Mechanistic Drivers of Flexibility in Summit Metabolic Rates of Small Birds

Flexible metabolic phenotypes allow animals to adjust physiology to better fit ecological or environmental demands, thereby influencing fitness. Summit metabolic rate (M(sum) = maximal thermogenic capacity) is one such flexible trait. Skeletal muscle and heart masses and myocyte metabolic intensity are potential drivers of M(sum) flexibility in birds. We examined correlations of skeletal muscle and heart masses and pectoralis muscle citrate synthase (CS) activity (an indicator of cellular metabolic intensity) with M(sum) in house sparrows (Passer domesticus) and dark-eyed juncos (Junco hyemalis) to determine whether these traits are associated with M(sum) variation. Pectoralis mass was positively correlated with M(sum) for both species, but no significant correlation remained for either species after accounting for body mass (M(b)) variation. Combined flight and leg muscle masses were also not significantly correlated with M(sum) for either species. In contrast, heart mass was significantly positively correlated with M(sum) for juncos and nearly so (P = 0.054) for sparrows. Mass-specific and total pectoralis CS activities were significantly positively correlated with M(sum) for sparrows, but not for juncos. Thus, myocyte metabolic intensity influences M(sum) variation in house sparrows, although the stronger correlation of total (r = 0.495) than mass-specific (r = 0.378) CS activity with M(sum) suggests that both pectoralis mass and metabolic intensity impact M(sum). In contrast, neither skeletal muscle masses nor pectoralis metabolic intensity varied with M(sum) in juncos. However, heart mass was associated with M(sum) variation in both species. These data suggest that drivers of metabolic flexibility are not uniform among bird species.