Comparative study of endocrine pancreatic tissue in bats: Assessing variations among frugivorous, insectivorous, and nectarivorous diets

Whether the endocrine pancreas exhibits structural features to couple with dietary patterns is not fully explored. Considering the lack of data comparing endocrine pancreas and islet cell distribution among different bat species in the same study, we considered this an opportunity to explore the topic, including five species within three different predominant diets. For this, we applied morphometric techniques to compare the islets of frugivorous Artibeus lituratus and Carollia perspicillata, insectivorous Molossus molossus and Myotis nigricans, and nectarivorous Glossophaga soricina bats. Data for islet size, cellularity, and mass were equivalent between frugivorous A. lituratus and nectarivorous G. soricina, which differed from insectivorous bats. The frugivorous C. perspicillata bat exhibited morphometric islet values between A. lituratus and the insectivorous species. A. lituratus and G. soricina but not C. perspicillata bats had higher islet mass than insectivorous species due to larger size, instead of a higher number of islets per area. Insectivorous bats, on the other hand, had a higher proportion of α-cells per islet. These differences in the endocrine pancreas across species with different eating habits indicate the occurrence of species-specific adjustments along the years of evolution, with the demand for α-cells higher in bats with higher protein intake.

Genomic insights into metabolic flux in hummingbirds

Hummingbirds are very well adapted to sustain efficient and rapid metabolic shifts. They oxidize ingested nectar to directly fuel flight when foraging but have to switch to oxidizing stored lipids derived from ingested sugars during the night or long-distance migratory flights. Understanding how this organism moderates energy turnover is hampered by a lack of information regarding how relevant enzymes differ in sequence, expression, and regulation. To explore these questions, we generated a chromosome-scale genome assembly of the ruby-throated hummingbird (A. colubris) using a combination of long- and short-read sequencing, scaffolding it using existing assemblies. We then used hybrid long- and short-read RNA sequencing of liver and muscle tissue in fasted and fed metabolic states for a comprehensive transcriptome assembly and annotation. Our genomic and transcriptomic data found positive selection of key metabolic genes in nectivorous avian species and deletion of critical genes (SLC2A4, GCK) involved in glucostasis in other vertebrates. We found expression of a fructose-specific version of SLC2A5 putatively in place of insulin-sensitive SLC2A5, with predicted protein models suggesting affinity for both fructose and glucose. Alternative isoforms may even act to sequester fructose to preclude limitations from transport in metabolism. Finally, we identified differentially expressed genes from fasted and fed hummingbirds, suggesting key pathways for the rapid metabolic switch hummingbirds undergo.

Application of Stable Isotope Techniques in Tracing the Sources of Atmospheric NOX and Nitrate

Nitrate is an important component of PM2.5, and its dry deposition and wet deposition can have an impact on ecosystems. Nitrate in the atmosphere is mainly transformed by nitrogen oxides (NOX = NO + NO2) through a number of photochemical processes. For effective management of the atmosphere’s environment, it is crucial to understand the sources of atmospheric NOX and the processes that produce atmospheric nitrate. The stable isotope method is an effective analytical method for exploring the sources of NO3− in the atmosphere. This study discusses the range and causes of δ15N data from various sources of NOX emissions, provides the concepts of stable isotope techniques applied to NOX traceability, and introduces the use of Bayesian mixture models for the investigation of NOX sources. The combined application of δ15N and δ18O to determine the pathways of nitrate formation is summarized, and the contribution of Δ17O to the atmospheric nitrate formation pathway and the progress of combining Δ17O simulations to reveal the atmospheric oxidation characteristics of different regions are discussed, respectively. This paper highlights the application results and development trend of stable isotope techniques in nitrate traceability, discusses the advantages and disadvantages of stable isotope techniques in atmospheric NOX traceability, and looks forward to its future application in atmospheric nitrate pollution. The research results could provide data support for regional air pollution control measures.

Dietary Diversification and Specialization in Neotropical Bats Facilitated by Early Molecular Evolution

Dietary adaptation is a major feature of phenotypic and ecological diversification, yet the genetic basis of dietary shifts is poorly understood. Among mammals, Neotropical leaf-nosed bats (family Phyllostomidae) show unmatched diversity in diet; from a putative insectivorous ancestor, phyllostomids have radiated to specialize on diverse food sources including blood, nectar, and fruit. To assess whether dietary diversification in this group was accompanied by molecular adaptations for changing metabolic demands, we sequenced 89 transcriptomes across 58 species and combined these with published data to compare ∼13,000 protein coding genes across 66 species. We tested for positive selection on focal lineages, including those inferred to have undergone dietary shifts. Unexpectedly, we found a broad signature of positive selection in the ancestral phyllostomid branch, spanning genes implicated in the metabolism of all major macronutrients, yet few positively selected genes at the inferred switch to plantivory. Branches corresponding to blood- and nectar-based diets showed selection in loci underpinning nitrogenous waste excretion and glycolysis, respectively. Intriguingly, patterns of selection in metabolism genes were mirrored by those in loci implicated in craniofacial remodeling, a trait previously linked to phyllostomid dietary specialization. Finally, we show that the null model of the widely-used branch-site test is likely to be misspecified, with the implication that the test is too conservative and probably under-reports true cases of positive selection. Our findings point to a complex picture of adaptive radiation, in which the evolution of new dietary specializations has been facilitated by early adaptations combined with the generation of new genetic variation.

Sucrose digestion capacity in birds shows convergent coevolution with nectar composition across continents

The major lineages of nectar-feeding birds (hummingbirds, sunbirds, honeyeaters, flowerpiercers, and lorikeets) are considered examples of convergent evolution. We compared sucrose digestion capacity and sucrase enzymatic activity per unit intestinal surface area among 50 avian species from the New World, Africa, and Australia, including 20 nectarivores. With some exceptions, nectarivores had smaller intestinal surfaces, higher sucrose hydrolysis capacity, and greater sucrase activity per unit intestinal area. Convergence analysis showed high values for sucrose hydrolysis and sucrase activity per unit intestinal surface area in specialist nectarivores, matching the high proportion of sucrose in the nectar of the plants they pollinate. Plants pollinated by generalist nectar-feeding birds in the Old and New Worlds secrete nectar in which glucose and fructose are the dominant sugars. Matching intestinal enzyme activity in birds and nectar composition in flowers appears to be an example of convergent coevolution between plants and pollinators on an intercontinental scale.

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Nectarivory has evolved independently in birds in the New and Old Worlds

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Nectarivorous birds have greater sucrose hydrolysis capacity than nonspecialists

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Nectarivorous birds have a smaller intestinal surface area than nonspecialists

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Capacity to digest sucrose and high nectar sucrose content coevolved independently

Glucose transporter expression and regulation following a fast in the ruby-throated hummingbird, Archilochus colubris

Hummingbirds, subsisting almost exclusively on nectar sugar, face extreme challenges to blood sugar regulation. The capacity for transmembrane sugar transport is mediated by the activity of facilitative glucose transporters (GLUTs) and their localisation to the plasma membrane (PM). In this study, we determined the relative protein abundance of GLUT1, GLUT2, GLUT3 and GLUT5 via immunoblot using custom-designed antibodies in whole-tissue homogenates and PM fractions of flight muscle, heart and liver of ruby-throated hummingbirds (Archilochus colubris). The GLUTs examined were detected in nearly all tissues tested. Hepatic GLUT1 was minimally present in whole-tissue homogenates and absent win PM fractions. GLUT5 was expressed in flight muscles at levels comparable to those of the liver, consistent with the hypothesised uniquely high fructose uptake and oxidation capacity of hummingbird flight muscles. To assess GLUT regulation, we fed ruby-throated hummingbirds 1 mol l-1 sucrose ad libitum for 24 h followed by either 1 h of fasting or continued feeding until sampling. We measured relative GLUT abundance and concentration of circulating sugars. Blood fructose concentration in fasted hummingbirds declined (∼5 mmol l-1 to ∼0.18 mmol l-1), while fructose-transporting GLUT2 and GLUT5 abundance did not change in PM fractions. Blood glucose concentrations remained elevated in fed and fasted hummingbirds (∼30 mmol l-1), while glucose-transporting GLUT1 and GLUT3 in flight muscle and liver PM fractions, respectively, declined in fasted birds. Our results suggest that glucose uptake capacity is dynamically reduced in response to fasting, allowing for maintenance of elevated blood glucose levels, while fructose uptake capacity remains constitutively elevated promoting depletion of blood total fructose within the first hour of a fast.

Low ambient temperature reduces the time for fuel switching in the ruby-throated hummingbird (Archilochus colubris)

Physiological adaptations that enhance flux through the sugar oxidation cascade permit hummingbirds to rapidly switch between burning lipids when fasted to burning ingested sugars when fed. Hummingbirds may be able to exert control over the timing and extent of use of ingested sugars by varying digestive rates when under pressure to accumulate energy stores or acquire energy in response to heightened energy demands. We hypothesized that hummingbirds would modulate the timing of a switch to reliance on ingested sugars differently when facing distinct energetic demands (cool versus warm ambient temperatures). The timing of the oxidation of a single nectar meal to fuel metabolism was assessed by open-flow respirometry, while the time to first excretion following the meal was used as a proxy for digestive throughput time. As predicted, birds showed a more rapid switch in respiratory exchange ratio (RER = rate of O₂ consumption/CO₂ production) and excreted earlier when held at cool temperatures compared to warm. In both cases, RER peaked barely above 1.0 indicating ingested sugar fueled ≈100% of resting metabolism. Our findings suggest that energetic demands modulate the rate of fuel switching through shifts of the sugar oxidation cascade. The speed of this shift may involve decreases in gut passage times which have previously been thought to be inflexible, or may be caused by changes in circulation as a result of low ambient temperature.

Evolution of digestive enzymes and dietary diversification in birds

As the most species-rich class of tetrapod vertebrates, Aves possesses diverse feeding habits, with multiple origins of insectivory, carnivory, frugivory, nectarivory, granivory and omnivory. Since digestive enzymes mediate and limit energy and nutrient uptake, we hypothesized that genes encoding digestive enzymes have undergone adaptive evolution in birds. To test this general hypothesis, we identified 16 digestive enzyme genes (including seven carbohydrase genes (hepatic amy, pancreatic amy, salivary amy, agl, g6pc, gaa and gck), three lipase genes (cyp7a1, lipf and pnlip), two protease genes (ctrc and pgc), two lysozyme genes (lyz and lyg) and two chitinase genes (chia and chit1)) from the available genomes of 48 bird species. Among these 16 genes, three (salivary amy, lipf and chit1) were not found in all 48 avian genomes, which was further supported by our synteny analysis. Of the remaining 13 genes, eight were single-copy and five (chia, gaa, lyz, lyg and pgc) were multi-copy. Moreover, the multi-copy genes gaa, lyg and pgc were predicted to exhibit functional divergence among copies. Positively selected sites were detected in all of the analyzed digestive enzyme genes, except agl, g6pc, gaa and gck, suggesting that different diets may have favored differences in catalytic capacities of these enzymes. Furthermore, the analysis also revealed that the pancreatic amylase gene and one of the lipase genes (cyp7a1) have higher ω (the ratio of nonsynonymous to the synonymous substitution rates) values in species consuming a larger amount of seeds and meat, respectively, indicating an intense selection. In addition, the gck carbohydrase gene in species consuming a smaller amount of seeds, fruits or nectar, and a lipase gene (pnlip) in species consuming less meat were found to be under relaxed selection. Thus, gene loss, gene duplication, functional divergence, positive selection and relaxed selection have collectively shaped the evolution of digestive enzymes in birds, and the evolutionary flexibility of these enzymes may have facilitated their dietary diversification.

Low energy reserves are associated with fasting susceptibility in Neotropical nectar bats Glossophaga soricina

Neotropical nectar-feeding bats consume large amounts of sugar and use most of their energy-rich diet directly from the bloodstream, suggesting an adaptation towards lower body energy reserves. Here we tested the hypothesis that bats Glossophaga soricina spare the energy costs of storing energy reserves, even if this would represent a risky susceptibility during fasting. Blood glucose concentrations in 18 h fasted bats showed a 40% decrease. Breast muscle and adipose tissue lipids, as well as carcass fatty acids and liver glycogen, were also decreased following fasting. The inability to keep normoglycemia following a short-term fasting (i.e. 28 h) confirm that nectar bats invest little on storing energy reserves and show a severe fasting susceptibility associated to this pattern. Our study also support the general hypothesis that evolutionary specializations towards nectar diets involve adaptations to allow a decreased body mass, which reduces the energy costs of flight while increases foraging time.

Metabolic Fates of Evening Crop-Stored Sugar in Ruby-Throated Hummingbirds (Archilochus colubris)

During the day, hummingbirds quickly metabolize floral nectar to fuel high metabolic demands, but are unable to feed at night. Though stored fat is the primary nocturnal metabolic fuel, it has been suggested that hummingbirds store nectar in their crop to offset fat expenditure in the night or to directly fuel their first foraging trip in the morning. We examine the use of crop-stored sugar in the nocturnal energy budget of ruby-throated hummingbirds (Archilochus colubris) using respirometry and 13C stable isotope analysis. Hummingbirds were fed a 13C-enriched sugar solution before lights-out and held in respirometry chambers overnight without food. Respirometry results indicate that the hummingbirds metabolized the sugar in the evening meal in less than 2 h, and subsequently primarily catabolized fat. Breath stable isotope signatures provide the key insight that the hummingbirds converted a substantial portion of an evening meal to fats, which they later catabolized to support their overnight metabolism and spare endogenous energy stores. These results show that the value of a hummingbird’s evening meal depends on how much of this energy was converted to fat. Furthermore, this suggests that evening hyperphagia is an important energy maximization strategy, especially during energetically expensive periods such as migration or incubation.

Integrative physiology of transcellular and paracellular intestinal absorption

Glucose absorption by the small intestine has been studied for nearly a century. Despite extensive knowledge about the identity, functioning and regulation of the relevant transporters, there has been and there remains controversy about how these transporters work in concert to determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) over a wide range of dietary and/or luminal concentrations. Our broader, integrative understanding of intestinal absorption requires more than the reductionist dissection of all the components and their elaboration at molecular and genetic levels. This Commentary emphasizes the integration of discrete molecular players and processes (including paracellular absorption) that, in combination, determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) and putative anti-nutrients (water-soluble toxins), and the integration of that absorption with other downstream processes related to metabolic demands. It identifies historic key advances, controversies and future research ideas, as well as important perspectives that arise through comparative as well as biomedical physiological research.

The Metabolic Flexibility of Hovering Vertebrate Nectarivores

Foraging hummingbirds and nectar bats oxidize both glucose and fructose from nectar at exceptionally high rates. Rapid sugar flux is made possible by adaptations to digestive, cardiovascular, and metabolic physiology affecting shared and distinct pathways for the processing of each sugar. Still, how these animals partition and regulate the metabolism of each sugar and whether this occurs differently between hummingbirds and bats remain unclear.

Cyclic bouts of extreme bradycardia counteract the high metabolism of frugivorous bats

Active flight requires the ability to efficiently fuel bursts of costly locomotion while maximizing energy conservation during non-flying times. We took a multi-faceted approach to estimate how fruit-eating bats (Uroderma bilobatum) manage a high-energy lifestyle fueled primarily by fig juice. Miniaturized heart rate telemetry shows that they use a novel, cyclic, bradycardic state that reduces daily energetic expenditure by 10% and counteracts heart rates as high as 900 bpm during flight. Uroderma bilobatum support flight with some of the fastest metabolic incorporation rates and dynamic circulating cortisol in vertebrates. These bats will exchange fat reserves within 24 hr, meaning that they must survive on the food of the day and are at daily risk of starvation. Energetic flexibly in U. bilobatum highlights the fundamental role of ecological pressures on integrative energetic networks and the still poorly understood energetic strategies of animals in the tropics.

To survive, all animals have to balance how much energy they take in and how much they use. They must find enough food to fuel the chemical processes that keep them alive – known as their metabolism – and store leftover fuel to use when food is not available. Bats, for example, have a fast metabolism and powerful flight muscles, which require a lot of energy. Some bat species, such as the tent-making bats, survive on fruit juice, and their food sources are often far apart and difficult to find. These bats are likely to starve if they go without food for more than 24 hours, and therefore need to conserve energy while they are resting.

To deal with potential food shortages, bats and other animals can enter a low-energy resting state called torpor. In this state, animals lower their body temperature and slow down their heart rate and metabolism so that they need less energy to stay alive. However, many animals that live in tropical regions, including tent-making bats, cannot enter a state of torpor, as it is too hot to sufficiently lower their body temperature. Until now, scientists did not fully understand how these bats control how much energy they use.

Now, O’Mara et al. studied tent-making bats in the wild by attaching small heart rate transmitters to four wild bats, and measured their heartbeats over several days. Since each heartbeat delivers oxygen and fuel to the rest of the body, measuring the bats’ heart rate indicates how much energy they are using. The experiments revealed for the first time that tent-making bats periodically lower their heart rates while resting (to around 200 beats per minute). This reduces the amount of energy they use each day by up to 10%, and helps counteract heart rates that can reach 900 beats per minute when the bats are flying.

Overall, these findings show that animals have evolved in various ways to control their use of energy. Future research should use similar technology to continue uncovering how wild animals have adapted to survive in different conditions. This knowledge will help us to understand how life has become so diverse in the tropics and the strategies that animals may use as climates change.

Sugar Metabolism in Hummingbirds and Nectar Bats

Hummingbirds and nectar bats coevolved with the plants they visit to feed on floral nectars rich in sugars. The extremely high metabolic costs imposed by small size and hovering flight in combination with reliance upon sugars as their main source of dietary calories resulted in convergent evolution of a suite of structural and functional traits. These allow high rates of aerobic energy metabolism in the flight muscles, fueled almost entirely by the oxidation of dietary sugars, during flight. High intestinal sucrase activities enable high rates of sucrose hydrolysis. Intestinal absorption of glucose and fructose occurs mainly through a paracellular pathway. In the fasted state, energy metabolism during flight relies on the oxidation of fat synthesized from previously-ingested sugar. During repeated bouts of hover-feeding, the enhanced digestive capacities, in combination with high capacities for sugar transport and oxidation in the flight muscles, allow the operation of the “sugar oxidation cascade”, the pathway by which dietary sugars are directly oxidized by flight muscles during exercise. It is suggested that the potentially harmful effects of nectar diets are prevented by locomotory exercise, just as in human hunter-gatherers who consume large quantities of honey.

Intestinal paracellular absorption is necessary to support the sugar oxidation cascade in nectarivorous bats

We made the first measurements of the capacity for paracellular nutrient absorption in intact nectarivorous bats. Leptonycteris yerbabuenae (20 g mass) were injected with or fed inert carbohydrate probes L-rhamnose and D(+)-cellobiose, which are absorbed exclusively by the paracellular route, and 3-O-methyl-D-glucose (3OMD-glucose), which is absorbed both paracellularly and transcellularly. Using a standard pharmacokinetic technique, we collected blood samples for 2 h after probe administration. As predicted, fractional absorption (f) of paracellular probes declined with increasing Mr in the order of rhamnose (f=0.71)>cellobiose (f=0.23). Absorption of 3OMD-glucose was complete (f=0.85; not different from unity). Integrating our data with those for glucose absorption and oxidation in another nectarivorous bat, we conclude that passive paracellular absorption of glucose is extensive in nectarivorous bat species, as in other bats and small birds, and necessary to support high glucose fluxes hypothesized for the sugar oxidation cascade.

Carbon stable-isotope tracking in breath for comparative studies of fuel use

Almost half a century ago, researchers demonstrated that the ratio of stable carbon isotopes in exhaled breath of rats and humans could reveal the oxidation of labeled substrates in vivo, opening a new chapter in the study of fuel use, the fate of ingested substrates, and aerobic metabolism. Until recently, the combined use of respirometry and stable-isotope tracer techniques had not been broadly employed to study fuel use in other animal groups. In this review, we summarize the history of this approach in human and animal research and define best practices that maximize its utility. We also summarize several case studies that use stable-isotope measurements of breath to explore the limits of aerobic metabolism and substrate turnover among several species and various physiological states. We highlight the importance of a comparative approach in revealing the profound effects that phylogeny, ecology, and behavior can have in shaping aerobic metabolism and energetics as well as the fundamental biological principles that underlie fuel use and metabolic function across taxa. New analytical equipment and refinement of methodology make the combined use of respirometry and stable-isotope tracer techniques simpler to perform, less costly, and more field ready than ever before.

Sugar flux through the flight muscles of hovering vertebrate nectarivores: a review

In most vertebrates, uptake and oxidation of circulating sugars by locomotor muscles rises with increasing exercise intensity. However, uptake rate by muscle plateaus at moderate aerobic exercise intensities and intracellular fuels dominate at oxygen consumption rates of 50% of maximum or more. Further, uptake and oxidation of circulating fructose by muscle is negligible. In contrast, hummingbirds and nectar bats are capable of fueling expensive hovering flight exclusively, or nearly completely, with dietary sugar. In addition, hummingbirds and nectar bats appear capable of fueling hovering flight completely with fructose. Three crucial steps are believed to be rate limiting to muscle uptake of circulating glucose or fructose in vertebrates: (1) delivery to muscle; (2) transport into muscle through glucose transporter proteins (GLUTs); and (3) phosphorylation of glucose by hexokinase (HK) within the muscle. In this review, we summarize what is known about the functional upregulation of exogenous sugar flux at each of these steps in hummingbirds and nectar bats. High cardiac output, capillary density, and blood sugar levels in hummingbirds and bats enhance sugar delivery to muscles (step 1). Hummingbird and nectar bat flight muscle fibers have relatively small cross-sectional areas and thus relatively high surface areas across which transport can occur (step 2). Maximum HK activities in each species are enough for carbohydrate flux through glycolysis to satisfy 100 % of hovering oxidative demand (step 3). However, qualitative patterns of GLUT expression in the muscle (step 2) raise more questions than they answer regarding sugar transport in hummingbirds and suggest major differences in the regulation of sugar flux compared to nectar bats. Behavioral and physiological similarities among hummingbirds, nectar bats, and other vertebrates suggest enhanced capacities for exogenous fuel use during exercise may be more wide spread than previously appreciated. Further, how the capacity for uptake and phosphorylation of circulating fructose is enhanced remains a tantalizing unknown.

Differences in the aerobic capacity of flight muscles between butterfly populations and species with dissimilar flight abilities

Habitat loss and climate change are rapidly converting natural habitats and thereby increasing the significance of dispersal capacity for vulnerable species. Flight is necessary for dispersal in many insects, and differences in dispersal capacity may reflect dissimilarities in flight muscle aerobic capacity. In a large metapopulation of the Glanville fritillary butterfly in the Åland Islands in Finland, adults disperse frequently between small local populations. Individuals found in newly established populations have higher flight metabolic rates and field-measured dispersal distances than butterflies in old populations. To assess possible differences in flight muscle aerobic capacity among Glanville fritillary populations, enzyme activities and tissue concentrations of the mitochondrial protein Cytochrome-c Oxidase (CytOx) were measured and compared with four other species of Nymphalid butterflies. Flight muscle structure and mitochondrial density were also examined in the Glanville fritillary and a long-distance migrant, the red admiral. Glanville fritillaries from new populations had significantly higher aerobic capacities than individuals from old populations. Comparing the different species, strong-flying butterfly species had higher flight muscle CytOx content and enzymatic activity than short-distance fliers, and mitochondria were larger and more numerous in the flight muscle of the red admiral than the Glanville fritillary. These results suggest that superior dispersal capacity of butterflies in new populations of the Glanville fritillary is due in part to greater aerobic capacity, though this species has a low aerobic capacity in general when compared with known strong fliers. Low aerobic capacity may limit dispersal ability of the Glanville fritillary.

Energy and metabolism

Although firmly grounded in metabolic biochemistry, the study of energy metabolism has gone well beyond this discipline and become integrative and comparative as well as ecological and evolutionary in scope. At the cellular level, ATP is hydrolyzed by energy-expending processes and resynthesized by pathways in bioenergetics. A significant development in the study of bioenergetics is the realization that fluxes through pathways as well as metabolic rates in cells, tissues, organs, and whole organisms are "system properties." Therefore, studies of energy metabolism have become, increasingly, experiments in systems biology. A significant challenge continues to be the integration of phenomena over multiple levels of organization. Body mass and temperature are said to account for most of the variation in metabolic rates found in nature. A mechanistic foundation for the understanding of these patterns is outlined. It is emphasized that evolution, leading to adaptation to diverse lifestyles and environments, has resulted in a tremendous amount of deviation from popularly accepted scaling "rules." This is especially so in the deep sea which constitutes most of the biosphere.

Glucose transporter expression in an avian nectarivore: the ruby-throated hummingbird (Archilochus colubris)

Glucose transporter (GLUT) proteins play a key role in the transport of monosaccharides across cellular membranes, and thus, blood sugar regulation and tissue metabolism. Patterns of GLUT expression, including the insulin-responsive GLUT4, have been well characterized in mammals. However, relatively little is known about patterns of GLUT expression in birds with existing data limited to the granivorous or herbivorous chicken, duck and sparrow. The smallest avian taxa, hummingbirds, exhibit some of the highest fasted and fed blood glucose levels and display an unusual ability to switch rapidly and completely between endogenous fat and exogenous sugar to fuel energetically expensive hovering flight. Despite this, nothing is known about the GLUT transporters that enable observed rapid rates of carbohydrate flux. We examined GLUT (GLUT1, 2, 3, & 4) expression in pectoralis, leg muscle, heart, liver, kidney, intestine and brain from both zebra finches (Taeniopygia guttata) and ruby-throated hummingbirds (Archilochus colubris). mRNA expression of all four transporters was probed using reverse-transcription PCR (RT-PCR). In addition, GLUT1 and 4 protein expression were assayed by western blot and immunostaining. Patterns of RNA and protein expression of GLUT1-3 in both species agree closely with published reports from other birds and mammals. As in other birds, and unlike in mammals, we did not detect GLUT4. A lack of GLUT4 correlates with hyperglycemia and an uncoupling of exercise intensity and relative oxidation of carbohydrates in hummingbirds. The function of GLUTs present in hummingbird muscle tissue (e.g. GLUT1 and 3) remain undescribed. Thus, further work is necessary to determine if high capillary density, and thus surface area across which cellular-mediated transport of sugars into active tissues (e.g. muscle) occurs, rather than taxon-specific differences in GLUT density or kinetics, can account for observed rapid rates of sugar flux into these tissues.

Backward flight in hummingbirds employs unique kinematic adjustments and entails low metabolic cost

Backward flight is a frequently used transient flight behavior among members of the species-rich hummingbird family (Trochilidae) when retreating from flowers, and is known from a variety of other avian and hexapod taxa, but the biomechanics of this intriguing locomotor mode have not been described. We measured rates of oxygen uptake (V(O2)) and flight kinematics of Anna's hummingbirds, Calypte anna (Lesson), within a wind tunnel using mask respirometry and high-speed videography, respectively, during backward, forward and hovering flight. We unexpectedly found that in sustained backward flight is similar to that in forward flight at equivalent airspeed, and is about 20% lower than hovering V(O2). For a bird that was measured throughout a range of backward airspeeds up to a speed of 4.5 m s(-1), the power curve resembled that of forward flight at equivalent airspeeds. Backward flight was facilitated by steep body angles coupled with substantial head flexion, and was also characterized by a higher wingbeat frequency, a flat stroke plane angle relative to horizontal, a high stroke plane angle relative to the longitudinal body axis, a high ratio of maximum:minimum wing positional angle, and a high upstroke:downstroke duration ratio. Because of the convergent evolution of hummingbird and some hexapod flight styles, flying insects may employ similar kinematics while engaged in backward flight, for example during station keeping or load lifting. We propose that backward flight behavior in retreat from flowers, together with other anatomical, physiological, morphological and behavioral adaptations, enables hummingbirds to maintain strictly aerial nectarivory.

Very low force-generating ability and unusually high temperature-dependency in hummingbird flight muscle fibers

Hummingbird flight muscle is estimated to have among the highest mass-specific power output among vertebrates, based on aerodynamic models. However, little is known about fundamental contractile properties of their remarkable flight muscles. We hypothesized that hummingbird pectoralis fibers generate relatively low force when activated in a tradeoff for high shortening speeds associated with the characteristic high wing beat frequencies that are required for sustained hovering. Our objective was to measure maximal force-generating ability (maximal force/cross-sectional area, Po/CSA) in single, skinned fibers from the pectoralis and supracoracoideus muscles, which power the wing downstroke and upstroke, respectively, in hummingbirds (Calypte anna) and in another similarly-sized species, zebra finch (Taeniopygia guttata), which also has a very high wingbeat frequency during flight but does not perform a sustained hover. Mean Po/CSA in hummingbird pectoralis fibers was very low - 1.6, 6.1 and 12.2 kN/m2, at 10, 15 and 20oC, respectively. Po/CSA in finch pectoralis fibers was also very low (for both species, ~5% of the reported Po/CSA of chicken pectoralis fast fibers at 15oC). Force generated at 20oC/force generated at 10oC ('Q10-force' value) was very high for hummingbird and finch pectoralis fibers (mean = 15.3 and 11.5, respectively), compared to rat slow and fast fibers (1.8 and 1.9, respectively). Po/CSA in hummingbird leg fibers was much higher than in pectoralis fibers, at each temperature, and the mean Q10-force was much lower. Thus, hummingbird and finch pectoralis fibers have an extremely low force-generating ability, compared to other bird and mammalian limb fibers, and an extremely high temperature-dependence of force generation. The extrapolated maximum force-generating ability of hummingbird pectoralis fibers in vivo (~48 kN/m2) is, however, substantially higher than the estimated requirements for hovering flight of C. anna. The unusually low Po/CSA of hummingbird and zebra finch pectoralis fibers may reflect a constraint imposed by a need for extremely high contraction frequencies, especially during hummingbird hovering.