Long-term fasting induced basal thermogenesis flexibility in female Japanese quails

Male Japanese quails (Coturnix japonica) have been found to exhibit a three-phase metabolic change when subjected to prolonged fasting, during which basal thermogenesis is significantly reduced. A study had shown that there is a significant difference in the body temperature between male and female Japanese quails. However, whether female Japanese quails also show the same characteristic three-phase metabolic change during prolonged fasting and the underlying thermogenesis mechanisms associated with such changes are still unclear. In this study, female Japanese quails were subjected to prolonged starvation, and the body mass, basal metabolic rate (BMR), body temperature, mass of tissues and organs, body fat content, the state-4 respiration (S4R) and cytochrome c oxidase (CCO) activity in the muscle and liver of these birds were measured to determine the status of metabolic changes triggered by the starvation. In addition, the levels of glucose, triglyceride (TG) and uric acid, and thyroid hormones (T3 and T4) in the serum and the mRNA levels of myostatin (MSTN) and avian uncoupling protein (av-UCP) in the muscle were also measured. The results revealed the existence of a three-phase stage similar to that found in male Japanese quails undergoing prolonged starvation. Fasting resulted in significantly lower body mass, BMR, body temperature, tissues masses and most organs masses, as well as S4R and CCO activity in the muscle and liver. The mRNA level of av-UCP decreased during fasting, while that of MSTN increased but only during Phase I and II and decreased significantly during Phase III. Fasting also significantly lowered the T3 level and the ratio of T3/T4 in the serum. These results indicated that female Japanese quails showed an adaptive response in basal thermogenesis at multiple hierarchical levels, from organismal to biochemical, enzyme and cellular level, gene and endocrine levels and this integrated adjustment could be a part of the adaptation used by female quails to survive long-term fasting.

Migration increases mitochondrial oxidative capacity without increasing reactive oxygen species emission in a songbird

Birds remodel their flight muscle metabolism prior to migration to meet the physiological demands of migratory flight, including increases in both oxidative capacity and defence against reactive oxygen species. The degree of plasticity mediated by changes in these mitochondrial properties is poorly understood but may be explained by two non-mutually exclusive hypotheses: variation in mitochondrial quantity or in individual mitochondrial function. We tested these hypotheses using yellow-rumped warblers (Setophaga coronata), a Nearctic songbird which biannually migrates 2000-5000 km. We predicted higher flight muscle mitochondrial abundance and substrate oxidative capacity, and decreased reactive oxygen species emission in migratory warblers captured during autumn migration compared with a short-day photoperiod-induced non-migratory phenotype. We assessed mitochondrial abundance via citrate synthase activity and assessed isolated mitochondrial function using high-resolution fluororespirometry. We found 60% higher tissue citrate synthase activity in the migratory phenotype, indicating higher mitochondrial abundance. We also found 70% higher State 3 respiration (expressed per unit citrate synthase) in mitochondria from migratory warblers when oxidizing palmitoylcarnitine, but similar H2O2 emission rates between phenotypes. By contrast, non-phosphorylating respiration was higher and H2O2 emission rates were lower in the migratory phenotype. However, flux through electron transport system complexes I-IV, II-IV and IV was similar between phenotypes. In support of our hypotheses, these data suggest that flight muscle mitochondrial abundance and function are seasonally remodelled in migratory songbirds to increase tissue oxidative capacity without increasing reactive oxygen species formation.

Genetics and Evolution of Bird Migration

Bird migration has long been a subject of fascination for humankind and is a behavior that is both intricate and multifaceted. In recent years, advances in technology, particularly in the fields of genomics and animal tracking, have enabled significant progress in our understanding of this phenomenon. In this review, we provide an overview of the latest advancements in the genetics of bird migration, with a particular focus on genomics, and examine various factors that contribute to the evolution of this behavior, including climate change. Integration of research from the fields of genomics, ecology, and evolution can enhance our comprehension of the complex mechanisms involved in bird migration and inform conservation efforts in a rapidly changing world.

Dietary fatty acids and flight-training influence the expression of the eicosanoid hormone prostacyclin in songbirds

Diet shifts can alter tissue fatty acid composition in birds, which is subsequently related to metabolic patterns. Eicosanoids, short-lived fatty acid-derived hormones, have been proposed to mediate these relationships but neither baseline concentrations nor the responses to diet and exercise have been measured in songbirds. We quantified a stable derivative of the vasodilatory eicosanoid prostacyclin in the plasma of male European Starlings (Sturnus vulgaris, N = 25) fed semisynthetic diets with either high (PUFA) or low (MUFA) amounts of n6 fatty acid precursors to prostacyclin. Plasma samples were taken from each bird before, immediately after, and two days following a 15-day flight-training regimen that a subset of birds (N = 17) underwent. We found elevated prostacyclin levels in flight-trained birds fed the PUFA diet compared to those fed the MUFA diet and a positive relationship between prostacyclin and body condition, indexed by fat score. Prostacyclin concentrations also significantly decreased at the final time point. These results are consistent with the proposed influences of precursor availability (i.e., dietary fatty acids) and regulatory feedback associated with exercise (i.e., fuel supply and inflammation), and suggest that prostacyclin may be an important mediator of dietary influence on songbird physiology.

Controlled expression of avian pre-migratory fattening influences indices of innate immunity

While immunity is frequently dampened when birds engage in strenuous migratory flights, whether and how immunity changes during the rapid accumulation of energy stores in preparation for migration remains largely unknown. Here we induced pre-migratory fattening through controlled changes of daylight in common quails (Coturnix coturnix) and regularly assessed changes in three markers of constitutive innate immunity (leukocyte coping capacity or LCC, hemagglutination and hemolysis titres) and measures of body composition (lean and fat mass). All the three markers showed similar changes over the pre-migratory fattening process. LCC responses, hemagglutination titres, and hemolysis titres, were on average higher in the mid-fattening phase compared to the peak-fattening phase, when values were similar to those observed prior the start of pre-migratory fattening. At mid-fattening, we found that the birds that showed a larger accumulation of fat mass (as % of body mass) had lower LCC peak responses and hemolysis titres. Reversibly, at mid-fattening, we also found that the birds that kept a higher proportion of lean mass (as % of body mass) had the highest LCC peaks. Our results indicate that migratory birds undergo changes in immune indices (over 8 weeks) as they accumulate energy stores for migration and propose that this could be due to competing or trade-off processes between metabolic remodelling and innate immune system function.

Sarcolipin relates to fattening, but not sarco/endoplasmic reticulum Ca2+-ATPase uncoupling, in captive migratory gray catbirds

In order to complete their energetically demanding journeys, migratory birds undergo a suite of physiological changes to prepare for long-duration endurance flight, including hyperphagia, fat deposition, reliance on fat as a fuel source, and flight muscle hypertrophy. In mammalian muscle, SLN is a small regulatory protein which binds to sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and uncouples Ca2+ transport from ATP hydrolysis, increasing energy consumption, heat production, and cytosolic Ca2+ transients that signal for mitochondrial biogenesis, fatigue resistance and a shift to fatty acid oxidation. Using a photoperiod manipulation of captive gray catbirds (Dumetella carolinensis), we investigated whether SLN may play a role in coordinating the development of the migratory phenotype. In response to long-day photostimulation, catbirds demonstrated migratory restlessness and significant body fat stores, alongside higher SLN transcription while SERCA2 remained constant. SLN transcription was strongly correlated with h-FABP and PGC1α transcription, as well as fat mass. However, SLN was not significantly correlated with HOAD or CD36 transcripts or measurements of SERCA activity, SR membrane Ca2+ leak, Ca2+ uptake rates, pumping efficiency or mitochondrial biogenesis. Therefore, SLN may be involved in the process of storing fat and shifting to fat as a fuel, but the mechanism of its involvement remains unclear.

White-throated sparrow (Zonotrichia albicollis) liver and pectoralis flight muscle transcriptomic changes in preparation for migration

Migratory songbirds undertake challenging journeys to reach their breeding grounds each spring. They accomplish these nonstop flapping feats of endurance through a suite of physiological changes, including the development of substantial fat stores and flight muscle hypertrophy and an increased capacity for fat catabolism. In addition, migratory birds may show large reductions in organ masses during flight, including the flight muscle and liver, which they must rapidly rebuild during their migratory stopover before replenishing their fat stores. However, the molecular basis of this capacity for rapid tissue remodeling and energetic output has not been thoroughly investigated. We performed RNA-sequencing analysis of the liver and pectoralis flight muscle of captive white-throated sparrows in experimentally photostimulated migratory and nonmigratory condition to explore the mechanisms of seasonal change to metabolism and tissue mass regulation that may facilitate these migratory journeys. Based on transcriptional changes, we propose that tissue-specific adjustments in preparation for migration may alleviate the damaging effects of long-duration activity, including a potential increase to the inflammatory response in the muscle. Furthermore, we hypothesize that seasonal hypertrophy balances satellite cell recruitment and apoptosis, while little evidence appeared in the transcriptome to support myostatin-, insulin-like growth factor 1-, and mammalian target of rapamycin-mediated pathways for muscle growth. These findings can encourage more targeted molecular studies on the unique integration of pathways that we find in the development of the migratory endurance phenotype in songbirds.NEW & NOTEWORTHY Migratory songbirds undergo significant physiological changes to accomplish their impressive migratory journeys. However, we have a limited understanding of the regulatory mechanisms underlying these changes. Here, we explore the transcriptomic changes to the flight muscle and liver of white-throated sparrows as they develop the migratory condition. We use these patterns to develop hypotheses about metabolic flexibility and tissue restructuring in preparation for migration.

Flight muscle size reductions and functional changes following long-distance flight under variable humidity conditions in a migratory warbler

Bird flight muscle can lose as much as 20% of its mass during a migratory flight due to protein catabolism, and catabolism can be further exacerbated under dehydrating conditions. However, the functional consequences of exercise and environment induced protein catabolism on muscle has not been examined. We hypothesized that prolonged flight would cause a decline in muscle mass, aerobic capacity, and contractile performance. This decline would be heightened for birds placed under dehydrating environmental conditions, which typically increases lean mass losses. Yellow-rumped warblers (Setophaga coronata) were exposed to dry or humid (12 or 80% relative humidity at 18°C) conditions for up to 6 h while at rest or undergoing flight. The pectoralis muscle was sampled after flight/rest or after 24 h of recovery, and contractile properties and enzymatic capacity for aerobic metabolism was measured. There was no change in lipid catabolism or force generation of the muscle due to flight or humidity, despite reductions in pectoralis dry mass immediately post-flight. However, there was a slowing of myosin-actin crossbridge kinetics under dry compared to humid conditions. Aerobic and contractile function is largely preserved after 6 h of exercise, suggesting that migratory birds preserve energy pathways and function in the muscle.

Concerted phenotypic flexibility of avian erythrocyte size and number in response to dietary anthocyanin supplementation

BACKGROUND

Endurance flight impose substantial oxidative costs on the avian oxygen delivery system. In particular, the accumulation of irreversible damage in red blood cells can reduce the capacity of blood to transport oxygen and limit aerobic performance. Many songbirds consume large amounts of anthocyanin-rich fruit, which is hypothesized to reduce oxidative costs, enhance post-flight regeneration, and enable greater aerobic capacity. While their antioxidant benefits appear most straightforward, the effects of anthocyanins on blood composition remain so far unknown. We fed thirty hand-raised European starlings (Sturnus vulgaris) two semisynthetic diets (with or without anthocyanin supplement) and manipulated the extent of flight activity in a wind tunnel (daily flying or non-flying for over two weeks) to test for their interactive effects on functionally important haematological variables.

RESULTS

Supplemented birds had on average 15% more and 4% smaller red blood cells compared to non-supplemented individuals and these diet effects were independent of flight manipulation. Haemoglobin content was 7% higher in non-supplemented flying birds compared to non-flying birds, while similar haemoglobin content was observed among supplemented birds that were flown or not. Neither diet nor flight activity influenced haematocrit.

CONCLUSION

The concerted adjustments suggest that supplementation generally improved antioxidant protection in blood, which could prevent the excess removal of cells from the bloodstream and may have several implications on the oxygen delivery system, including improved gas exchange and blood flow. The flexible haematological response to dietary anthocyanins may also suggest that free-ranging species preferentially consume anthocyanin-rich fruits for their natural blood doping, oxygen delivery-enhancement effects.

Altered dynamics of mitochondria and reactive oxygen species in the erythrocytes of migrating red-headed buntings

Background: Blood antioxidants provide propensity to mitigate reactive oxygen species (ROS) apart from other oxidative challenges during a high-energy state of migration in night migratory songbirds. The study investigated the modulation of erythrocytes, mitochondrial abundance, hematocrit changes, and relative expression of fat transport-related genes during migration in red-headed buntings (Emberiza bruniceps). We hypothesized an increase in antioxidants along with the mitigation of mitochondria-related reactive oxygen species elevation and consequential apoptosis occurring during migration. Methods: Male red-headed buntings (n = 6) were placed under short days (8 h of light and 16 h of dark, 8L:16D)/long days (14L:10D) and photo induced to simulated non-migratory, nMig; pre-migratory, pMig; and migratory, Mig, states. Erythrocyte shape, reactive oxygen species production, mitochondrial membrane potential (MMP), reticulocyte proportion, and apoptosis were analyzed using flow cytometry and relative expression of fat metabolizing and antioxidant genes was measured by using qPCR. Results: There was a significant increase in hematocrit, erythrocyte area, and mitochondrial membrane potential. Reactive oxygen species and apoptotic erythrocyte proportion declined in the Mig state. The changes in antioxidant genes (SOD1 and NOS2), fatty acid translocase (CD36), and metabolic (FABP3, DGAT2, GOT2, and ATGL) genes showed a significant increment during the Mig state. Conclusion: These results suggested that adaptive changes occur in mitochondrial behavior and apoptosis of erythrocytes. The transition in erythrocytes, antioxidant genes, and fatty acid metabolism gene expressions suggested differences in regulatory strategies at the cellular/transcriptional level during different states of simulated migration in birds.

The effects of training, acute exercise and dietary fatty acid composition on muscle lipid oxidative capacity in European starlings

Migratory birds undergo seasonal changes to muscle biochemistry. Nonetheless, it is unclear to what extent these changes are attributable to the exercise of flight itself versus endogenous changes. Using starlings (Sturnus vulgaris) flying in a wind tunnel, we tested the effects of exercise training, a single bout of flight and dietary lipid composition on pectoralis muscle oxidative enzymes and lipid transporters. Starlings were either unexercised or trained over 2 weeks to fly in a wind tunnel and sampled either immediately following a long flight at the end of this training or after 2 days recovery from this flight. Additionally, they were divided into dietary groups that differed in dietary fatty acid composition (high polyunsaturates versus high monounsaturates) and amount of dietary antioxidant. Trained starlings had elevated (19%) carnitine palmitoyl transferase and elevated (11%) hydroxyacyl-CoA dehydrogenase in pectoralis muscle compared with unexercised controls, but training alone had little effect on lipid transporters. Immediately following a long wind-tunnel flight, starling pectoralis had upregulated lipid transporter mRNA (heart-type fatty acid binding protein, H-FABP, 4.7-fold; fatty acid translocase, 1.9-fold; plasma membrane fatty acid binding protein, 1.6-fold), and upregulated H-FABP protein (68%). Dietary fatty acid composition and the amount of dietary antioxidants had no effect on muscle catabolic enzymes or lipid transporter expression. Our results demonstrate that birds undergo rapid upregulation of catabolic capacity that largely becomes available during flight itself, with minor effects due to training. These effects likely combine with endogenous seasonal changes to create the migratory phenotype observed in the wild.

Skeletal muscle and metabolic flexibility in response to changing energy demands in wild birds

Phenotypically plastic responses of animals to adjust to environmental variation are pervasive. Reversible plasticity (i.e., phenotypic flexibility), where adult phenotypes can be reversibly altered according to prevailing environmental conditions, allow for better matching of phenotypes to the environment and can generate fitness benefits but may also be associated with costs that trade-off with capacity for flexibility. Here, we review the literature on avian metabolic and muscle plasticity in response to season, temperature, migration and experimental manipulation of flight costs, and employ an integrative approach to explore the phenotypic flexibility of metabolic rates and skeletal muscle in wild birds. Basal (minimum maintenance metabolic rate) and summit (maximum cold-induced metabolic rate) metabolic rates are flexible traits in birds, typically increasing with increasing energy demands. Because skeletal muscles are important for energy use at the organismal level, especially to maximum rates of energy use during exercise or shivering thermogenesis, we consider flexibility of skeletal muscle at the tissue and ultrastructural levels in response to variations in the thermal environment and in workloads due to flight exercise. We also examine two major muscle remodeling regulatory pathways: myostatin and insulin-like growth factor -1 (IGF-1). Changes in myostatin and IGF-1 pathways are sometimes, but not always, regulated in a manner consistent with metabolic rate and muscle mass flexibility in response to changing energy demands in wild birds, but few studies have examined such variation so additional study is needed to fully understand roles for these pathways in regulating metabolic flexibility in birds. Muscle ultrastrutural variation in terms of muscle fiber diameter and associated myonuclear domain (MND) in birds is plastic and highly responsive to thermal variation and increases in workload, however, only a few studies have examined ultrastructural flexibility in avian muscle. Additionally, the relationship between myostatin, IGF-1, and satellite cell (SC) proliferation as it relates to avian muscle flexibility has not been addressed in birds and represents a promising avenue for future study.

Transcriptome signature changes in the liver of a migratory passerine

The liver plays a principal role in avian migration. Here, we characterised the liver transcriptome of a long-distance migrant, the Northern Wheatear (Oenanthe oenanthe), sampled at different migratory stages, looking for molecular processes linked with adaptations to migration. The analysis of the differentially expressed genes suggested changes in the periods of the circadian rhythm, variation in the proportion of cells in G1/S cell-cycle stages and the putative polyploidization of this cell population. This may explain the dramatic increment in the liver's metabolic capacities towards migration. Additionally, genes involved in anti-oxidative stress, detoxification and innate immune responses, lipid metabolism, inflammation and angiogenesis were regulated. Lipophagy and lipid catabolism were active at all migratory stages and increased towards the fattening and fat periods, explaining the relevance of lipolysis in controlling steatosis and maintaining liver health. Our study clears the way for future functional studies regarding long-distance avian migration.

Lack of variation in nuclear DNA content in avian muscle

The avian pectoralis muscle demonstrates plasticity with regard to size, so that temperate birds facing winter conditions or birds enduring a migration bout tend to have significant increases in the size and mass of this tissue due to muscular hypertrophy. Myonuclear domain (MND), the volume of cytoplasm a myonuclei services, in the pectoralis muscle of birds seems to be altered during thermal stress or changing seasons. However, there is no information available regarding muscle DNA content or ploidy level within the avian pectoralis. Changes in muscle DNA content can be used in this tissue to aid in size and mass changes. Here, we hypothesized that long-distance migrants or temperate residents would use the process of endoreduplication to aid in altering muscle size. Mostly contradictory to our hypotheses, we found no differences in the mean muscle DNA content in any of the 62 species of birds examined in this study. We also found no correlations between mean muscle DNA content and other muscle structural measurements, such as the number of nuclei per millimeter of fiber, myonuclear domain, and fiber cross-sectional area. Thus, while avian muscle seems more phenotypically plastic than mammalian muscle, the biological processes surrounding myonuclear function may be more closely related to those seen in mammals.

Migratory state and patterns of steroid hormone regulation in the pectoralis muscle of a nomadic migrant, the pine siskin (Spinus pinus)

The endocrine system is known to mediate responses to environmental change and transitions between different life stages (e.g., a non-breeding to a breeding life stage). Previous works from the field of environmental endocrinology have primarily focused on changes in circulating hormones, but a comprehensive understanding of endocrine signaling pathways requires studying changes in additional endocrine components (e.g., receptor densities) in a diversity of contexts and life stages. Migratory birds, for instance, can exhibit dramatic changes in their physiology and behavior, and both sex steroids as well as glucocorticoids are proposed mediators of the transition into a migratory state. However, the role of changes in endocrine signaling components within integral target tissues, such as flight muscles, in modulating the transition into a migratory state remains poorly understood. Here, we examined changes in gene expression levels of and correlational patterns (i.e., integration) between 8 endocrine signaling components associated with either glucocorticoids or sex steroid signaling in the pectoralis muscles of a nomadic migratory bird, the pine siskin (Spinus pinus). The pectoralis muscle is essential to migratory flight and undergoes conspicuous changes in preparation for migration, including hypertrophy. We focus on endocrine receptors and enzymes (e.g., 5α-reductase) that modulate the signaling capacity of circulating hormones within target tissues and may influence either catabolic or anabolic functioning within the pectoralis. Endocrine signaling components were compared between captive birds sampled prior to the expression of vernal migratory preparation and during the expression of a vernal migratory state. While birds exhibited differences in the size and color of the flight muscle and behavioral shifts indicative of a migratory state (i.e., zugunruhe), none of the measured endocrine components differed before and after the transition into the migratory state. Patterns of integration amongst all genes did, however, differ between the two life stages, suggesting the contrasting demands of different life stages may shape entire endocrine signaling networks within target tissues rather than individual components. Our work aligns with previous endocrine studies on pine siskins and, viewed together, suggest additional studies are needed to understand the endocrine system's role in mediating the development and progression of the vernal migratory state in this species. Further, the patterns observed in pine siskins, a nomadic migrant, differ from previous studies on obligate migrants and suggest that different mechanisms or interactions between endocrine signaling components may mediate the migratory transition in nomadic migrants.

Dietary linoleic acid, antioxidants, and flight training influence the activity of oxidative enzymes in European Starlings (Sturnus vulgaris)

Multiple studies have demonstrated that diet (e.g., fatty acid composition, antioxidants) and exercise training affect the metabolic performance of songbirds during aerobic activity, although the physiological mechanisms that cause such an effect remain unclear. We tested the hypothesis that elevated proportions of dietary linoleic acid (18:2n6) and amounts of dietary anthocyanins (a hydrophilic antioxidant class) influence the activity and protein expression of oxidative enzymes in flight and leg muscle of European Starlings (Sturnus vulgaris N = 96), a subset of which were flown over 15 days in a wind tunnel. Carnitine palmitoyl transferase (CPT) and citrate synthase (CS) activity displayed 18:2n6-dependent relationships with soluble protein concentration. Lactate dehydrogenase (LDH) was similarly related to protein concentration although also dependent on both dietary anthocyanins and flight training. 3-Hydroxyacyl CoA Dehydrogenase (HOAD) activity increased throughout the experiment in flight muscle, whereas this relationship was dependent on dietary anthocyanins in the leg muscle. Soluble protein concentration also increased throughout the experiment in the flight muscle, but was unrelated to date in the leg muscle, instead being influenced by both dietary anthocyanins and flight training. Training also produced additive increases in CPT and leg muscle HOAD activity. FAT/CD36 expression was related to both dietary 18:2n6 and training and changed over the course of the experiment. These results demonstrate a notable influence of our diet manipulations and flight training on the activity of these key oxidative enzymes, and particularly CPT and CS. Such influence suggests a plausible mechanism linking diet quality and metabolic performance in songbirds.

Ambient temperature affects multiple drivers of physiology and behaviour: adaptation for timely departure of obligate spring migrants

We investigated the role of ambient temperature in departure from wintering areas of migratory black-headed buntings in spring. Birds transferred at 22 and 35°C to long days were compared with one another and with controls held on short days for indices of readiness to migrate (Zugunruhe, fattening, mass gain), levels of testosterone and gonadal recrudescence. Temperature affected the development of migratory behaviour and physiology: buntings under long days at 35°C, compared with those at 22°C, showed altered migratory behaviour (daily activity and Zugunruhe onset), and enhanced muscle growth and plasma testosterone levels, but showed no effect on testis growth. Temperature was perceived at both peripheral and central levels, and affected multiple molecular drivers culminating into the migratory phenotype. This was evidenced by post-mortem comparison of the expression of 13 genes with known functions in the skin (temperature-sensitive TRP channels: trpv4 and trpm8), hypothalamus and/or midbrain (migration-linked genes: th, ddc, adcyap1 and vps13a) and flight muscles (muscle growth associated genes: ar, srd5a3, pvalb, mtor, myod, mstn and hif1a). In photostimulated birds, the expression of trpv4 in skin, th in the hypothalamus and midbrain, and srd5a3, ar, pvalb and mtor in flight muscle, in parallel with testosterone levels, was greater at 35°C than at 22°C. These results demonstrate the role of ambient temperature in development of the spring migration phenotype, and suggest that transcriptional responsiveness to temperature is a component of the overall adaptive strategy in latitudinal songbird migrants for timely departure from wintering areas in spring.

Structural plasticity of the avian pectoralis: a case for geometry and the forgotten organelle

The avian pectoralis muscle demonstrates incredible plasticity. This muscle is the sole thermogenic organ of small passerine birds, and many temperate small passerines increase pectoralis mass in winter, potentially to increase heat production. Similarly, this organ can double in size prior to migration in migratory birds. In this Commentary, following the August Krogh principle, I argue that the avian pectoralis is the perfect tissue to reveal general features of muscle physiology. For example, in both mammals and birds, skeletal muscle fiber diameter is generally accepted to be within 10–100 µm. This size constraint is assumed to include reaction-diffusion limitations, coupled with metabolic cost savings associated with fiber geometry. However, avian muscle fiber structure has been largely ignored in this field, and the extensive remodeling of the avian pectoralis provides a system with which to investigate this. In addition, fiber diameter has been linked to whole-animal metabolic rates, although this has only been addressed in a handful of bird studies, some of which demonstrate previously unreported levels of plasticity and flexibility. Similarly, myonuclei, which are responsible for protein turnover within the fiber, have been forgotten in the avian literature. The few studies that have addressed myonuclear domain (MND) changes in avian muscle have found rates of change not previously seen in mammals. Both fiber diameter and MND have strong implications for aging rates; most aging mammals demonstrate muscular atrophy (a decrease in fiber diameter) and changes in MND. As I discuss here, these features are likely to differ in birds.

Evaluating endoplasmic reticulum stress and unfolded protein response through the lens of ecology and evolution

Considerable progress has been made in understanding the physiological basis for variation in the life-history patterns of animals, particularly with regard to the roles of oxidative stress and hormonal regulation. However, an underappreciated and understudied area that could play a role in mediating inter- and intraspecific variation of life history is endoplasmic reticulum (ER) stress, and the resulting unfolded protein response (UPR^ER ). ER stress response and the UPR^ER maintain proteostasis in cells by reducing the intracellular load of secretory proteins and enhancing protein folding capacity or initiating apoptosis in cells that cannot recover. Proper modulation of the ER stress response and execution of the UPR^ER allow animals to respond to intracellular and extracellular stressors and adapt to constantly changing environments. ER stress responses are heritable and there is considerable individual variation in UPR^ER phenotype in animals, suggesting that ER stress and UPR^ER phenotype can be subjected to natural selection. The variation in UPR^ER phenotype presumably reflects the way animals respond to ER stress and environmental challenges. Most of what we know about ER stress and the UPR^ER in animals has either come from biomedical studies using cell culture or from experiments involving conventional laboratory or agriculturally important models that exhibit limited genetic diversity. Furthermore, these studies involve the assessment of experimentally induced qualitative changes in gene expression as opposed to the quantitative variations that occur in naturally existing populations. Almost all of these studies were conducted in controlled settings that are often quite different from the conditions animals experience in nature. Herein, we review studies that investigated ER stress and the UPR^ER in relation to key life-history traits including growth and development, reproduction, bioenergetics and physical performance, and ageing and senescence. We then ask if these studies can inform us about the role of ER stress and the UPR^ER in mediating the aforementioned life-history traits in free-living animals. We propose that there is a need to conduct experiments pertaining to ER stress and the UPR^ER in ecologically relevant settings, to characterize variation in ER stress and the UPR^ER in free-living animals, and to relate the observed variation to key life-history traits. We urge others to integrate multiple physiological systems and investigate how interactions between ER stress and oxidative stress shape life-history trade-offs in free-living animals.

The effects of dietary linoleic acid and hydrophilic antioxidants on basal, peak, and sustained metabolism in flight-trained European starlings

Dietary micronutrients have the ability to strongly influence animal physiology and ecology. For songbirds, dietary polyunsaturated fatty acids (PUFAs) and antioxidants are hypothesized to be particularly important micronutrients because of their influence on an individual's capacity for aerobic metabolism and recovery from extended bouts of exercise. However, the influence of specific fatty acids and hydrophilic antioxidants on whole-animal performance remains largely untested. We used diet manipulations to directly test the effects of dietary PUFA, specifically linoleic acid (18:2n6), and anthocyanins, a hydrophilic antioxidant, on basal metabolic rate (BMR), peak metabolic rate (PMR), and rates of fat catabolism, lean catabolism, and energy expenditure during sustained flight in a wind tunnel in European starlings (Sturnus vulgaris). BMR, PMR, energy expenditure, and fat metabolism decreased and lean catabolism increased over the course of the experiment in birds fed a high (32%) 18:2n6 diet, while birds fed a low (13%) 18:2n6 diet exhibited the reverse pattern. Additionally, energy expenditure, fat catabolism, and flight duration were all subject to diet-specific effects of whole-body fat content. Dietary antioxidants and diet-related differences in tissue fatty acid composition were not directly related to any measure of whole-animal performance. Together, these results suggest that the effect of dietary 18:2n6 on performance was most likely the result of the signaling properties of 18:2n6. This implies that dietary PUFA influence the energetic capabilities of songbirds and could strongly influence songbird ecology, given their availability in terrestrial systems.

Phenotypic flexibility of glucocorticoid signaling in skeletal muscles of a songbird preparing to migrate

Glucocorticoids are commonly associated with responses to stress, but other important functions include homeostatic regulation, energy metabolism and tissue remodeling. At low circulating levels, glucocorticoids bind to high-affinity mineralocorticoid receptors (MR) to activate tissue repair and homeostasis (anabolic pathways), whereas at elevated levels, glucocorticoids bind to glucocorticoid receptors (GR) to activate catabolic pathways. Long distance migrations, such as those performed by Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), require modification of anatomy, physiology and behavior. Plasma corticosterone (CORT) increases in association with impending departure and flight and may promote muscle-specific anabolic states. To test this idea, we explored glucocorticoid signaling in the pectoralis (flight) and gastrocnemius (leg) muscles of male sparrows on the wintering grounds at three stages leading up to spring departure: winter (February), pre-nuptial molt (March), and pre-departure (April). CORT was detected in plasma and in both muscles, but measures of CORT signaling differed across muscles and stages. Expression of 11β-hydroxysteroid dehydrogenase (11β-HSD) Type 2 (inactivates CORT) increased in the pectoralis at pre-departure, whereas 11β-HSD Type 1 (regenerates CORT) did not change. Neither of the two 11β-HSD isoforms was detectable in the gastrocnemius. Expression of MR, but not GR, was elevated in the pectoralis at pre-departure, while only GR expression was elevated at pre-nuptial molt in gastrocnemius. These data suggest that anabolic functions predominate in the pectoralis only while catabolic activity is undetected in either muscle at pre-departure.

Acute effects of intense exercise on the antioxidant system in birds: does exercise training help?

The acute effects of an energy-intensive activity such as exercise may alter an animal's redox homeostasis, although these short-term effects may be ameliorated by chronic exposure to that activity, or training, over time. Although well documented in mammals, how energy-intensive training affects the antioxidant system and damage by reactive species has not been investigated fully in flight-trained birds. We examined changes to redox homeostasis in zebra finches exposed to energy-intensive activity (60 min of perch-to-perch flights twice a day), and how exercise training over many weeks affected this response. We measured multiple components of the antioxidant system: an enzymatic antioxidant (glutathione peroxidase, GPx) and non-enzymatic antioxidants (measured by the OXY-adsorbent test) as well as a measure of oxidative damage (d-ROMs). At no point during the experiment did oxidative damage change. We discovered that exposure to energy-intensive exercise training did not alter baseline levels of GPx, but induced exercise-trained birds to maintain a higher non-enzymatic antioxidant status as compared with untrained birds. GPx activity was elevated above baseline in trained birds immediately after completion of the second 1 h flight on each of the three sampling days, and non-enzymatic antioxidants were acutely depleted during flight after 13 and 44 days of training. The primary effect of exercise training on the acute response of the antioxidant system to 2 h flights was increased coordination between the enzymatic (GPx) and non-enzymatic components of the antioxidant system of birds that reduced oxidative damage associated with exercise.

Seasonal muscle ultrastructure plasticity and resistance of muscle structural changes during temperature increases in resident black-capped chickadees and rock pigeons

Resident birds in temperate zones respond to seasonally fluctuating temperatures by adjusting their physiology, such as changes in basal metabolic rate or peak metabolic rate during cold exposure, or altering their organ sizes, so as to match the thermogenic requirements of their current environment. Climate change is predicted to cause increases in the frequency of heat and cold wave events, which could increase the likelihood that birds will face an environmental mismatch. Here, we examined seasonality and the effects of acute and chronic heat shock to 33°C and subsequent recovery from heat shock on the ultrastructure of the superficial pectoralis muscle fiber diameter, myonuclear domain (MND) and capillary density in two temperate bird species of differing body mass, the black-capped chickadee (Poecile atricapillus) and the rock pigeon (Columba livia). We found that muscle fiber ultrastructure did not change with heat treatment. However, in black-capped chickadees, there was a significant increase in fiber diameter in spring phenotype birds compared with summer phenotype birds. In rock pigeons, we saw no differences in fiber diameter across seasons. Capillary density did not change as a function of fiber diameter in black-capped chickadees, but did change seasonally, as did MND. Across seasons, as fiber diameter decreased, capillary density increased in the pectoralis muscle of rock pigeons. For both species in this study, we found that as fiber diameter increased, so did MND. Our findings imply that these two temperate birds employ different muscular growth strategies that may be metabolically beneficial to each.

Preparing to migrate: expression of androgen signaling molecules and insulin-like growth factor-1 in skeletal muscles of Gambel's white-crowned sparrows

Migratory birds, including Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), exhibit profound modifications of skeletal muscles prior to migration, notably hypertrophy of the pectoralis muscle required for powered flight. Muscle growth may be influenced by anabolic effects of androgens; however, prior to spring departure, circulating androgens are low in sparrows. A seasonal increase in local androgen signaling may occur within muscle to promote remodeling. We measured morphological parameters, plasma and tissue levels of testosterone, as well as mRNA expression levels of androgen receptor, 5α-reductase (converts testosterone to 5α-dihydrotestosterone), and the androgen-dependent myotrophic factor insulin-like growth factor-1. We studied the pectoralis muscle as well as the gastrocnemius (leg) muscle of male sparrows across three stages on the wintering grounds: winter (February), pre-nuptial molt (March), and pre-departure (April). Testosterone levels were low, but detectable, in plasma and muscles at all three stages. Androgen receptor mRNA and 5α-reductase Type 1 mRNA increased at pre-departure, but did so in both muscles. Notably, mRNA levels of insulin-like growth factor-1, an androgen-dependent gene critical for muscle remodeling, increased at pre-departure in the pectoralis but decreased in the gastrocnemius. Taken together, these data suggest a site-specific molecular basis for muscle remodeling that may serve to enable long-distance flight.

Relative Contributions of Myostatin and the GH/IGF-1 Axis in Body Composition and Muscle Strength

Myostatin, a negative regulator of muscle growth, is considered a potential therapeutic agent for individuals suffering from various muscle wasting and strength declining diseases because inhibiting Mstn signaling leads to muscular hypertrophy. In this study we investigate the interaction between myostatin and the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis in muscle function and strength. To this end, we measured hind limb grip strength and myostatin levels in two mouse models of GH gene manipulation; GH receptor knockout (GHR^-/-) mice which have reduced GH/IGF-1 action, and bovine GH transgenic (bGH) mice which have excess GH/IGF-1 action. We found that specific muscle force was significantly reduced in bGH mice, and significantly increased in GHR^-/- mice, compared to their respective littermate wild type controls. The expression of the mature form of myostatin was significantly increased in bGH mice, and unchanged in GHR^-/- mice. In the bGH mice, the high levels of mature myostatin were accompanied by increase body weight and lean mass, consistent with other published results indicating that the IGF-1 signaling pathway is dominant over that of Mstn. Our results also suggest that in these mouse models there is an inverse relationship between muscle strength and levels of myostatin and GH, since constitutive overexpression of GH resulted in elevated levels of mature myostatin in muscle, accompanied by a reduction in strength. By contrast, in the GHR^-/- mice with reduced levels of IGF-1, mature myostatin levels were unchanged and muscle strength was increased.

Do Bar-Headed Geese Train for High Altitude Flights?

SYNOPSIS

Exercise at high altitude is extremely challenging, largely due to hypobaric hypoxia (low oxygen levels brought about by low air pressure). In humans, the maximal rate of oxygen consumption decreases with increasing altitude, supporting progressively poorer performance. Bar-headed geese (Anser indicus) are renowned high altitude migrants and, although they appear to minimize altitude during migration where possible, they must fly over the Tibetan Plateau (mean altitude 4800 m) for much of their annual migration. This requires considerable cardiovascular effort, but no study has assessed the extent to which bar-headed geese may train prior to migration for long distances, or for high altitudes. Using implanted loggers that recorded heart rate, acceleration, pressure, and temperature, we found no evidence of training for migration in bar-headed geese. Geese showed no significant change in summed activity per day or maximal activity per day. There was also no significant change in maximum heart rate per day or minimum resting heart rate, which may be evidence of an increase in cardiac stroke volume if all other variables were to remain the same. We discuss the strategies used by bar-headed geese in the context of training undertaken by human mountaineers when preparing for high altitude, noting the differences between their respective cardiovascular physiology.

Understanding variation in migratory movements: A mechanistic approach

Spatial and temporal fluctuations in resource availability have led to the evolution of varied migration patterns. In order to appropriately time movements in relation to resources, environmental cues are used to provide proximate information for timing and the endocrine system serves to integrate these external cues and behavioral and physiological responses. Yet, the regulatory mechanisms underlying migratory timing have rarely been compared across a broad range of migratory patterns. First, we offer an updated nomenclature of migration using a mechanistic perspective to clarify terminology describing migratory types in relation to ecology, behavior and endocrinology. We divide migratory patterns into three types: obligate, nomadic, and fugitive. Obligate migration is characterized by regular and directed annual movements between locations, most commonly for breeding and overwintering, where resources are predictable and sufficient. Nomadic migrations occur less predictably than do obligate migrations as animals make use of potentially rich but ephemeral resources that occur unpredictably in space or time. Fugitive migrations move animals away from an area in response to severe disruption of environmental conditions and occur as part of an emergency life history stage. We also consider partially migratory populations, which include a mix of sedentary and migratory individuals; the movement patterns of partial migrants are expected to fall into one of the three types above. For these various forms of migration, we review our understanding of the environmental cues and endocrine mechanisms that underlie the expression of a migratory state. Several common hormonal mechanisms exist across the varied migratory forms, but there are also important areas where further investigations are needed in order to gain broad insight into the origin of movements and the diversity of migratory patterns. We propose that taking a comparative approach across the migratory types that considers endocrine mechanisms will advance a new understanding of migration biology.

INVITED REVIEW: Inhibitors of myostatin as methods of enhancing muscle growth and development

With the increasing demand for affordable, high-quality meat, livestock and poultry producers must continually find ways to maximize muscle growth in their animals without compromising palatability of the meat products. Muscle mass relies on myoblast proliferation during prenatal or prehatch stages and fiber hypertrophy through protein synthesis and nuclei donation by satellite cells after birth or hatch. Therefore, understanding the cellular and molecular mechanisms of myogenesis and muscle development is of great interest. Myostatin is a well-known negative regulator of muscle growth and development that inhibits proliferation and differentiation in myogenic cells as well as protein synthesis in existing muscle fibers. In this review, various inhibitors of myostatin activity or signaling are examined that may be used in animal agriculture for enhancing muscle growth. Myostatin inhibitors are relevant as potential therapies for muscle-wasting diseases and muscle weakness in humans and animals. Currently, there are no commercial myostatin inhibitors for agriculture or biomedical purposes because the safest and most effective option has yet to be identified. Further investigation of myostatin inhibitors and administration strategies may revolutionize animal production and the medical field.

Context-dependent regulation of pectoralis myostatin and lipid transporters by temperature and photoperiod in dark-eyed juncos

A prominent example of seasonal phenotypic flexibility is the winter increase in thermogenic capacity (=summit metabolism, [Formula: see text]) in small birds, which is often accompanied by increases in pectoralis muscle mass and lipid catabolic capacity. Temperature or photoperiod may be drivers of the winter phenotype, but their relative impacts on muscle remodeling or lipid transport pathways are little known. We examined photoperiod and temperature effects on pectoralis muscle expression of myostatin, a muscle growth inhibitor, and its tolloid-like protein activators (TLL-1 and TLL-2), and sarcolemmal and intracellular lipid transporters in dark-eyed juncos Junco hyemalis. We acclimated winter juncos to four temperature (3 °C or 24 °C) and photoperiod [short-day (SD) = 8L:16D; long-day (LD) = 16L:8D] treatments. We found that myostatin, TLL-1, TLL-2, and lipid transporter mRNA expression and myostatin protein expression did not differ among treatments, but treatments interacted to influence lipid transporter protein expression. Fatty acid translocase (FAT/CD36) levels were higher for cold SD than for other treatments. Membrane-bound fatty acid binding protein (FABPpm) levels, however, were higher for the cold LD treatment than for cold SD and warm LD treatments. Cytosolic fatty acid binding protein (FABP_c) levels were higher on LD than on SD at 3 °C, but higher on SD than on LD at 24 °C. Cold temperature groups showed upregulation of these lipid transporters, which could contribute to elevated M_sum compared to warm groups on the same photoperiod. However, interactions of temperature or photoperiod effects on muscle remodeling and lipid transport pathways suggest that these effects are context-dependent.

Experimentally reducing corticosterone mitigates rapid captivity effects on behavior, but not body composition, in a wild bird

Wild animals and captives display physiological and behavioral differences, and it has been hypothesized, but rarely tested, that these differences are caused by sustained elevation of the hormone corticosterone. We used repeated computed tomography (CT) imaging to examine body composition changes in breeding male and female wild house sparrows (Passer domesticus; n=20) in response to two weeks of captivity, and assessed behavioral changes using video recordings. Half of the birds received the drug mitotane, which significantly decreased stress-induced corticosterone titers compared to controls. Based on the CT images, fat volumes increased, and pectoralis muscle density and heart and testes volumes decreased, over the two weeks of captivity in both groups of birds. However, beak-wiping, a behavior that can indicate anxiety and aggression, showed increased occurrence in controls compared to mitotane-treated birds. While our results do not support the hypothesis that these body composition changes were primarily driven by stress-induced corticosterone, our data suggest that experimentally reducing stress-induced corticosterone may mitigate some captivity-induced behavioral changes. Broadly, our results emphasize that researchers should take behavioral and physiological differences between free-living animals and captives into consideration when designing studies and interpreting results. Further, time in captivity should be minimized when birds will be reintroduced back to the wild.

Altered expression of pectoral myosin heavy chain isoforms corresponds to migration status in the white-crowned sparrow (Zonotrichia leucophrys gambelii)

Birds undergo numerous changes as they progress through life-history stages, yet relatively few studies have examined how birds adapt to both the dynamic energetic and mechanical demands associated with such transitions. Myosin heavy chain (MyHC) expression, often linked with muscle fibre type, is strongly correlated with a muscle's mechanical power-generating capability, thus we examined several morphological properties, including MyHC expression of the pectoralis, in a long-distance migrant, the white-crowned sparrow (Zonotrichia leucophrys gambelii) throughout the progression from winter, spring departure and arrival on breeding grounds. White-crowned sparrows demonstrated significant phenotypic flexibility throughout the seasonal transition, including changes in prealternate moult status, lipid fuelling, body condition and flight muscle morphology. Pectoral MyHC expression also varied significantly over the course of the study. Wintering birds expressed a single, newly classified adult fast 2 isoform. At spring departure, pectoral isoform expression included two MyHC isoforms: the adult fast 2 isoform along with a smaller proportion of a newly present adult fast 1 isoform. By spring arrival, both adult fast isoforms present at departure remained, yet expression had shifted to a greater relative proportion of the adult fast 1 isoform. Altering pectoral MyHC isoform expression in preparation for and during spring migration may represent an adaptation to modulate muscle mechanical output to support long-distance flight.

Hepatocyte Growth Factor and Satellite Cell Activation

Satellite cells are the "currency" for the muscle growth that is critical to meat production in many species, as well as to phenotypic distinctions in development at the level of species or taxa, and for human muscle growth, function and regeneration. Careful research on the activation and behaviour of satellite cells, the stem cells in skeletal muscle, including cross-species comparisons, has potential to reveal the mechanisms underlying pathological conditions in animals and humans, and to anticipate implications of development, evolution and environmental change on muscle function and animal performance.

Cross-training in birds: cold and exercise training produce similar changes in maximal metabolic output, muscle masses and myostatin expression in house sparrows (Passer domesticus)

Maximal metabolic outputs for exercise and thermogenesis in birds presumably influence fitness through effects on flight and shivering performance. Because both summit (Msum, maximum thermoregulatory metabolic rate) and maximum (MMR, maximum exercise metabolic rate) metabolic rates are functions of skeletal muscle activity, correlations between these measurements and their mechanistic underpinnings might occur. To examine whether such correlations occur, we measured the effects of experimental cold and exercise training protocols for 3 weeks on body (Mb) and muscle (Mpec) masses, basal metabolic rate (BMR), Msum, MMR, pectoralis mRNA and protein expression for myostatin, and mRNA expression of TLL-1 and TLL-2 (metalloproteinase activators of myostatin) in house sparrows (Passer domesticus). Both training protocols increased Msum, MMR, Mb and Mpec, but BMR increased with cold training and decreased with exercise training. No significant differences occurred for pectoralis myostatin mRNA expression, but cold and exercise increased the expression of TLL-1 and TLL-2. Pectoralis myostatin protein levels were generally reduced for both training groups. These data clearly demonstrate cross-training effects of cold and exercise in birds, and are consistent with a role for myostatin in increasing pectoralis muscle mass and driving organismal increases in metabolic capacities.

Transcriptome analysis of post-hatch breast muscle in legacy and modern broiler chickens reveals enrichment of several regulators of myogenic growth

Agriculture provides excellent model systems for understanding how selective pressure, as applied by humans, can affect the genomes of plants and animals. One such system is modern poultry breeding in which intensive genetic selection has been applied for meat production in the domesticated chicken. As a result, modern meat-type chickens (broilers) exhibit enhanced growth, especially of the skeletal muscle, relative to their legacy counterparts. Comparative studies of modern and legacy broiler chickens provide an opportunity to identify genes and pathways affected by this human-directed evolution. This study used RNA-seq to compare the transcriptomes of a modern and a legacy broiler line to identify differentially enriched genes in the breast muscle at days 6 and 21 post-hatch. Among the 15,945 genes analyzed, 10,841 were expressed at greater than 0.1 RPKM. At day 6 post-hatch 189 genes, including several regulators of myogenic growth and development, were differentially enriched between the two lines. The transcriptional profiles between lines at day 21 post-hatch identify 193 genes differentially enriched and still include genes associated with myogenic growth. This study identified differentially enriched genes that regulate myogenic growth and differentiation between the modern and legacy broiler lines. Specifically, differences in the ratios of several positive (IGF1, IGF1R, WFIKKN2) and negative (MSTN, ACE) myogenic growth regulators may help explain the differences underlying the enhanced growth characteristics of the modern broilers.

Regulatory mechanisms of metabolic flexibility in the dark-eyed junco (Junco hyemalis)

Small temperate birds reversibly modify their aerobic performance to maintain thermoregulatory homeostasis under seasonally changing environmental conditions and these physiological adjustments may be attributable to changes in the expression of genes in the underlying regulatory networks. Here, we report the results of an experimental procedure designed to gain insight into the fundamental mechanisms of metabolic flexibility in the dark-eyed junco (Junco hyemalis). We combined genomic transcriptional profiles with measures of metabolic enzyme activities and whole-animal thermogenic performance from juncos exposed to four 6-week acclimation treatments that varied in temperature (cold, 3°C; warm, 24°C) and photoperiod (short day, 8 h light:16 h dark; long day, 16 h light:8 h dark). Cold-acclimated birds increased thermogenic capacity compared with warm-acclimated birds, and this enhanced performance was associated with upregulation of genes involved in muscle hypertrophy, angiogenesis, and lipid transport and oxidation, as well as with catabolic enzyme activities. These physiological changes occurred over ecologically relevant timescales, suggesting that birds make regulatory adjustments to interacting, hierarchical pathways in order to seasonally enhance thermogenic capacity.

Phenotypic flexibility of skeletal muscle and heart masses and expression of myostatin and tolloid-like proteinases in migrating passerine birds

Migrant birds require large flight muscles and hearts to enhance aerobic capacity and support sustained flight. A potential mechanism for increasing muscle and heart masses during migration in birds is the muscle growth inhibitor myostatin and its metalloproteinase activators, tolloid-like proteinases (TLL-1 and TLL-2). We hypothesized that myostatin, TLL-1 and TLL-2 are downregulated during migration in pectoralis and hearts of migratory passerines to promote hypertrophy. We measured seasonal variation of tissue masses, mRNA expression of myostatin, TLL-1, and TLL-2, and myostatin protein levels in pectoralis muscle and heart for yellow warblers (Setophaga petechia), warbling vireos (Vireo gilvus), and yellow-rumped warblers (Setophaga coronata). Pectoralis mass was greatest in spring for warbling vireos and yellow warblers, but was stable between spring and fall for yellow-rumped warblers. Heart mass was higher in spring than in fall for yellow-rumped warblers, lowest in fall for warbling vireos, and seasonally stable for yellow warblers. Pectoralis and heart mRNA expression of myostatin and the TLLs did not differ significantly for any of the three species, offering little support for our hypothesis for a prominent role for myostatin in regulating migration-induced variation in pectoralis and heart masses. In contrast, pectoralis myostatin protein levels were lowest in spring for all three species, consistent with our hypothesis. Myostatin protein levels in heart, however, were seasonally stable for warbling vireos and yellow warblers, and increased in spring relative to fall for yellow-rumped warblers. These data offer mixed support for our hypothesis for the pectoralis, but suggest that myostatin is not a prominent regulator of migration-induced heart hypertrophy. Moreover, the different seasonal patterns for pectoralis mRNA and protein expression suggest that post-transcriptional modification of myostatin may contribute to pectoralis mass regulation during migration.

Phenotypic flexibility in passerine birds: seasonal variation in fuel storage, mobilization and transport

Winter acclimatization in small birds living in cold climates produces a winter phenotype characterized by upregulation of metabolic rates to meet enhanced thermoregulatory demands. We measured several key aspects of fuel storage, mobilization and transport in summer and winter to determine whether black-capped chickadees (Poecile atricapillus), white-breasted nuthatches (Sitta carolinensis), and house sparrows (Passer domesticus) seasonally modulate these attributes to meet enhanced winter thermoregulatory demands. In addition, we exposed birds to thermoneutral (control) and severe cold exposure treatments to determine whether acute cold exposure influenced fuel storage, mobilization or transport. Carcass lipid mass and pectoralis intramuscular lipid did not vary significantly between seasons or temperature treatments for any of the study species. Muscle glycogen varied significantly seasonally only for chickadee supracoracoideus and leg muscles, and did not vary among warm or cold treatments for any species. Pectoralis fatty acid binding protein (FABPc) was significantly elevated in winter for chickadees and nuthatches, but not for sparrows. Plasma metabolites showed little consistent variation in response to season or acute cold exposure. Thus, fuel storage and mobilization do not appear to be major targets of adjustment associated with seasonal metabolic flexibility in these species, but modulation of intracellular lipid transport by FABPc may be an important contributor to seasonal phenotypes in some species of small birds.

Seasonal variation in pectoralis muscle and heart myostatin and tolloid-like proteinases in small birds: a regulatory role for seasonal phenotypic flexibility?

Seasonally variable environments produce seasonal phenotypes in small birds such that winter birds have higher thermogenic capacities and pectoralis and heart masses. One potential regulator of these seasonal phenotypes is myostatin, a muscle growth inhibitor, which may be downregulated under conditions promoting increased energy demand. We examined summer-to-winter variation in skeletal muscle and heart masses and used qPCR and Western blots to measure levels of myostatin and its metalloproteinase activators TLL-1 and TLL-2 for two small temperate-zone resident birds, American goldfinches (Spinus tristis) and black-capped chickadees (Poecile atricapillus). Winter pectoralis and heart masses were significantly greater than in summer for American goldfinches. Neither myostatin expression nor protein levels differed significantly between seasons for goldfinch pectoralis. However, myostatin levels in goldfinch heart were significantly greater in summer than in winter, although heart myostatin expression was seasonally stable. In addition, expression of both metalloproteinase activators was greater in summer than in winter goldfinches for both pectoralis and heart, significantly so except for heart TLL-2 (P = 0.083). Black-capped chickadees showed no significant seasonal variation in muscle or heart masses. Seasonal patterns of pectoralis and heart expression and/or protein levels for myostatin and its metalloproteinase activators in chickadees showed no consistent seasonal trends, which may help explain the absence of significant seasonal variation in muscle or heart masses for chickadees in this study. These data are partially consistent with a regulatory role for myostatin, and especially myostatin processing capacity, in mediating seasonal metabolic phenotypes of small birds.

Growth prior to thermogenesis for a quick fledging of Adélie penguin chicks (Pygoscelis adeliae)

The evolutionary trade-off between tissue growth and mature function restricts the post natal development of polar birds. The present study uses an original integrative approach as it includes gene expression, plus biochemical and physiological analysis to investigate how Adélie penguin chicks achieve a rapid growth despite the energetic constraints linked to the cold and the very short breeding season in Antarctica. In pectoralis muscle, the main thermogenic tissue in birds, our data show that the transition from ectothermy to endothermy on Day 15 post- hatching is associated with substantial and coordinated changes in the transcription of key genes. While the early activation of genes controlling cell growth and differentiation (avGHR, avIGF-1R, T3Rβ) is rapidly down-regulated after hatching, the global increase in the relative expression of genes involved in thermoregulation (avUCP, avANT, avLPL) and transcriptional regulation (avPGC1α, avT3Rβ) underlie the muscular acquisition of oxidative metabolism. Adélie chicks only become real endotherms at 15 days of age with the development of an oxidative muscle phenotype and the ability to shiver efficiently. The persistent muscular expression of IGF-1 throughout growth probably acts as a local mediator to adjust muscle size and its oxidative capacity to anticipate the new physiological demands of future Dives in cold water. The up-regulation of T3Rβ mRNA levels suggests that circulating T3 may play an important role in the late maturation of skeletal muscle by reinforcing, at least in part, the paracrine action of IGF-1. From day 30, the metabolic shift from mixed substrate to lipid metabolism, with the markedly increased mRNA levels of muscle avLPL, avANT and avUCP, suggests the late development of a fatty acid-enhanced muscle non-shivering thermogenesis mechanism. This molecular control is the key to this finely-tuned strategy by which the Adélie penguin chick successfully heads for the sea on schedule.

Individual variation in thermogenic capacity is correlated with flight muscle size but not cellular metabolic capacity in American goldfinches (Spinus tristis)

Abstract Cold tolerance and overwinter survival are positively correlated with organismal thermogenic capacity (=summit metabolic rate [Msum]) in endotherms. Msum varies seasonally in small-bird populations and may be mechanistically associated with variation in flight muscle size or cellular metabolic capacity, but the relative roles of these traits as drivers of individual variation in thermogenic performance are poorly known. We measured flight muscle size by ultrasonography, pectoralis and supracoracoideus muscle masses, and muscular activities of key aerobic enzymes (citrate synthase, carnitine palmitoyl transferase, and β-hydroxyacyl-CoA dehydrogenase) and correlated these measurements with Msum for individual American goldfinches (Spinus tristis) to test the hypotheses that muscle size and/or cellular metabolic capacity serve as prominent drivers of individual variation in organismal metabolic capacity. Ultrasonographic flight muscle size was weakly positively correlated with Msum ([Formula: see text]). Both log10-transformed Msum and flight muscle mass were significantly correlated with log10 body mass, so we calculated allometric residuals for log Msum and for log flight muscle mass to test their correlation independent of body mass. Flight muscle mass residuals were significantly positively correlated with Msum residuals, and this correlation was primarily driven by variation in pectoralis muscle mass. In contrast, none of the mass-specific activities of any enzyme in any muscle were significantly correlated with Msum. These data suggest that flight muscle size, not cellular metabolic capacity, is the primary driver of individual variation in thermogenic performance in goldfinches. This is consistent with the idea that phenotypic flexibility of flight muscle mass is a general mechanism mediating variation in metabolic performance in response to changing energy demands in birds.

Androgens regulate gene expression in avian skeletal muscles

Circulating androgens in adult reproductively active male vertebrates influence a diversity of organ systems and thus are considered costly. Recently, we obtained evidence that androgen receptors (AR) are expressed in several skeletal muscles of three passeriform birds, the golden-collared manakin (Manacus vitellinus), zebra finch (Taenopygia guttata), and ochre-bellied flycatcher (Mionectes oleagieus). Because skeletal muscles that control wing movement make up the bulk of a bird’s body mass, evidence for widespread effects of androgen action on these muscles would greatly expand the functional impact of androgens beyond their well-characterized effects on relatively discrete targets throughout the avian body. To investigate this issue, we use quantitative PCR (qPCR) to determine if androgens alter gene mRNA expression patterns in wing musculature of wild golden-collared manakins and captive zebra finches. In manakins, the androgen testosterone (T) up-regulated expression of parvalbumin (PV) and insulin-like growth factor I (IGF-I), two genes whose products enhance cellular Ca²⁺ cycling and hypertrophy of skeletal muscle fibers. In T-treated zebra finches, the anti-androgen flutamide blunted PV and IGF-I expression. These results suggest that certain transcriptional effects of androgen action via AR are conserved in passerine skeletal muscle tissue. When we examined wing muscles of manakins, zebra finches and ochre-bellied flycatchers, we found that expression of PV and IGF-I varied across species and in a manner consistent with a function for AR-dependent gene regulation. Together, these findings imply that androgens have the potential to act on avian muscle in a way that may enhance the physicality required for successful reproduction.

Does immune suppression during stress occur to promote physical performance?

Two adaptationist hypotheses have been proposed to explain why stress, particularly elevation of stress hormones (i.e., glucocorticoids), tends to suppress immune functions. One is that immune suppression represents efforts to minimize autoimmune responses to self-antigens released as organisms cope with stressors (i.e., the autoimmune-avoidance hypothesis). The other is that immune suppression occurs to promote a shunting of resources to life processes more conducive to survival of the stressor (i.e., the re-allocation hypothesis). Here in wild-caught house sparrows (Passer domesticus), we tested the second hypothesis, asking whether sustained elevation of baseline glucocorticoids, due to captivity, caused a greater rate of decline in immune functions than flight performance. A greater decline in immune functions than flight performance would support the re-allocation hypothesis. As in previous studies, we found that captivity tended to alter baseline corticosterone, suggesting that house sparrows experience captivity as a stressor. Captivity also affected several constitutive and induced innate immune metrics: bacterial (Escherichia coli) killing activity (BKA) of blood and oxidative burst of leukocytes both changed in a manner consistent with immune disregulation. In contrast, breast muscle size and vertical flight (hovering) duration improved over captivity. Collectively, these changes provide indirect support for the re-allocation hypothesis, although within individuals, changes in immune and physical performance were unrelated.