Central Limits to Sustainable Metabolic Rate Have No Role in Cold Acclimation of the Short-Tailed Field Vole (Microtus agrestis)

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Abstract
We investigated the extent to which the changes in basal metabolic rate (BMR), gut morphology, and food intake (FI) that typically occur during cold acclimation in small mammals can be explained by the concept of alimentary-mediated limits to sustainable metabolic energy expenditure. Adult short-tailed field voles (Microtus agrestis) were cold stressed by continuous exposure to 5° C. Exposure for 10, 20, 50, and 100 d (n = 6 in all cases) produced significant changes in oxygen consumption ( ), mass, FI, and the dry weight of a variety of morphological parameters when compared with voles that were not cold exposed (n = 8). At 10°, 20°, and 25° C, (mL · min⁻¹) increased significantly with the duration of cold exposure. After 100 d cold exposure, had increased by more than 50% at each test temperature. Food intake (g · d⁻¹) increased significantly by 106% after 10 d cold exposure but did not increase further with increased exposure time. The ratio of FI (J · h⁻¹) to BMR (J · h⁻¹) was 1.2 in controls and increased to 2.7 after 10 d cold exposure. Thereafter the ratio decreased, and after 100 d cold exposure FI:BMR was not significantly different from control levels. The mass of cold-exposed voles increased significantly with duration of exposure. Masses of the following morphological parameters increased with increasing duration of cold exposure; whole body mass, carcass, skeleton, pelage, subcutaneous fat, liver, kidney, lung, and interscapular brown fat (BAT). There was no significant relationship between duration of cold exposure and the masses of muscle, large intestine, heart, and brain. Using stepwise multiple regression analysis we showed that variation in BMR was linked mostly to changes in skeletal mass. However, when skeletal mass was removed as an independent variable from the analysis, BAT, muscle, and gut mass entered as significant predictors, together explaining 55.5% of the variation in BMR. Although FI increased during cold exposure, the increase (106%) was apparently insufficient to precipitate a hypertrophic response in the gut. Nevertheless BMR did increase as duration of cold exposure increased, probably linked to an increase in BAT mass and thus thermogenic capacity. We cannot support the hypothesis that the changes that typically occur in BMR, food intake, and gut morphology during cold acclimation are a consequence of alimentary-mediated limits to sustainable metabolic rate.