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The purpose of this study was to determine whether chronic cold exposure would increase the aerobic capacity of skeletal muscle in UCP-dta mice, a transgenic line lacking brown adipose tissue (BAT). Wild type and UCPdta mice were acclimated to either warm (23 °C), or cold (4 °C) conditions. Cold increased muscle oxidative capacity nearly equivalently in wild-type and UCP-dta mice, but did not affect the respiratory function of isolated mitochondria. Summit metabolism (V̇ O2summit) and norepinephrine-induced thermogenesis (V̇ O2NST) were significantly lower in UCP-dta mice relative to wild-type mice regardless of temperature treatment, but both were significantly higher in cold relative to warm acclimated mice. BAT mass was significantly higher in the cold relative to warm acclimated wild-type mice, but not in cold acclimated UCP-dta mice. BAT citrate synthase activity was lower in transgenic animals regardless of acclimation temperature and BAT citrate synthase activity per depot was significantly higher only in the cold acclimated wild-type mice. Muscle citrate synthase activity was increased in both genotypes. As defects in muscle oxidative function have been observed with obesity and type 2 diabetes, these results suggest that chronic cold exposure is a useful intervention to drive skeletal muscle oxidative capacity in mouse models of obesity.

Seasonal acclimatization permits organisms to maintain function in the face of environmental change. Tadpoles of the green frog (Lithobates clamitans) overwinter as tadpoles in much of their range. Because they are active in winter, we hypothesized that green frog tadpoles would display acclimatization of metabolic and locomotor function. We collected tadpoles in Sewanee, Tennessee (35.2°N) in winter and summer. Tadpoles collected during each season were tested at both winter (8°C) and summer (26°C) temperatures. Winter tadpoles were able to maintain swimming performance at both temperatures, whereas swimming performance decreased at cold temperatures in summer tadpoles. There was no evidence for seasonal acclimatization of whole-animal metabolic rate. Although whole-animal metabolic acclimatization was not observed, the activities of cytochrome c oxidase, citrate synthase, and lactate dehydrogenase measured in skeletal muscle homogenates showed higher activity in winter-acclimatized tadpoles indicating compensation for temperature. Further, the composition of muscle membranes of winter tadpoles had less saturated and more monounsaturated fatty acids and a higher ω-3 balance, unsaturation index, and peroxidation index than summer tadpoles. These data indicate that reversible phenotypic plasticity of thermal physiology occurs in larval green frog tadpoles. They appear to compensate for colder temperatures to maintain burst-swimming velocity and the ability to escape predators without the cost of maintaining a constant, higher standard metabolic rate in the winter.

Temperate and tropical birds possess divergent life history strategies. Physiological parameters including energy metabolism correlate with the life history such that tropical species with a slower 'pace of life' have lower resting and maximal metabolic rates than temperate congeners. To better understand the physiological mechanisms underlying these differences, we investigated the relationship of metabolic capacity, muscle oxidative capacity and activity patterns to variation in life history patterns in American robins (Turdus migratorius), while resident in central North America and Clay-colored robins (Turdus grayi) resident in Panama. We measured summit metabolism [Formula: see text] in birds from both tropical and temperate habitats and found that the temperate robins have a 60 % higher metabolic capacity. We also measured the field metabolic rate (FMR) of free-living birds using heart rate (HR) telemetry and found that temperate robins' daily energy expenditure was also 60 % higher. Thus, [Formula: see text] and FMR both reflect life history differences between the species. Further, both species operate at a nearly identical ~50 % of their thermogenic capacity throughout a given day. As a potential mechanism to explain differences in activity and metabolic capacity, we ask whether oxidative properties of flight muscle are altered in accordance with life history variation and found minimal differences in oxidative capacity of skeletal muscle. These data demonstrate a close relationship between thermogenic capacity and daily activity in free-living birds. Further, they suggest that the slow pace of life in tropical birds may be related to the maintenance of low activity rather than functional capacity of the muscle tissue.

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