In endotherms, metabolic performance is associated with a wide array of ecological traits, including species distribution. Researchers have previously suggested that the northern boundaries of North American passerines are limited by their ability to sustain high metabolic rates required for thermoregulation. Black-capped chickadees (Poecile atricapillus, BC) are year-round residents in most of Canada and the northern half of the United States, whereas Carolina chickadees (Poecile carolinensis, CA) are found exclusively in the southeastern United States. These species hybridize along a narrow contact zone, which has been moving northward at a rate of about 1.6 km per decade, coincident with warming temperatures in Ohio. The location of the chickadee hybrid zone in Ohio closely matches air temperature isotherms, further suggesting that metabolic rate may correlate with distribution in these species.
We tested the hypothesis that distribution patterns of chickadees are linked with their rate of metabolism. For populations of BC and CA chickadees, we measured basal (BMR) and cold-induced peak metabolic rates (PMR) from areas that differ in winter temperatures and supplemented this information with data from other studies. Our findings suggest a general relationship between colder temperatures and higher metabolic rates, though this trend was not robust among all locations.
Within Ohio, hybrids had a significantly higher mass-corrected BMR than either parental species. We suggest that the mtDNA-nDNA mismatch of hybrids may produce less efficient mitochondrial protein complexes, which in turn affects the efficiency of ATP production, thereby increasing rate of oxygen consumption to meet ATP demands.
In chickadees, heat production during cold exposure is primarily achieved through shivering thermogenesis, an energetically expensive process that requires high rates of ATP production. Thus, we predicted that northern populations would exhibit higher PMR compared to southern populations, facilitated by an elevation in enzyme activity of oxidative pathways. To explore this relationship, we measured in vitro activity of three pectoral muscle metabolic enzymes that catalyze non-equilibrium, highly exergonic reactions: citrate synthase (CS), phosphofructokinase (PK), and L-3-hydroxyacyl-CoA-dehydrogenase (HOAD).
While we did not find any differences in mass- or protein-corrected enzyme activities between the two species, we did observe a higher level of total muscle CS activity in BC chickadees. In addition, PMR in BC chickadees was significantly correlated with pectoralis mass and total muscle enzyme activity of CS, PFK, and HOAD. We concluded that among BC chickadees, higher metabolic rates were primarily a result of greater pectoralis mass, rather than an increase in tissue-specific metabolic enzyme activity.