Flight respiration and energetics

Host inbreeding increases susceptibility to ectoparasitism

Inbreeding, which increases homozygosity throughout the genome by increasing the proportion of alleles that are identical by descent, is expected to compromise resistance against parasitism. Here, we demonstrate that host inbreeding increases susceptibility to ectoparasitism in a natural fruit fly (Drosophila nigrospiracula) - mite (Macrocheles subbadius) association, and that this effect depends on host genetic background. Moreover, flies generated from reciprocal crosses between susceptible inbred lines exhibited elevated levels of resistance similar to that in the mass-bred base population, confirming in reverse direction the causative link between expected heterozygosity and resistance. We also show that inbreeding reduces the host's ability to sustain energetically expensive behaviours, and that host exhaustion dramatically increases susceptibility. These findings suggest that inbreeding depression for resistance results from an inability to sustain defensive behaviours because of compromised physiological competence.

Does the metabolic rate-flight speed relationship vary among geometrically similar birds of different mass?

Based on aerodynamic considerations, the energy use-flight speed relationship of all airborne animals and aircraft should be U-shaped. However, measures of the metabolic rate-flight speed relationship in birds have been available since Tucker's pioneering experiments with budgerigars nearly forty years ago, but this classic work remains the only study to have found a clearly U-shaped metabolic power curve. The available data suggests that the energetic requirements for flight within this species are unique, yet the metabolic power curve of the budgerigar is widely considered representative of birds in general. Given these conflicting results and the observation that the budgerigar's mass is less than 50% of the next smallest species to have been studied, we asked whether large and small birds have metabolic power curves of different shapes. To address this question we measured the rates of oxygen uptake and wingbeat kinematics in budgerigars and cockatiels flying within a variable-speed wind tunnel. These species are close phylogenetic relatives, have similar flight styles, wingbeat kinematics, and are geometrically similar but have body masses that differ by a factor of two. In contrast to our expectations, we found the metabolic rate-flight speed relationship of both species to be acutely U-shaped. We also found that neither budgerigars nor cockatiels used their normal intermittent flight style while wearing a respirometric mask. We conclude that species size differences alone do not explain the previously unique metabolic power curve of the budgerigar; however, due to the absence of comparable data we cannot evaluate whether the mask-related kinematic response we document influences the metabolic rate-flight speed relationship of these parrots, or whether the energetics of flight differ between this and other avian clades.

Bee foraging ranges and their relationship to body size

Bees are the most important pollinator taxon; therefore, understanding the scale at which they forage has important ecological implications and conservation applications. The foraging ranges for most bee species are unknown. Foraging distance information is critical for understanding the scale at which bee populations respond to the landscape, assessing the role of bee pollinators in affecting plant population structure, planning conservation strategies for plants, and designing bee habitat refugia that maintain pollination function for wild and crop plants. We used data from 96 records of 62 bee species to determine whether body size predicts foraging distance. We regressed maximum and typical foraging distances on body size and found highly significant and explanatory nonlinear relationships. We used a second data set to: (1) compare observed reports of foraging distance to the distances predicted by our regression equations and (2) assess the biases inherent to the different techniques that have been used to assess foraging distance. The equations we present can be used to predict foraging distances for many bee species, based on a simple measurement of body size.

Relationship between n-3 PUFA content and energy metabolism in the flight muscles of a migrating shorebird: evidence for natural doping

During their fall migration from the Arctic to South America, semipalmated sandpipers Calidris pusilla stop in the Bay of Fundy (east coast of Canada) before flying non-stop for ∼4500 km across the ocean. Refueling birds double their body mass by feeding on Corophium volutator, an amphipod containing high amounts of n-3 polyunsaturated fatty acids (n-3 PUFA), particularly eicosapentaenoic (20:5) and docosahexaenoic acid (22:6). In mammals, high dietary intake of n-3 PUFA is known to increase capacity for oxidative metabolism. Therefore, we hypothesized that tissue incorporation of n-3 PUFA would be associated with increases in the activity of key muscle enzymes to upregulate energy metabolism for prolonged exercise.

Birds were collected at various stages of fat loading to monitor changes in lipid composition and flight muscle enzymes simultaneously. Enzymes were measured to assess oxidative capacity [citrate synthase (CS)],β-oxidation [carnitine palmitoyl transferase (CPT) and 3-hydroxyacyl dehydrogenase (HOAD)] and glycolytic capacity [lactate dehydrogenase (LDH)]. Changes in the fatty acid composition of muscle membranes (phospholipids) and fuel reserves (neutral lipids) were measured separately to distinguish between membrane-related and systemic effects of n-3 PUFA. Results show that muscle CS and HOAD are stimulated during refueling and that their activities are correlated with n-3 PUFA content in phospholipids (22:6 for CS, 20:5 for HOAD)and in neutral lipids (20:5 for CS). This suggests that 20:5 and 22:6 have different effects on energy metabolism and that they act via changes in membrane structure and systemic mechanisms. CPT and LDH did not change during refueling, but LDH activity was significantly related to the n-3 PUFA content of fuel reserves. This study shows that oxidative capacity increases rapidly during refueling and supports the idea that dietary n-3 PUFA are used as molecular signals to prime flight muscles of some long-distance migrants for extreme exercise.

Carbohydrate metabolism genes and pathways in insects: insights from the honey bee genome

Carbohydrate-metabolizing enzymes may have particularly interesting roles in the honey bee, Apis mellifera, because this social insect has an extremely carbohydrate-rich diet, and nutrition plays important roles in caste determination and socially mediated behavioural plasticity. We annotated a total of 174 genes encoding carbohydrate-metabolizing enzymes and 28 genes encoding lipid-metabolizing enzymes, based on orthology to their counterparts in the fly, Drosophila melanogaster, and the mosquito, Anopheles gambiae. We found that the number of genes for carbohydrate metabolism appears to be more evolutionarily labile than for lipid metabolism. In particular, we identified striking changes in gene number or genomic organization for genes encoding glycolytic enzymes, cellulase, glucose oxidase and glucose dehydrogenases, glucose-methanol-choline (GMC) oxidoreductases, fucosyltransferases, and lysozymes.

The significance of spiracle conductance and spatial arrangement for flight muscle function and aerodynamic performance in flyingDrosophila

During elevated locomotor activity such as flight, Drosophilasatisfies its increased respiratory demands by increasing the total spiracle opening area of the tracheal gas exchange system. It has been assumed that in a diffusion-based system, each spiracle contributes to oxygen flux into and carbon dioxide flux out of the tracheal system according to the size of its opening. We evaluated this hypothesis by determining how a reduction in size and interference with the spatial distribution of gas exchange areas impair flight muscle function and aerodynamic force production in the small fruit fly Drosophila melanogaster. This was done by selectively blocking thoracic spiracles of tethered flies flying inside a flight simulator. Flow-through respirometry and simultaneous measurements of flight force production and wing kinematics revealed a negligible functional safety margin for respiration. Maximum locomotor performance was only achieved by unmanipulated flies, supporting the general assumption that at the animal's maximum locomotor capacity, maximum spiracle opening area matches respiratory need. The maximum total buffer capacity for carbon dioxide in Drosophila amounts to approximately 33.5 μl g–1body mass, estimated from the temporal integral of carbon dioxide release rate during the resting period after flight. By comparing flight variables in unmanipulated and `spiracle-blocked' flies at comparable flight forces, we found that (i) stroke amplitude, stroke frequency and the chemo-mechanical conversion efficiency of the indirect flight musculature were broadly independent of the arrangement of spiracle conductance, while (ii) muscle mechanical power significantly increased, and (iii) mean lift coefficient and aerodynamic efficiency significantly decreased up to approximately 50% with an increasing number of blocked spiracles. The data suggest that Drosophila apparently maximizes the total efficiency of its locomotor system for flight by allowing oxygen delivery to the flight musculature through multiple spiracles of the thorax.

Flight activity, mortality rates, and lipoxidative damage in Drosophila

In this study, the effect of flight activity on mortality rates and lipoxidative damage in Drosophila was determined to identify mechanisms through which oxidative damage affects life span. The results showed that flies allowed flying throughout life had higher mortality rates and decreased median and maximum life spans compared to controls. The mortality rate of the flight activity group could be lowered, but not completely reversed by switching to control conditions; and the accrued oxidative damage could not be eliminated. The levels of reactive oxygen species produced by mitochondria isolated from high activity and control flies did not differ significantly. However, the high activity flies had altered membrane fatty acid compositions, which made them prone to increased lipid peroxidation. The effect of flight activity on insect life span differs considerably from the beneficial effects of exercise in mammals; these differences may be caused by physiological differences between the two taxa.

Unconventional mechanisms control cyclic respiratory gas release in flying Drosophila

The high power output of flight muscles places special demands on the respiratory gas exchange system in insects. In small insects, respiration relies on diffusion, and for elevated locomotor performance such as flight, instantaneous gas exchange rates typically co-vary with the animal's metabolic activity. By contrast, under certain conditions, instantaneous release rate of carbon dioxide from the fruit fly Drosophila flying in a virtual-reality flight arena may oscillate distinctly at low frequency (0.37+/-0.055 Hz), even though flight muscle mechanical power output requires constant metabolic activity. Cross-correlation analysis suggests that this uncoupling between respiratory and metabolic rate is not driven by conventional types of convective flow reinforcement such as abdominal pumping, but might result from two unusual mechanisms for tracheal breathing. Simplified analytical modeling of diffusive tracheal gas exchange suggests that cyclic release patterns in the insect occur as a consequence of the stochastically synchronized control of spiracle opening area by the four large thoracic spiracles. Alternatively, in-flight motion analysis of the abdomen and proboscis using infra-red video imaging suggests utilization of the proboscis extension reflex (PER) for tracheal convection. Although the respiratory benefit of synchronized spiracle opening activity in the fruit fly is unclear, proboscis-induced tracheal convection might potentially help to balance the local oxygen supply between different body compartments of the flying animal.

Beyond the '3/4-power law': variation in the intra- and interspecific scaling of metabolic rate in animals

In this review I show that the '3/4-power scaling law' of metabolic rate is not universal, either within or among animal species. Significant variation in the scaling of metabolic rate with body mass is described mainly for animals, but also for unicells and plants. Much of this variation, which can be related to taxonomic, physiological, and/or environmental differences, is not adequately explained by existing theoretical models, which are also reviewed. As a result, synthetic explanatory schemes based on multiple boundary constraints and on the scaling of multiple energy-using processes are advocated. It is also stressed that a complete understanding of metabolic scaling will require the identification of both proximate (functional) and ultimate (evolutionary) causes. Four major types of intraspecific metabolic scaling with body mass are recognized [based on the power function R=aMb, where R is respiration (metabolic) rate, a is a constant, M is body mass, and b is the scaling exponent]: Type I: linear, negatively allometric (b<1); Type II: linear, isometric (b=1); Type III: nonlinear, ontogenetic shift from isometric (b=1), or nearly isometric, to negatively allometric (b<1); and Type IV: nonlinear, ontogenetic shift from positively allometric (b>1) to one or two later phases of negative allometry (b<1). Ontogenetic changes in the metabolic intensity of four component processes (i.e. growth, reproduction, locomotion, and heat production) appear to be important in these different patterns of metabolic scaling. These changes may, in turn, be shaped by age (size)-specific patterns of mortality. In addition, major differences in interspecific metabolic scaling are described, especially with respect to mode of temperature regulation, body-size range, and activity level. A 'metabolic-level boundaries hypothesis' focusing on two major constraints (surface-area limits on resource/waste exchange processes and mass/volume limits on power production) can explain much, but not all of this variation. My analysis indicates that further empirical and theoretical work is needed to understand fully the physiological and ecological bases for the considerable variation in metabolic scaling that is observed both within and among species. Recommended approaches for doing this are discussed. I conclude that the scaling of metabolism is not the simple result of a physical law, but rather appears to be the more complex result of diverse adaptations evolved in the context of both physico-chemical and ecological constraints.

Ontogenetic effects on aerobic and anaerobic metabolism during jumping in the American locust, Schistocerca americana

Developing vertebrates increase both their locomotory power output and endurance due to ontogenetic improvements in anaerobic and aerobic metabolic capacities. Do similar patterns hold for insect locomotion, or do longer tracheal lengths create problems for oxygen delivery in older animals? We forced developing American locust grasshoppers (Schistocerca americana) to jump repeatedly and examined the effect of development on power output, endurance, lactate concentration, oxygen consumption and the oxygen sensitivity of jump performance. As previously shown, power outputs, relative leg lengths and leg cuticular content increased with age. A key finding of this study is that both lactate concentration and aerobic metabolic rate of the jumping muscle increase with age, explaining how the increased leg cuticular stiffness can result in increased power output. After two minutes of jumping, grasshoppers rely completely on aerobic ATP production. The rise in mass-specific, active aerobic metabolic rates with age indicates that problems with longer tracheae can be overcome; however, the reduced endurance, higher lactate concentrations and increased oxygen sensitivity of locomotory performance in older animals indicate that larger/older grasshoppers have smaller safety margins for oxygen delivery during hopping.

Ontogeny of tracheal dimensions and gas exchange capacities in the grasshopper, Schistocerca americana

How does body size affect the structure and gas exchange capacities of insect tracheae? Do insects become more oxygen-limited as they grow? We addressed these questions by measuring the dimensions of two transverse tracheae within the abdomen of American locusts of different ages, and evaluating the potential for diffusion or convection to provide adequate gas exchange. The grasshopper abdomen has longitudinal tracheae that run along the midgut, heart, nerve cord, and lateral body wall. Transverse tracheae run from each spiracle to the longitudinal tracheae. Dorsal air sacs attach near each spiracle. In both transverse tracheae studied, diffusive capacities increased more slowly than metabolic rates with age, and calculated oxygen gradients necessary to supply oxygen by diffusion increased exponentially with age. However, surgical studies demonstrated that transport of gas through these transverse tracheae occurred by convection, at least in adults. Convective capacities paralleled metabolic rates with age, and the calculated pressure gradients required to sustain oxygen consumption rates by convection were independent of age. Thus, in growing grasshoppers, tracheal capacities matched tissue oxygen needs. Our morphological and physiological data together suggest that use of convection allows older grasshoppers to overcome potential limitations on size imposed by diffusion through tracheal systems.

Oxygen partial pressure effects on metabolic rate and behavior of tethered flying locusts

Resting insects are extremely tolerant of hypoxia. However, oxygen requirements increase dramatically during flight. Does the critical atmospheric P (O)(2) (P(c)) increase strongly during flight, or does increased tracheal conductance allow even flying insects to possess large safety margins for oxygen delivery? We tested the effect of P(O)(2) on resting and flying CO(2) emission, as well as on flight behavior and vertical force production in flying locusts, Schistocerca americana. The P(c) for CO(2) emission of resting animals was less than 1 kPa, similar to prior studies. The P(c) for flight bout duration was between 10 and 21 kPa, the P(c) for vertical force production was between 3 and 5 kPa, and the P(c) for CO(2) emission was between 10 and 21 kPa. Our study suggests that the P(c) for steady-state oxygen consumption is between 10 and 21 kPa (much higher than for resting animals), and that tracheal oxygen stores allowed brief flights in 5 and 10 kPa P(O)(2) atmospheres to occur. Thus, P(c) values strongly increased during flight, consistent with the hypothesis that the excess oxygen delivery capacity observed in resting insects is substantially reduced during flight.

Functional and structural optimization of the respiratory system of the batTadarida brasiliensis(Chiroptera, Molossidae): does airway geometry matter?

We studied structure and function of the respiratory system in the bat Tadarida brasiliensis and compared it with those of two species of rodents, Abrothrix andinus and A. olivaceus. Tadarida brasiliensis had lower resting oxygen consumption, but higher maximum oxygen consumption and aerobic scope, than the rodents. The blood–gas barrier of the bat was thinner and its relative lung size was larger; however,alveolar surface density was similar among the three species. In consequence, T. brasiliensis has an oxygen diffusion capacity two or three times higher than that of the rodents. In Tadarida brasiliensis the characteristics of the lung were accompanied by geometrical changes in the proximal airway, such as high physical optimization as a consequence of small variations in the symmetry and the scaling ratio of the bronchial diameters. These may constitute an efficient way to save energy in respiratory mechanics and are the first report of airway adjustments to decrease entropy generation in bats.

Ontogeny of tracheal system structure: A light and electron-microscopy study of the metathoracic femur of the American locust,Schistocerca americana

Does oxygen delivery become more challenging for insects as they increase in size? To partially test this hypothesis, we used quantitative light and electron microscopy to estimate the oxygen delivery capacity for two steps of tracheal oxygen delivery within the metathoracic femur (jumping leg) for 2nd instar (about 47 mg) and adult (about 1.7 g) locusts, Schistocerca americana. The fractional cross-sectional areas of the major tracheae running longitudinally along the leg were similar in adults and 2nd instars; however, since the legs of adults are longer, the mass-specific diffusive conductances of these tracheae were 4-fold greater in 2nd instars. Diffusive gas exchange longitudinally along the leg is easily possible for 2nd instars but not adults, who have many air sacs within the femur. Mitochondrial content fell proximally to distally within the femur in 2nd instars but not adults, supporting the hypothesis that diffusion was more important for the former. Lateral diffusing capacities of the tracheal walls were 12-fold greater in adults than 2nd instars. This was primarily due to differences in the smallest tracheal class (tracheoles), which had thinner epidermal and cuticular layers, greater surface to volume ratios, and greater mass-specific surface areas in adults. Adults also had greater mitochondrial contents, larger cell sizes and more intracellular tracheae. Thus, larger insects do not necessarily face greater problems with oxygen delivery; adult grasshoppers have superior oxygen delivery systems and greater mass-specific aerobic capacities in their legs than smaller/younger insects.

Metabolic rate variation in Glossina pallidipes (Diptera: Glossinidae): gender, ageing and repeatability

Despite the importance of metabolic rate in determining flight time of tsetse and in mediating the influence of abiotic variables on life history parameters (and hence abundance and distribution), metabolic rate measurements and their repeatability have not been widely assessed in these flies. We investigate age-related changes in standard metabolic rate (SMR) and its repeatability, using flow-through respirometry, for a variety of feeding, gender and pregnancy classes during early adult development in laboratory-reared individuals of the tsetse fly, Glossina pallidipes. Standard metabolic rate (144-635 microW) was generally within 22% of previous estimates, though lower than the values found using closed system respirometry. There was no significant difference between the genders, but metabolic rate increased consistently with age, probably owing to flight muscle development. Repeatability of metabolic rate was generally high (r=0.6-.09), but not in younger teneral adults and pregnant females (r approximately equal to 0.05-0.4). In these individuals, low repeatability values are a consequence of muscle or in utero larval development. Tsetse and other flies generally have a much higher metabolic rate, for a given size, than do other insect species investigated to date.

Allometry of kinematics and energetics in carpenter bees (Xylocopa varipuncta)hovering in variable-density gases

We assessed the energetic and aerodynamic limits of hovering flight in the carpenter bee Xylocopa varipuncta. Using normoxic, variable-density mixtures of O2, N2 and He, we were able to elicit maximal hovering performance and aerodynamic failure in the majority of bees sampled. Bees were not isometric regarding thorax mass and wing area, both of which were disproportionately lower in heavier individuals. The minimal gas density necessary for hovering (MGD) increased with body mass and decreased with relative thoracic muscle mass. Only the four bees in our sample with the highest body mass-specific thorax masses were able to hover in pure heliox. Wingbeat frequency and stroke amplitude during maximal hovering were significantly greater than in normodense hovering, increased significantly with body mass during normodense hovering but were mass independent during maximal hovering. Reserve capacity for wingbeat frequency and stroke amplitude decreased significantly with increasing body mass, although reserve capacity in stroke amplitude (10–30%) exceeded that of wingbeat frequency(0–8%). Stroke plane angle during normodense hovering was significantly greater than during maximal hovering, whereas body angle was significantly greater during maximal hovering than during normodense hovering. Power production during normodense hovering was significantly less than during maximal hovering. Metabolic rates were significantly greater during maximal hovering than during normodense hovering and were inversely related to body mass during maximal and normodense hovering. Metabolic reserve capacity averaged 34% and was independent of body mass. Muscle efficiencies were slightly higher during normodense hovering. The allometry of power production,power reserve capacity and muscle efficiency were dependent on the assumed coefficient of drag (CD), with significant allometries most often at lower values of CD. Larger bees operate near the envelope of maximal performance even in normodense hovering due to smaller body mass-specific flight muscles and limited reserve capacities for kinematics and power production.

Metabolic rate and jump performance in seven species of desert fleas

We hypothesized that sexual and interspecific differences in jumping performance of fleas found in our previous study are correlated with differences in resting metabolic rate (RMR) between sexes and among species. To test this hypothesis, we measured RMR of seven flea species (Xenopsylla conformis mycerini, Xenopsylla ramesis, Xenopsylla dipodilli, Parapulex chephrenis, Synosternus cleopatrae pyramidis, Nosopsyllus iranus theodori and Stenoponia tripectinata medialis). We compared RMR between sexes and among species and examined whether there is intra- and interspecific correlation between RMR and jumping ability. Both mass-specific and mass-independent RMR were the highest in female S. t. medialis, whereas mass-specific RMR was the lowest in male X. dipodilli and mass-independent RMR was the lowest in three Xenopsylla species and P. chephrenis. Mass-specific and mass-independent RMR were significantly higher in females than in males in all fleas except S. t. medialis. Differences in jumping ability between males and females were found to be correlated with sexual differences in mass-specific or mass-independent RMR. Interspecific comparison showed that the length of jump in both male and female fleas was strongly affected by their mass-specific and mass-independent RMR.

Ultrastructure meets reproductive success: performance of a sphecid wasp is correlated with the fine structure of the flight-muscle mitochondria

Organisms show a remarkable inter-individual variation in reproductive success. The proximate causes of this variation are not well understood. We hypothesized that the ultrastructure of costly or complex tissues or organelles might affect reproductive performance. We tested this hypothesis in females of a sphecid wasp, the European beewolf, Philanthus triangulum (Hymenoptera, Sphecidae), that show considerable variation in reproductive success. The most critical component of reproduction in beewolf females is flying with paralysed honeybees, which more than double their weight. Because of the high energetic requirements for flight, we predicted that the ultrastructure of the flight-muscle mitochondria might influence female success. We determined the density of mitochondria and the density of the inner mitochondrial membranes (DIMM) of the flight muscles as well as age, body size and fat content. Only DIMM had a significant influence on female reproductive success, which might be mediated by an elevated adenosine triphosphate (ATP) supply. The variation in DIMM might result from differences in larval provisions or from an accumulation of mutations in the mitochondrial genome. Our results support the hypothesis that the organization of complex structures contributes to inter-individual variation in reproductive success.

Respiratory water loss in insects

The contribution of respiratory transpiration to overall water loss in insects is contentious. Misgivings concerning the importance of this route of water loss have arisen largely as a consequence of work on discontinuous gas exchange cycles (DGC). Most studies have found that respiratory water loss constitutes only a small proportion of total water loss. Thus, it has been argued that modulation of metabolic rate and/or the components of the DGC is unlikely to constitute a fitness benefit. In contrast to these intraspecific studies, interspecific comparative data suggest that, at least in xeric species, respiratory transpiration is an important component of water loss. However, these arguments are confounded by several factors. In DGC-based studies, these include multiple effects of the experimental treatments, the absence of a null expectation for the contribution of respiratory to total water loss, and problems with the use of proportions as a way of assessing the importance of respiratory water loss. The interspecific studies are confounded by the likely significance of influences other than water conservation on metabolic rate, the absence of analyses of phylogenetic independent contrasts, and little information on behavioral differences between species. Future work should be based on a strong inference approach and designed in such a way that these problems can be resolved. Moreover, in the case of the DGC it should be recognized that several factors are likely to influence this gas exchange pattern, and that they probably act in concert, especially during dormancy.

Environmental and genetic influences on flight metabolic rate in the honey bee, Apis mellifera

Flying honey bees demonstrate highly variable metabolic rates. The lowest reported values (approximately 0.3 Wg(-1)) occur in tethered bees generating the minimum lift to support their body weight, free-flying 2-day old bees, winter bees, or bees flying at high air temperatures (45 degrees C). The highest values (approximately 0.8 Wg(-1)) occur in foragers that are heavily loaded or flying in low-density air. In different studies, flight metabolic rate has increased, decreased, or remained constant with air temperature. Current research collectively suggests that this variation occurs because flight metabolic rates decrease at thorax temperatures above or below 38 degrees C. At 30 degrees C, approximately 30% of colonial energy is spent during typical foraging, so variation in flight metabolic rate can strongly affect colony-level energy balance. Higher air temperatures tend to increase colonial net gain rates, efficiencies and honey storage rates due to lower metabolic rates during flight and in the hive. Variation in flight metabolism has a clear genetic basis. Different genetic strains of honey bees often differ in flight metabolic rate, and these differences in flight physiology can be correlated with foraging effort, suggesting a possible pathway for selection effects on flight metabolism.

Myocyte ploidy in heart chambers of birds with different locomotor activity

The ploidy levels of atrio- and ventriculocytes were determined by means of cytofluorimetry in 31 species of birds. The obtained data were collated with postnatal growth rate, heart mass index, and relative masses of heart chambers. The difference between mean ploidy of cardiomyocytes in the left and right atrium is small (7.9+/-0.6%) and comparable to the difference in the masses of these chambers (10.5+/-0.8%). The difference between mean ploidy of atrio- and ventriculocytes is most pronounced for the left and right parts of heart (23.9+/-1.4% and 24.0+/-1.3%, respectively) and corresponds to considerable differences in the average masses of atria and ventricles (4.5-fold and 2.1-fold, respectively). The mean cardiomyocyte ploidy levels in the left and right ventricles differ only slightly, as in the case of atria (by 8.1+/-0.5%), whereas the average mass of the left ventricle is greater by 237+/-16%. This discord can be explained by peculiarities of the growth, which is nonproportionally faster in the left ventricle during the last stage of proliferative heart growth as compared to other chambers. The cardiomyocyte ploidy is higher in birds with a relatively small heart and lower ability to flight. Birds with a high locomotor activity in the adult state have an athletic heart (mass index >1%); they are fast growing, altricial species with a low heart workload in the early postnatal ontogenesis. Birds with a low locomotor activity at the adult state are precocial; they grow slowly and have a high locomotor activity from the first minutes of life. Thus, notwithstanding the fact that a greater elevation of cardiomyocyte ploidy level is acquired under a higher functional load (ventricles vs. atria, left vs. right part of the heart), it is associated with a lower functional potential of the organ at the adult state. The level of somatic polyploidy can be considered an indicator of developmental tensions arising due to a high workload during the growth of a given organ and deficiency of resources invested into this growth. J. Exp. Zool. 293:427-441, 2002.