Catabolic capacity of the muscles of shorebird chicks: maturation of function in relation to body size

Wings versus legs in the avian bauplan: development and evolution of alternative locomotor strategies

Wings have long been regarded as a hallmark of evolutionary innovation, allowing insects, birds, and bats to radiate into aerial environments. For many groups, our intuitive and colloquial perspective is that wings function for aerial activities, and legs for terrestrial, in a relatively independent manner. However, insects and birds often engage their wings and legs cooperatively. In addition, the degree of autonomy between wings and legs may be constrained by tradeoffs, between allocating resources to wings versus legs during development, or between wing versus leg investment and performance (because legs must be carried as baggage by wings during flight and vice versa). Such tradeoffs would profoundly affect the development and evolution of locomotor strategies, and many related aspects of animal ecology. Here, we provide the first evaluation of wing versus leg investment, performance and relative use, in birds-both across species, and during ontogeny in three precocial species with different ecologies. Our results suggest that tradeoffs between wing and leg modules help shape ontogenetic and evolutionary trajectories, but can be offset by recruiting modules cooperatively. These findings offer a new paradigm for exploring locomotor strategies of flying organisms and their extinct precursors, and thereby elucidating some of the most spectacular diversity in animal history.

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.

Differential muscular myosin heavy chain expression of the pectoral and pelvic girdles during early growth in the king penguin (Aptenodytes patagonicus) chick

Continuous growth, associated with a steady parental food supply, is a general pattern in offspring development. So that young chicks can acquire their locomotor independence, this period is usually marked by a fast maturation of muscles, during which different myosin heavy chain (MyHC) isoforms are expressed. However, parental food provisioning may fluctuate seasonally, and offspring therefore face a challenge to ensure the necessary maturation of their tissues when energy is limited. To address this trade-off we investigated muscle maturation in both the pectoral and pelvic girdles of king penguin chicks. This species has an exceptionally long rearing period (1 year), which is prolonged when parental food provisioning is drastically reduced during the sub-Antarctic winter. Approximately 1 month post hatching, chicks acquire a functional pedestrian locomotion, which uses pelvic muscles, whereas swimming, which uses the pectoral muscles, only occurs 1 year later. We therefore tested the hypothesis that the MyHC content of the leg muscles reaches a mature state before those of the pectoral muscles. We found that leg muscle MyHC composition changed with the progressive acquisition of pedestrian locomotion, whereas pectoral muscle fibres reached their mature MyHC profile as early as hatching. Contrary to our predictions, the acquisition of the adult profile in pectoral muscles could be related to an early maturation of the contractile muscular proteins, presumably associated with early thermoregulatory capacities of chicks, necessary for survival in their cold environment. This differential maturation appears to reconcile both the locomotor and environmental constraints of king penguin chicks during growth.

Postnatal development of muscle biochemistry in nursing harbor seal (Phoca vitulina) pups: limitations to diving behavior?

Adult marine mammal muscles rely upon a suite of adaptations for sustained aerobic metabolism in the absence of freely available oxygen (O(2)). Although the importance of these adaptations for supporting aerobic diving patterns of adults is well understood, little is known about postnatal muscle development in young marine mammals. However, the typical pattern of vertebrate muscle development, and reduced tissue O(2) stores and diving ability of young marine mammals suggest that the physiological properties of harbor seal (Phoca vitulina) pup muscle will differ from those of adults. We examined myoglobin (Mb) concentration, and the activities of citrate synthase (CS), beta-hydroxyacyl coA dehydrogenase (HOAD), and lactate dehydrogenase (LDH) in muscle biopsies from harbor seal pups throughout the nursing period, and compared these biochemical parameters to those of adults. Pups had reduced O(2) carrying capacity ([Mb] 28-41% lower than adults) and reduced metabolically scaled catabolic enzyme activities (LDH/RMR 20-58% and CS/RMR 29-89% lower than adults), indicating that harbor seal pup muscles are biochemically immature at birth and weaning. This suggests that pup muscles do not have the ability to support either the aerobic or anaerobic performance of adult seals. This immaturity may contribute to the lower diving capacity and behavior in younger pups. In addition, the trends in myoglobin concentration and enzyme activity seen in this study appear to be developmental and/or exercise-driven responses that together work to produce the hypoxic endurance phenotype seen in adults, rather than allometric effects due to body size.

Increased myogenic responsiveness of skeletal muscle arterioles with juvenile growth

Previous studies from this laboratory suggest that during juvenile growth, structural changes in the arteriolar network are accompanied by changes in some of the mechanisms responsible for regulation of tissue blood flow. To test the hypothesis that arteriolar myogenic behavior is altered with growth, we studied gracilis muscle arterioles isolated from Sprague-Dawley rats at two ages: 21-28 and 42-49 days. When studied at their respective in vivo pressures, the myogenic index (instantaneous slope of the active pressure-diameter curve) of arterioles from 42-49-day-old rats was more negative than that of arterioles from 21-28-day-old rats, indicating greater myogenic responsiveness. Endothelial denudation, or prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) receptor antagonism without denudation, significantly reduced the myogenic responsiveness of arterioles from the older rats over a wide range of pressures but had no consistent effects on the myogenic responsiveness of arterioles from the younger rats. The heme oxygenase inhibitor chromium (III) mesoporphyrin IX chloride had no effect on the myogenic activity of arterioles from either age group. These findings indicate that microvascular growth in young animals is accompanied by an increase in the myogenic behavior of arterioles, possibly because PGH(2) or TxA(2) assumes a role in reinforcing myogenic activity over this period. As a result, the relative contribution of myogenic activity to blood flow regulation in skeletal muscle may increase during rapid juvenile growth.

Does growth rate determine the rate of metabolism in shorebird chicks living in the Arctic?

We measured resting and peak metabolic rates (RMR and PMR, respectively) during development of chicks of seven species of shorebirds: least sandpiper (Calidris minutilla; adult mass 20-22 g), dunlin (Calidris alpina; 56-62 g), lesser yellowlegs (Tringa flavipes; 88-92 g), short-billed dowitcher (Limnodromus griseus; 85-112 g), lesser golden plover (Pluvialis dominicana; 150-156 g), Hudsonian godwit (Limosa haemastica; 205-274 g), and whimbrel (Numenius phaeopus; 380 g). We tested two opposing hypotheses: the growth rate-maturity hypothesis, which posits that growth rate in chicks is inversely related to functional maturity of tissues, and the fast growth rate-high metabolism hypothesis, which suggests that rapid growth is possible only with a concomitant increase in either RMR or PMR. We have found no evidence that chicks of shorebirds with fast growth rates have lower RMRs or lower PMRs, as would be predicted by the growth rate-maturity hypothesis, but our data suggested that faster-growing chest muscles resulted in increased thermogenic capacity, consistent with the fast growth-high metabolism hypothesis. The development of homeothermy in smaller species is a consequence primarily of greater metabolic intensities of heat-generating tissues. The maximum temperature gradient between a chick's body and environment that can be maintained in the absence of a net radiative load increased rapidly with body mass during development and was highest in least sandpipers and lowest among godwits. Chicks of smaller species could maintain a greater temperature gradient at a particular body mass because of their higher mass-specific maximum metabolic rates.

Growth rate, protein accumulation, and catabolic enzyme activity of skeletal muscles of galliform birds

We measured the mass and several potential indices of functional capacity of the leg and pectoral muscles through 21 d of age in chicks of three species of galliform birds and the domesticated turkey. The study was conducted to test the hypothesis that the growth rate of a tissue is inversely related to its capacity for mature function across species. We measured the proportion of protein and the activities of the catabolic enzymes citrate synthase (CS), pyruvate kinase (PK), and beta -hydroxy-acyl-CoA-dehydrogenase (HOAD) and estimated exponential growth rate (EGR) from growth increments. EGR was negatively related to proportion of protein, PK, and HOAD and positively related to CS activity. In a multiple regression, EGR was uniquely related only to proportion of protein; it was higher in pectoral muscles and increased in this order: wild turkey<pheasant<domestic turkey<chukar. Thus, among wild species, the muscles of smaller species grew more rapidly for a given proportion of protein, but domestication and selection for rapid growth and large muscle size in turkeys resulted in substantially elevated growth rate. When the proportion of protein was normalized by its maximum value for each species and muscle type, the relationship between EGR and normalized protein did not differ significantly among species or muscle type. Thus, if we accept the proportion of protein relative to the mature level as an index of functional capacity--presumably representing the assembly of the contractile apparatus--then growth rate is consistently inversely related to a muscle's capacity for mature function, that is, force generation.

Flight muscle enzyme activities do not differ between pelagic and near-shore foraging seabird species

Common terns (Sterna hirundo), sooty terns (S. fuscata) and brown noddies (Anous stolidus) are phylogenetically related seabirds that differ in field activity levels and daily energy expenditure. To test whether muscle metabolic capacities co-evolve with activity levels and energy expenditure, we collected pectoral muscle biopsies from members of each species, and measured the activities of key enzymes in oxidative metabolism (citrate synthase, CS), anaerobic metabolism (lactate dehydrogenase, LDH), glycolysis (pyruvate kinase, PK), fatty acid oxidation (3-hydroxyacyl CoA dehydrogenase) and phosphocreatine hydrolysis (creatine phosphokinase, CPK). We hypothesized that temperate-breeding common terns would have higher enzyme activities than the two tropical species (sooty terns and brown noddies); consistent with the higher activity level of common terns. There were no differences in enzyme activities among adults of the three species. Common tern chicks within 2-3 days of flight had two-fold higher pyruvate kinase activity than adults, suggesting an increased glycolytic capacity in the chicks. Given the lack of difference among species at the enzymatic level, our results support the notion that behavior and whole organism performance can evolve considerably before there are detectable changes in underlying lower-level physiological/biochemical traits.

Growth rate and thermoregulation in reared king quails (Coturnix chinensis)

Growth rate was investigated in king quails between 1st and 60th day of life. Gompertz growth constants were 0.075 in males and 0.056 in females. Colonic temperature (Tb) was measured in quails divided into four age groups (1-3, 7-10, 16-19, and 44-59 days old) in ambient temperatures set separately for each group. Metabolic rate was measured only in 44-59-day-old birds. The mean value of the thermoneutral body temperature (Tb at TNZ) in the active phase in the youngest quails was 39.0 degrees C. In 44-59-day-old quails, the resting metabolic rate in the thermoneutral zone (RMR at TNZ) was on average 9.44 mW g(-1) (1.66 cm3 O2 g(-1)h(-1)), without sex-specific differences. No such differences were found in this age group neither in Tb at TNZ, nor in minimal thermal conductance (Cmin). However, differences were found in the rate of metabolic heat production below the thermoneutral zone, even when mass-independent units were used. The maximum metabolic rate (Mmax) in 2-month-old males was 34.08 mW g(-1) (5.98 cm3 O2 g(-1)h(-1)), while in females 29.73 mW g(-1) (5.21 cm3 O2 g(-1)h(-1)). Heat-stressed 44-59-day-old quails elevated their Tb to as much as an average 44.1 degrees C in Ta of about 45 degrees C. The obtained growth model and a gradual development of the body temperature regulation mechanism in king quails followed the known strategy of development, typical for precocial birds. The sexual size dimorphism in the studied quails did not result in differences in thermoregulation parameters between the sexes, except for the rate of metabolic rate below thermoneutral zone.

Energy metabolism and thermoregulation in hand-reared chukars (Alectoris chukar)

Metabolic rate and colonic temperature were measured in chukars between 1st and 108th day of life (divided into six age groups: 1-3-days old, 1, 2, 3, and 4-weeks old and 3.5-months old) in ambient temperatures set separately for each group and ranging from -12 to 41 degrees C. The Gompertz growth constant for growing chukars (0.042) was close to the value obtained in earlier study for the grey partridge. Similarly as in other species of Galliformes, newly hatched chukar chicks had lower T(b) at TNZ (39.5 degrees C) than that found in older birds (41.3 degrees C in 4-weeks old). The body temperatures taken at TNZ in 2-weeks old chicks and older fitted neatly within allometrically predicted limits of body temperatures for adult birds. The values of RMR at TNZ followed closely a biphasic pattern, with the second phase correlating strongly with the body mass. The value of metabolic scope (the level of metabolic efficiency) for the youngest group was high (3.2) and exceeded the values obtained in earlier studies for other gallinaceous species, including the grey partridge. The obtained values of minimum wet thermal conductance for growing chukar chicks exceeded the predicted values by approximately 40% but the slopes of both lines were very similar. In chukars, the key stage in the expression of fully developed thermoregulatory capacities comes immediately after the first week of life (maintaining somewhat constant body temperature, evident drop in the value of RMR at TNZ and minimal thermal conductance). The model of gradual development of thermoregulation which could be derived from the experiments on chukar chicks was characteristic for typical precocial birds.

Metabolic responses of shorebird chicks to cold stress: hysteresis of cooling and warming phases

We developed a protocol for determining the maximum rate of oxygen consumption of shorebird chicks (Scolopacidae and Charadriidae) in response to cold challenge. We first subjected the chicks to gradually decreasing temperatures until their metabolism peaked and began to decrease. We ended the cooling phase of a trial when a chick's body temperature T(b) had declined typically to 32-34 degrees C. After this point, we gradually increased the temperature in the metabolism chamber until normal T(b) values and thermoneutral resting metabolism were restored. We refer to this cycle as the down-up (DU) protocol. We estimated instantaneous oxygen consumption ((O(2))) using the equation of Bartholomew et al. (1981). (O(2)) and T(b) were monitored continuously during the trials. Here, we illustrate typical temperature and metabolism dynamics of the DU protocol by describing several trials in detail, and we discuss the implications of these results for the control of metabolism and regulation of T(b). Chicks subjected to the DU protocol exhibited three distinct phases of metabolic response to ambient temperature (T(a)). In Phase I, (O(2)) increase was directly related to the gradient between T(b) and T(a), consistent with a Newtonian response to cooling. During Phase II, chicks sustained a maximum level of (O(2)) that decreased as T(b) dropped, exhibiting a Q(10) of approximately 2. Based on the slope of the relationship between (O(2)) and T(b) during Phase II, we were able to estimate maximum (O(2)) at a standardized high T(b). Phase II continued until chick T(b) began to rise as a result of the gradually increasing T(a). During Phase III, the T(b)-adjusted rate of oxygen consumption decreased from the maximum level at low T(b) to the resting level at high T(b) in the thermoneutral zone. Further trials with faster and slower rates of chamber cooling showed that (O(2)) during Phase I varied in proportion to the difference between T(b) and T(a) (DeltaT), whereas during Phase III it responded to T(b). Even though chicks may be capable of generating enough heat to regulate T(b) during the early part of Phase I of the DU protocol, the constantly decreasing T(a) created a time lag between T(a) and the chick's metabolic response, leading to body cooling. The hysteresis observed between Phase I and Phase III suggests that chicks rewarm passively while being brooded following the decrease in T(b) experienced during active foraging. The results of the DU protocol suggest that T(b) should be measured continuously during measurements of maximum oxygen consumption, and that peak values should be adjusted by T(b) to make them comparable with other studies.

Foraging behavior and physiological changes in precocial quail chicks in response to low temperatures

We examined whether low ambient temperatures influence foraging behavior of precocial Japanese quail chicks and alter the balance between investment in growth and thermogenic function. To test this, one group of chicks was exposed to 7 degrees C and one group to 24 degrees C during foraging throughout the developmental stage. Chicks adapted well to the temperatures through a high flexibility in behavior and physiological development. In response to cold, chicks shortened foraging bout lengths two- to sixfold, and increased cycle lengths (i.e. length of a brooding bout plus subsequent foraging bout), resulting in a decrease in total foraging time. Body temperature during foraging was lower in cold-exposed chicks but did not drop below 37.8 degrees C, suggesting that the direct cause to end a foraging bout was not body temperature. The metabolic rate of cold-exposed chicks was reduced by 48% to 60% when switching from foraging to brooding, which may be an important factor in determining foraging behavior of precocial chicks. Mass-specific food intake of the cold-exposed chicks exceeded that of warm chicks by 15%. Cold-exposed chicks doubled foraging efficiency to reach this intake during their reduced foraging time. Metabolic rates initially were similar in both groups, but increased rapidly and were elevated in cold-exposed chicks from 15 days of age onward. Growth rate initially was reduced in cold-exposed chicks, and exceeded growth of warm chicks only after 21 days of age. These results suggest that in response to cold, a shift occurs in the balance between growth rate and thermoregulatory function in favor of thermoregulatory function.

Environmental influences on the evolution of growth and developmental rates in passerines

The reasons why growth and developmental rates vary widely among species have remained unclear. Previous examinations of possible environmental influences on growth rates of birds yielded few correlations, leading to suggestions that young may be growing at maximum rates allowed within physiological constraints. However, estimations of growth rates can be confounded by variation in relative developmental stage at fledging. Here, we reestimate growth rates to control for developmental stage. We used these data to examine the potential covariation of growth and development with environmental variation across a sample of 115 North American passerines. Contrary to previous results, we found that growth rates of altricial nestlings were strongly positively correlated to daily nest predation rates, even after controlling for adult body mass and phylogeny. In addition, nestlings of species under stronger predation pressure remained in the nest for a shorter period, and they left the nest at lower body mass relative to adult body mass. Thus, nestlings both grew faster and left the nest at an earlier developmental stage in species with higher risk of predation. Growth patterns were also related to food, clutch size, and latitude. These results support a view that growth and developmental rates of altricial nestlings are strongly influenced by the environmental conditions experienced by species, and they generally lend support to an adaptive view of interspecific variation in growth and developmental rates.

Metabolic response to wind of downy chicks of Arctic-breeding shorebirds(Scolopacidae)

Wind is a significant factor in the thermoregulation of chicks of shorebirds on the Arctic tundra. We investigated the effect of wind at speeds typical of near-surface conditions (0.1-3 ms-1) on metabolic heat production, evaporative cooling and thermal conductance of 1- to 3- week-old downy scolopacid chicks (least sandpiper Calidris minutilla;short-billed dowitcher Limnodromus griseus; whimbrel Numenius phaeopus). Body mass ranged from 9 to 109 g. To accurately measure the interacting effects of air temperature and wind speed, we used two or more air temperatures between 15° and 30°C that produced cold stress at all wind speeds, but allowed chicks to maintain normal body temperature(approximately 39°C). Thermal conductance increased by 30-50% as wind speed increased from 0.1 to 3 ms-1. Conductance in these chicks is somewhat lower than that of 1-day-old mallard ducklings of similar mass, but higher than values reported for downy capercaillie and Xantus' murrelet chicks, as well as for adult shorebirds. Evaporative water loss was substantial and increased with mass and air temperature. We developed a standard operative temperature scale for shorebird chicks. The ratio of evaporative cooling to heat production varied with wind speed and air temperature.

Seasonal dynamics of flight muscle fatty acid binding protein and catabolic enzymes in a migratory shorebird

We developed an ELISA to measure heart-type fatty acid binding protein (H-FABP) in muscles of the western sandpiper (Calidris mauri), a long-distance migrant shorebird. H-FABP accounted for almost 11% of cytosolic protein in the heart. Pectoralis H-FABP levels were highest during migration (10%) and declined to 6% in tropically wintering female sandpipers. Premigratory birds increased body fat, but not pectoralis H-FABP, indicating that endurance flight training may be required to stimulate H-FABP expression. Juveniles making their first migration had lower pectoralis H-FABP than adults, further supporting a role for flight training. Aerobic capacity, measured by citrate synthase activity, and fatty acid oxidation capacity, measured by 3-hydroxyacyl-CoA-dehydrogenase and carnitine palmitoyl transferase activities, did not change during premigration but increased during migration by 6, 12, and 13%, respectively. The greater relative induction of H-FABP (+70%) with migration than of catabolic enzymes suggests that elevated H-FABP is related to the enhancement of uptake of fatty acids from the circulation. Citrate synthase, 3-hydroxyacyl-CoA-dehydrogenase, and carnitine palmitoyl transferase were positively correlated within individuals, suggesting coexpression, but enzyme activities were unrelated to H-FABP levels.