Exercise and reduced muscle mass in starlings

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.

Ultraviolet-reflective film applied to windows reduces the likelihood of collisions for two species of songbird

Perhaps a billion birds die annually from colliding with residential and commercial windows. Therefore, there is a societal need to develop technologies that reduce window collisions by birds. Many current window films that are applied to the external surface of windows have human-visible patterns that are not esthetically preferable. BirdShades have developed a short wavelength (ultraviolet) reflective film that appears as a slight tint to the human eye but should be highly visible to many bird species that see in this spectral range. We performed flight tunnel tests of whether the BirdShades external window film reduced the likelihood that two species of song bird (zebra finch, Taeniopygia guttata and brown-headed cowbird, Molothrus ater) collide with windows during daylight. We paid particular attention to simulate the lighting conditions that birds will experience while flying during the day. Our results indicate a 75-90% reduction in the likelihood of collision with BirdShades-treated compared with control windows, in forced choice trials. In more ecologically relevant comparison between trials where all windows were either treated or control windows, the estimated reduction in probability of collision was 30-50%. Further, both bird species slow their flight by approximately 25% when approaching windows treated with the BirdShades film, thereby reducing the force of collisions if they were to happen. Therefore, we conclude that the BirdShades external window film will be effective in reducing the risk of and damage caused to populations and property by birds' collision with windows. As this ultraviolet-reflective film has no human-visible patterning to it, the product might be an esthetically more acceptable low cost solution to reducing bird-window collisions. Further, we call for testing of other mitigation technologies in lighting and ecological conditions that are more similar to what birds experience in real human-built environments and make suggestions for testing standards to assess collision-reducing technologies.

Morphological determinants of jumping performance in the Iberian green frog

Predation is one of the main selective forces in nature, frequently selecting potential prey for developing escape strategies. Escape ability is typically influenced by several morphological parameters, such as morphology of the locomotor appendices, muscular capacity, body mass, or fluctuating asymmetry, and may differ between sexes and age classes. In this study, we tested the relationship among these variables and jumping performance in 712 Iberian green frogs Pelophylax perezi from an urban population. The results suggest that the main determinant of jumping capacity was body size (explaining 48% of variance). Larger frogs jumped farther, but jumping performance reached an asymptote for the largest frogs. Once controlled by structural body size, the heaviest frogs jumped shorter distances, suggesting a trade-off between fat storage and jumping performance. Relative hind limb length also determined a small but significant percentage of variance (2.4%) in jumping performance—that is, the longer the hind limbs, the greater the jumping capacity. Juveniles had relatively shorter and less muscular hind limbs than adults (for a given body size), and their jumping performance was poorer. In our study population, the hind limbs of the frogs were very symmetrical, and we found no effect of fluctuating asymmetry on jumping performance. Therefore, our study provides evidence that jumping performance in frogs is not only affected by body size, but also by body mass and hind limb length, and differ between age classes.

Artificial selection reveals the energetic expense of producing larger eggs

BACKGROUND

The amount of resources provided by the mother before birth has important and long-lasting effects on offspring fitness. Despite this, there is a large amount of variation in maternal investment seen in natural populations. Life-history theory predicts that this variation is maintained through a trade-off between the benefits of high maternal investment for the offspring and the costs of high investment for the mother. However, the proximate mechanisms underlying these costs of reproduction are not well understood. Here we used artificial selection for high and low maternal egg investment in a precocial bird, the Japanese quail (Coturnix japonica) to quantify costs of maternal reproductive investment.

RESULTS

We show that females from the high maternal investment lines had significantly larger reproductive organs, which explained their overall larger body mass, and resulted in a higher resting metabolic rate (RMR). Contrary to our expectations, this increase in metabolic activity did not lead to a higher level of oxidative damage.

CONCLUSIONS

This study is the first to provide experimental evidence for metabolic costs of increased per offspring investment.

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.

Exposure to cold but not exercise increases carbon turnover rates in specific tissues of a passerine

Carbon turnover differs between tissues within an animal, but the extent to which ecologically relevant increases in metabolism affect carbon turnover rates is largely unknown. We tested the energy expenditure and protein turnover hypotheses that predict increased carbon turnover, either in association with increased daily energy expenditure, or in concert with tissue-specific increased protein metabolism. We used stable-isotope-labeled diets to quantify the rate of carbon turnover in 12 different tissues for three groups of zebra finches (Taeniopygia guttata): cold-exposed birds kept at ambient temperatures below their thermoneutral zone, exercised birds that were flown for 2 h per day in a flight arena, and control birds that were kept at ambient temperatures within their thermoneutral zone and that were not exercised. We found that increases in metabolism associated with cold-exposure but not exercise produced measurable increases in carbon turnover rate of, on average, 2.4±0.3 days for pectoral muscle, gizzard, pancreas and heart, even though daily energy intake was similar for exercised and cold-exposed birds. This evidence does not support the energy expenditure hypothesis, and we invoke two physiological processes related to protein metabolism that can explain these treatment effects: organ mass increase and tissue-specific increase in activity. Such changes in carbon turnover rate associated with cold temperatures translate into substantial variation in the estimated time window for which resource use is estimated and this has important ecological relevance.

Seasonal variation of myostatin gene expression in pectoralis muscle of house sparrows (Passer domesticus) is consistent with a role in regulating thermogenic capacity and cold tolerance

Winter acclimatization in small birds overwintering in cold climates, including house sparrows (Passer domesticus), is associated with improved cold tolerance, elevated summit metabolic rates (M(sum) = maximum cold-induced metabolic rate), and increased pectoralis muscle mass compared to summer birds. Myostatin is a potent autocrine/paracrine inhibitor of skeletal muscle growth in mammals and birds and is a potential candidate for regulation of seasonal phenotypic flexibility in birds. As a first step toward examining such a role for myostatin in small birds, we measured summer and winter gene expression of myostatin and its potential metalloproteinase activators TLL-1 and TLL-2 in house sparrows from southeastern South Dakota. Gene expression of myostatin decreased significantly in winter, with summer values exceeding winter values by 1.52-fold. Moreover, gene expression of TLL-1 was also significantly reduced in winter, with summer values exceeding winter values by 1.55-fold. These data are consistent with the hypothesis that the winter increases in pectoralis muscle mass, M(sum), and cold tolerance in house sparrows are mediated by reduced levels of myostatin and its activator TLL-1, and they suggest the possibility that myostatin may be a common mediator of phenotypic flexibility of muscle mass in birds.

Morphological and physiological aspects of takeoff aptitudes of female common eiders (Somateria mollissima) during the pre-laying period

In many bird species, follicular growth and the accumulation of body reserves may impair flight takeoff capability of breeding females. The pre-laying period in common eiders, Somateria mollissima (L., 1758), is characterized by a dramatic sex-specific takeoff impairment, and we tested the hypothesis that pre-laying female common eiders compensate to diminish or delay their takeoff impairment. We addressed our question at three organizational levels: (1) measurement of wing morphology, (2) mass of flight musculature, and (3) measurement of the activity of key enzymes of aerobic and glycolytic capacity in one locomotor muscle and one structural muscle. All measurements were performed on individuals of both sexes. We found that wing area was similar between sexes despite males being structurally larger. Our results also indicate that female mass gain is partly compensated by higher pectoralis mass, while the catalytic capacity of the tissue remains unchanged. We conclude that females diminish and delay their takeoff impairment differently according to organizational levels.

Continued divergence in VO2max of rats artificially selected for running endurance is mediated by greater convective blood O2 delivery

We previously showed that after seven generations of artificial selection of rats for running capacity, maximal O2 uptake (VO2max) was 12% greater in high-capacity (HCR) than in low-capacity runners (LCR). This difference was due exclusively to a greater O2 uptake and utilization by skeletal muscle of HCR, without differences between lines in convective O2 delivery to muscle by the cardiopulmonary system (QO2max). The present study in generation 15 (G15) female rats tested the hypothesis that continuing improvement in skeletal muscle O2 transfer must be accompanied by augmentation in QO2max to support VO2max of HCR. Systemic O2 transport was studied during maximal normoxic and hypoxic exercise (inspired PO2 approximately 70 Torr). VO2max divergence between lines increased because of both improvement in HCR and deterioration in LCR: normoxic VO2max was 50% higher in HCR than LCR. The greater VO2max in HCR was accompanied by a 41% increase in QO2max: 96.1 +/- 4.0 in HCR vs. 68.1 +/- 2.5 ml stpd O2 x min(-1) x kg(-1) in LCR (P < 0.01) during normoxia. The greater G15 QO2max of HCR was due to a 48% greater stroke volume than LCR. Although tissue O2 diffusive conductance continued to increase in HCR, tissue O2 extraction was not significantly different from LCR at G15, because of the offsetting effect of greater HCR blood flow on tissue O2 extraction. These results indicate that continuing divergence in VO2max between lines occurs largely as a consequence of changes in the capacity to deliver O2 to the exercising muscle.

Circulatory variables and the flight performance of birds

When considering the 'burst' flight performance of birds, such as during take-off, one of the most important structural variables is the ratio of the mass of the flight muscle myofibrils with respect to body mass. However, when considering 'prolonged' flight performance the variable of interest should be the body mass ratio of the mass of the flight muscle myofibrils that can be perfused sustainably with metabolites via the blood supply. The latter variable should be related to blood flow (ml min(-1)), which in turn has been shown to be a function of heart muscle mass, the value of which is more easily obtainable for different species than that for the mass of perfused muscle. The limited empirical evidence available suggests that for birds and mammals the rate of maximum oxygen consumption scales with heart mass (Mh) as Mh 0.88 and that for birds Mh scales with body mass (Mb) as Mb 0.92, leading to the conclusion that the rate of maximum oxygen consumption in birds scales with an exponent of around Mb 0.82. A similar exponent would be expected for the rate of maximum oxygen consumption with respect to the flight muscle mass of birds. This suggests that the sustainable power output from the flight muscles may ultimately be limiting the flight performance of very large flying animals, but as a result of circulatory constraints rather than biomechanical considerations of the flight muscles per se. Under the particular circumstances of sustainable flight performance, calculations of rates of metabolic energy consumed by the flight muscles can be compared directly with the estimates of biomechanical power output required, as calculated using various aerodynamic models. The difference between these calculated values for rates of energy input and output from the muscles is equivalent to the 'apparent' mechanochemical conversion efficiency. The results of one such analysis, of the maximum sustainable flight performance of migratory birds, leads to the conclusion that the efficiency of the flight muscles appears to scale as Mb 0.14. However, much of this apparent scaling may be an artefact of the application and assumptions of the models. The resolution of this issue is only likely to come from studying bird species at either extreme of the size range.

Myofiber adaptational response to exercise in a mouse model of nemaline myopathy

In some muscle diseases, such as muscular dystrophy, exercise can increase muscle damage and alter myofiber adaptation. We determined whether this is also true for the congenital muscle disease nemaline myopathy using our mouse model of this disease. Nemaline mice expressing a mutant alpha-tropomyosinslow protein [alpha-Tmslow(Met9Arg)] in skeletal muscle underwent 4 weeks of treadmill exercise. Exercise increased slow/oxidative myofibers, but different fibers were involved in these transformations in nemaline mice. Despite similar expression of the mutant alpha-Tmslow protein in muscles of the nemaline mouse, muscles responded in a unique manner that did not reflect fiber-type composition. For example, the particular fibers involved in fast-to-slow transformation were specific for each muscle examined. In contrast to the muscular dystrophies, exercise did not result in muscle damage nor did it cause an increase in rod-containing fibers; however, the fiber-type distribution of rod-containing fibers was altered in a muscle-specific fashion. That exercise did not exacerbate the pathology (i.e., nemaline rod formation) supports its use in nemaline myopathy patients. This study shows that fibers of a similar type respond to increased activity differently in different muscles and suggests that fibers of similar type may be functionally distinct in different muscles.

Plasticity in Body Composition in Breeding Birds: What Drives the Metabolic Costs of Egg Production?

Body composition in vertebrates is known to show phenotypic plasticity, and changes in organ masses are usually rapid and reversible. One of the most rapid and reversible changes is the transformation of the female avian reproductive organs before breeding. This provides an excellent system to investigate the effects of plasticity in organ size on basal metabolic rate (BMR) through relationships between organ masses and BMR. We compared body composition of female European starlings (Sturnus vulgaris) during various reproductive stages over 3 yr and investigated the pattern of changes in reproductive and nonreproductive organ mass during follicular development and ovulation. Furthermore, we analyzed the relationship between organ mass and resting metabolic rate (RMR) in nonbreeding, laying, and chick-rearing females. Our analysis revealed marked variation in organ masses between breeding stages but no consistent pattern among years except for kidney and pectoralis muscle. Furthermore, changes in nonreproductive organs did not parallel the cycle of growth and regression of the reproductive organs. The oviduct gained 62% of its 22-fold increase in mass in only 3 d, and oviduct regression was just as rapid and began even before the final egg of the clutch was laid, with 42% of the oviduct mass lost before laying of the final egg. In laying females, 18% of variation in mass-corrected RMR was explained by the mass of the oviduct (r2=0.18, n=80, P<0.0005), while pectoralis muscle mass in nonbreeding individuals and liver and gizzard mass in chick-rearing females were the only organs significantly related to RMR (r2=0.31-0.44). We suggest that the nonreproductive organs are affected more by changes in local ecological conditions than the reproductive state itself and that the activity and maintenance cost of the oviduct is high enough that selection has led to a very tight size-function relationship for this organ.

Responses of body components to changes in the energetic demand throughout the breeding stages of rhinoceros auklets

When rearing chicks, seabirds increase their daily energy expenditures during commuting flights between foraging areas and breeding colonies, owing to the heavy food loads. At this time, parents are expected to enlarge the size of their energy-supplying organs in response to the increased energy demands but reduce their total body mass to minimize the energetic cost of flight. The changes in body components of 40 incubating and chick-rearing rhinoceros auklets (Cerorhinca monocerata) were examined. Chick-rearing auklets did not have larger energy-supplying organs and breast muscles than incubating ones. However, chick-rearing auklets had greater ash composition, but smaller lipid contents, of breast muscles than incubating ones, whereas the former had a mass of water and protein similar to the latter. Male and female auklets lost a mean of 32.6 and 32.1 g in body mass between incubation and chick-rearing stages, mainly via loss of lipid reserves, which consequently reduces flight costs by 9.9 and 9.1%, respectively. Performance of commuting flight could be improved through changes in breast muscle compositions and reductions in total body mass. Although auklets did not enlarge their energy-supplying organs, their body conditions could be maintained within the same phase between the breeding stages.

Tetranectin in slow intra- and extrafusal chicken muscle fibers

Tetranectin is a C-type lectin that occurs in the mammalian musculoskeletal system. In the present report we describe the first studies on an avian tetranectin. A full-length chicken tetranectin cDNA was isolated. Comparison of the deduced amino acid sequence of chicken tetranectin with mouse and human tetranectin showed an identity of 67 and 68%, respectively. Northern blot analysis demonstrated broad expression of chicken tetranectin mRNA, which was first detected on embryonic day 4. Tetranectin protein was detected in chicken serum and egg yolk. Since muscle is one of few tissues in which tetranectin protein is retained, we examined the distribution of tetranectin in various muscle types in chicken. Myofibers strongly positive for tetranectin were observed in several muscles including m. tibialis ant. and m. sartorius (from embryonic day 10 to adult). Using antibodies to fast and slow myosin heavy chains (MHC) and double immunostaining techniques, we found that tetranectin was restricted to slow (type I) muscle fibers. Similarly only slow intrafusal fibers accumulated tetranectin. The pattern of immunostaining in chickens differs markedly from that seen in mouse muscles, indicating that tetranectin performs a role in muscle that is not associated with a hitherto recognized muscle type or function.

Body building and concurrent mass loss: flight adaptations in tree sparrows

Environmental changes are responsible for the evolution of flexible physiology and the extent of phenotypic plasticity in the regulation of birds' organ size has not been appreciated until recently. Rapid reversible physiological changes during different life-history stages are virtually only known from long-distance migrants, and few studies have focused on less extreme aspects of organ flexibility. During moult, birds suffer from increased wing loading due to wing-area reductions, which may impair flight ability. A previous study found that tree sparrows' escape flight (Passer montanus) is unaffected during moult, suggesting compensatory aptness. We used non-invasive techniques to study physiological adaptations to increased wing loading in tree sparrows. As wing area was reduced during natural moult the ratio of pectoral-muscle size to body mass increased. When moult was completed this ratio decreased. We show experimentally a novel, strategic, organ-flexibility pattern. Unlike the general pattern, where body mass is positively coupled to pectoral-muscle size, tree sparrows responded within 7 days to reductions in wing area by reducing body mass concurrently with an increase in pectoral-muscle size. This rapid flexibility in a non-migratory species probably reflects the paramount importance and long history of flight in birds.