Effect of cold-acclimation on house finch metabolism and organ sizes

Two of the most intensively studied processes in the field of ecological physiology are the limits to metabolic performance and the energetics of temperature acclimation. A major emphasis currently is the examination of relationships among basal metabolic rate (BMR), maximum metabolic rate (MMR), and the sizes of vital organs and tissues such as kidneys, liver, heart, and skeletal muscle. We examined these relationships in house finches (Carpodacus mexicanus) that were maintained in the laboratory at two different acclimation temperatures. Birds in the cold-acclimation (CA) group were maintained at [5.sup.f] C, and those in the warm-acclimation (WA) control group at [25.sup.f] C, for six weeks following capture in New York in June. All metabolic rate values were measured as oxygen consumption in an open-circuit respirometry system. MMR was determined by two methods for each animal: exercise maximum in a running wheel chamber and thermogenic maximum during exposure to cold temperatures in a helium-oxygen (heliox) atmosphere. The body mass of CA birds (20.1 [+ or -] 0.5 g) was significantly greater (p [is less than] 0.001) than that of WA birds (18.4 [+ or -] 0.3 g). Even when adjusted for differences in body mass, the BMR of CA birds was 15% greater than that of WA individuals (p [is less than] 0.01). However, there was no effect of acclimation temperature on either exercise or cold-induced MMR. Exercise-induced MMR values were 17.5% greater than those obtained using the heliox protocol (p [is less than] 0.01). The relationships among organ sizes and metabolic measurements will be discussed.