastern distribution. Michigan and Colorado house finches, which are exposed to more severe winter conditions, do show seasonal acclimatization, but southern California individuals do not. Interestingly, the mechanism for seasonal acclimatization appears to be different in Michigan birds from those in Colorado. The magnitude of geographic variation evident in house finch populations reinforces the potential dangers in widely distributed species of extrapolating findings about mechanisms for seasonal acclimatization from single populations to a larger scale.