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Using red knots (Calidris canutus) as a model, we determined how changes in mass and metabolic activity of organs relate to temperature-induced variation in metabolic performance. In cold-acclimated birds, we expected large muscles and heart as well as improved oxidative capacity and lipid transport, and we predicted that this would explain variation in maximal thermogenic capacity (Msum). We also expected larger digestive and excretory organs in these same birds and predicted that this would explain most of the variation in basal metabolic rate (BMR). Knots kept at 5°C were 20% heavier and maintained 1.5 times more body fat than individuals kept in thermoneutral conditions (25°C). The birds in the cold also had a BMR up to 32% higher and a Msum 16% higher than birds at 25°C. Organs were larger in the cold, with muscles and heart being 9-20% heavier and digestive and excretory organs being 21-36% larger than at thermoneutrality. Rather than the predicted digestive and excretory organs, the cold-induced increase in BMR correlated with changes in mass of the heart, pectoralis, and carcass. Msum varied positively with the mass of the pectoralis, supracoracoideus, and heart, highlighting the importance of muscles and cardiac function in cold endurance. Cold-acclimated knots also expressed upregulated capacity for lipid transport across mitochondrial membranes [carnitine palmitoyl transferase (CPT)] in their pectoralis and leg muscles, higher lipid catabolism capacity in their pectoralis muscles [β-hydroxyacyl CoA-dehydrogenase (HOAD)], and elevated oxidative capacity in their liver and kidney (citrate synthase). These adjustments may have contributed to BMR through changes in metabolic intensity. Positive relationships among Msum, CPT, and HOAD in the heart also suggest indirect constraints on thermogenic capacity through limited cardiac capacity. [ABSTRACT FROM AUTHOR]

Small birds inhabiting regions with cold winter climates show seasonally flexible metabolic phenotypes, with the winter phenotype characterized by increments of summit metabolic rate (M) and cold tolerance. In the study reported here, we focused on variations in M as a metric of metabolic performance because it is positively correlated with cold tolerance in birds and positively related to overwinter survival in small mammals, although the latter has yet to be demonstrated in birds. Temperature appears to be a prominent driver of seasonal metabolic phenotypes in birds, as evidenced by the correlation between inter- and intra-seasonal variation in M and temperature variation, by recent temperature variables serving as better predictors of M variation than long-term climate variables, and by the induction of M variation by experimental cold exposure. In contrast, photoperiod and social status do not appear to be prominent drivers of metabolic flexibility in birds studied to date. Because skeletal muscle is the primary thermogenic tissue in birds, studies of the mechanistic underpinnings of metabolic flexibility have focused on skeletal muscles, particularly flight muscles. At the level of the skeletal muscle, two potential mechanisms exist for increasing thermogenic capacity, namely, muscle hypertrophy and elevated cellular metabolic intensity. Correlative studies suggest consistent winter increments in flight muscle size, with a potential regulatory role for the muscle growth inhibitor myostatin. Recent experimental studies in small birds, including modification of flight costs, cold acclimation, and exercise training, also suggest that muscle size is an important driver of metabolic flexibility in birds. Therefore, the focus of our study was on the seasonal regulation of muscle size and its contribution to metabolic flexibility. Future studies should address fitness consequences of M variation, the relative roles of muscle hypertrophy, and other factors (e.g., oxygen and substrate transport, cellular metabolic intensity) promoting M variation, as well as the molecular mechanisms underlying seasonal phenotypes. [ABSTRACT FROM AUTHOR]

Reaction norms reflect an organisms' capacity to adjust its phenotype to the environment and allows for identifying trait values associated with physiological limits. However, reaction norms of physiological parameters are mostly unknown for endotherms living in natural conditions. Black-capped chickadees (Poecile atricapillus) increase their metabolic performance during winter acclimatization and are thus good model to measure reaction norms in the wild. We repeatedly measured basal (BMR) and summit (Msum) metabolism in chickadees to characterize, for the first time in a free-living endotherm, reaction norms of these parameters across the natural range of weather variation. BMR varied between individuals and was weakly and negatively related to minimal temperature. Msum varied with minimal temperature following a Z-shape curve, increasing linearly between 24°C and −10°C, and changed with absolute humidity following a U-shape relationship. These results suggest that thermal exchanges with the environment have minimal effects on maintenance costs, which may be individual-dependent, while thermogenic capacity is responding to body heat loss. Our results suggest also that BMR and Msum respond to different and likely independent constraints. [ABSTRACT FROM AUTHOR]

The body condition of free-ranging animals affects their response to stress, decisions, ability to fulfil vital needs and, ultimately, fitness. However, this key attribute in ecology remains difficult to assess, and there is a clear need for more integrative measures than the common univariate proxies., We propose a systems biology approach that positions individuals along a gradient from a 'normal/optimal' to 'abnormal/suboptimal' physiological state based on Mahalanobis distance computed from physiological biomarkers. We previously demonstrated the validity of this approach for studying ageing in humans; here, we illustrate its broad potential for ecological studies., As an example, we used biomarker data on shorebirds and found that birds with an abnormal condition had a lower maximal thermogenic capacity and higher scores of inflammation, with important implications for their ecology and health. Moreover, Mahalanobis distance captured a signal of condition not detected by the individual biomarkers., Overall, our results on birds and humans show that individuals with abnormal physiologies are indeed in worse condition. Moreover, our approach appears not to be particularly sensitive to which set of biomarkers is used to assess condition. Consequently, it could be applied easily to existing ecological data sets., Our approach provides a general, powerful way to measure condition that helps resolve confusion as to how to deal with complex interactions and interdependence among multiple physiological and condition measures. It can be applied directly to topics such as the effect of environmental quality on body condition, risks of health outcomes, mechanisms of adaptive phenotypic plasticity, and mechanisms behind long-term processes such as senescence. [ABSTRACT FROM AUTHOR]

The plasma metabolites triglycerides ( TRIG) and β-hydroxybutyrate ( BUTY) are used as indices of nutritional condition in migrating birds during refuelling and can provide a measure of relative fattening rates in individual birds. Because non-migratory birds wintering at northern latitudes also fatten on a daily basis to support their overnight fast, blood metabolites could provide a useful tool to measure individual performance in energy acquisition. However, daily patterns of metabolite change may differ between species and could be affected by thermoregulatory requirements. We studied daily variation in TRIG and BUTY over a complete winter in Black-capped Chickadees to determine the pattern of daily and seasonal change in these markers. We also assessed how short-term variation (up to 7 days) in weather parameters that influence heat exchange may affect TRIG and BUTY levels. In contrast to a linear gain of body mass, TRIG increased non-linearly during the day, with a rapid increase in the morning that levelled off in the afternoon, whereas BUTY did not change significantly. Metabolites varied with sampling time and the seasonal change in day length, suggesting higher fat catabolism and fattening rates in mid-winter. TRIG and BUTY also differed between capture sites, possibly due to differences in shelter quality. Weather variation did not affect TRIG levels and had a significant but marginal effect on BUTY, explaining at best 3% of the variation. Our results suggest that these markers can be used as indicators of energy turnover in resident wintering passerines. [ABSTRACT FROM AUTHOR]

The metabolic cost of egg production in birds (passerines) has been measured as a 16-27% increase in basal or resting metabolic rate (BMR and RMR, respectively) when comparing non-breeding values with those in egg-producing individuals. However, available data to date have been obtained in free-living birds and may thus be confounded by the effect of variable ecological conditions on non-reproductive physiological machinery (organ mass or metabolic activity) which might contribute to measured variation in RMR. Here, we show that in captive, controlled conditions, the process of egg formation induces a 22% increase in RMR in female zebra finches Taeniopygia guttata. Among individuals, variation in laying RMR is independent of egg mass, clutch size or total clutch mass. Importantly, we show that individual variation in both non-breeding and laying RMR is repeatable over periods of at least 8-10 months, i.e. individual variation in RMR remained constant over time for any given physiological state. This suggests that the metabolic cost of egg formation should respond to selection. However, we also show that in males, but not females, repeatability of RMR declines over time even when birds are kept in constant controlled conditions. [ABSTRACT FROM AUTHOR]

Little is known about the energy costs of egg production in birds. We showed in previous papers that, during egg production, European starlings (Sturnus vulgaris) undergo a 22% increase in resting metabolic rate (RMR) and that the maintenance and activity costs of the oviduct are responsible for 18% of the variation in elevated laying RMR. Therefore, other energy-consuming physiological mechanisms must be responsible for the remaining unexplained variation in elevated laying RMR. Yolk precursor [vitellogenin (VTG) and very-low-density lipoprotein (VLDL)] production is likely to be costly because it signifies a marked increase in the biosynthetic activity of the liver. We documented the pattern of yolk precursor production in response to daily injections of 17β-estradiol (E[SUB2]) in zebra finches (Taeniopygia guttata). Based on this pattern we carried out an experiment in order to evaluate the metabolic costs of producing VTG and VLDL. Our E[SUB2] treatment resulted in a significant increase in plasma VTG and VLDL levels within the natural breeding range for the species. Although RMR was measured during the period of active hepatic yolk precursor production, it did not differ significantly within individuals in response to the treatment or when comparing E[SUB2]-treated birds with sham-injected birds. This could mean that yolk precursor production represents low energy investment. However, we discuss these results in light of possible adjustments between organs that could result in energy compensation. [ABSTRACT FROM AUTHOR]