Higher Mass-specific Metabolic Rates Research Articles

Mitochondrial metabolism in blood more reliably predicts whole-animal energy needs compared to other tissues

Understanding energy metabolism in free-ranging animals is crucial for ecological studies. In birds, red blood cells (RBCs) offer a minimally invasive method to estimate metabolic rate (MR). In this study with European starlings Sturnus vulgaris, we examined how RBC oxygen consumption relates to oxygen use in key tissues (brain, liver, heart, and pectoral muscle) and versus the whole organism measured at basal levels. The pectoral muscle accounted for 34%-42% of organismal MR, while the heart and liver, despite their high mass-specific metabolic rate, each contributed 2.5%-3.0% to organismal MR. Despite its low contribution to organismal MR (0.03%-0.04%), RBC MR best predicted organismal MR (r= 0.70). Oxygen consumption of the brain and pectoralis was also associated with whole-organism MR, unlike that of heart and liver. Overall, our findings demonstrate that the metabolism of a systemic tissue like blood is a superior proxy for organismal energy metabolism than that of other tissues.

Native lagomorphs facilitate noxious weeds in a semi-arid rangeland

Rangeland management has traditionally focused on the grazing effects of livestock, or on wild ungulates because they are potential competitors with livestock. However, herbivory by smaller herbivores, such as lagomorphs, could play a much greater role in shaping the plant community than is commonly thought. Being selective feeders with high mass-specific metabolic rates, we hypothesized that lagomorphs impose an herbivory regime on the rangeland that differs from that of ungulates and drives the plant community toward a different composition. We used an 8-year exclosure experiment with three plot types (complete exclosure, partial exclosure, open rangeland), each 5.95 m2, to separate the effects of grazing by lagomorphs (jackrabbits and cottontails) from the effects of the large grazers (mainly cattle and bison) on vegetation structure and plant community composition. We replicated the experiment across 20 sites in a semiarid rangeland in the western US. The plant community in the complete exclosure plots (lagomorphs and ungulates excluded) developed a fivefold greater standing crop of grass than the open rangeland but supported the lowest biomass, density, and aerial cover of forbs. Partial exclosure plots (ungulates excluded, lagomorphs included) developed a community with higher forb phytomass (33.6% increase relative to open rangeland) and lower forb species richness (16.4% decrease relative to open rangeland). This effect was mainly caused by the encroachment of noxious weeds, including Salsola tragus (prickly Russian thistle/tumbleweed), which covered up to 37% of the ground in plots that lagomorphs had exclusive access to. We infer that selective grazing by lagomorphs facilitates noxious weeds that are elsewhere controlled to some extent by (a) the coarse grazing and trampling effects of large ungulates and (b) competition with native herbaceous species. For semiarid rangelands that have been degraded by livestock grazing, our complete and partial exclosures show that a reduction in stocking rate might not return the rangeland to its former state due to a hysteresis effect resulting from the facilitation of noxious weeds by lagomorphs. Rangeland management should be informed by an understanding of the entire food web, which includes the direct and indirect grazing effects of small herbivores such as lagomorphs.

Free-living Allen’s hummingbirds (Selasphorus sasin) rarely use torpor while nesting

For reproducing animals, maintaining energy balance despite thermoregulatory challenges is important for surviving and successfully raising offspring. This is especially apparent in small endotherms that exhibit high mass-specific metabolic rates and live in unpredictable environments. Many of these animals use torpor, substantially reducing their metabolic rate and often body temperature to cope with high energetic demands during non-foraging periods. In birds, when the incubating parent uses torpor, the lowered temperatures that thermally sensitive offspring experience could delay development or increase mortality risk. We used thermal imaging to noninvasively explore how nesting female hummingbirds sustain their own energy balance while effectively incubating their eggs and brooding their chicks. We located 67 active Allen’s hummingbird (Selasphorus sasin) nests in Los Angeles, California and recorded nightly time-lapse thermal images at 14 of these nests for 108 nights using thermal cameras. We found that nesting females usually avoided entering torpor, with one bird entering deep torpor on two nights (2% of nights), and two other birds possibly using shallow torpor on three nights (3% of nights). We also modeled nightly energetic requirements of a bird experiencing nest temperatures vs. ambient temperature and using torpor or remaining normothermic, using data from similarly-sized broad-billed hummingbirds. Overall, we suggest that the warm environment of the nest, and possibly shallow torpor, help brooding female hummingbirds reduce their own energy requirements while prioritizing the energetic demands of their offspring.

Digital 3D models of theropods for approaching body-mass distribution and volume

The aim of this work is to obtain diverse morphometric data from digitized 3D models of scientifically accurate palaeoreconstructions of theropods from eight representative families. The analysed polyvinyl chloride (PVC) models belong to the genera Coelophysis, Dilophosaurus, Ceratosaurus, Allosaurus, Baryonyx, Carnotaurus, Giganotosaurus, and Tyrannosaurus. The scanned 3D models were scaled considering different body-size estimations of the literature. The 3D analysis of these genera provides information on the skull length and body length that allows for recognition of major evolutionary trends. The skull length/body length in the studied genera increases according with the size of the body from the smallest Coelophysis with a ratio of 0.093 to ratios of 0.119–0.120 for Tyrannosaurus and Giganotosaurus, the largest study theropods. The study of photogrammetric 3D models also provides morphometric information that cannot be obtained from the study of bones alone, but knowing that all reconstructions begin from the fossil bones, such as the surface/volume ratio (S/V). For the studied theropod genera surface/volume ratio ranges from 35.21 for Coelophysis to 5.55 for Tyrannosaurus. This parameter, closely related to the heat dissipation, help in the characterization of the metabolism of extinct taxa. Accordingly, slender primitive forms of the Early Jurassic (i.e. Coelophysis and Dilophosaurus) had relatively smaller skulls and higher mass-specific metabolic rates than the robust large theropods of the Cretaceous (i.e. Giganotosaurus and Tyrannosaurus). This work presents a technique that, when applied to proper dinosaur models, provides extent and accurate data that may help in diverse study areas within the dinosaur palaeontology and palaeobiology.

Body size allometry impacts flight-related morphology and metabolic rates in the solitary bee Megachile rotundata

Body size is related to many aspects of life history, including foraging distance and pollination efficiency. In solitary bees, manipulating the amount of larval diet produces intraspecific differences in adult body size. The goal of this study was to determine how body size impacts metabolic rates, allometry, and flight-related morphometrics in the alfalfa leafcutting bee, Megachile rotundata. By restricting or providing excess food, we produced a range of body sizes, which allowed us to test the effect of body size on allometry, the power required for flight, and amount of energy produced, as measured indirectly through CO2 emission. The power required during flight was predicted using the flight biomechanical formulas for wing loading and excess power index. We found larger bees had higher absolute metabolic rates at rest and during flight, but smaller bees had higher mass-specific metabolic rates at rest. During flight, bees did not have size-related differences in mass-specific metabolic rate. As bees increase in size, their thorax and abdomens become disproportionately larger, while their wings (area, and length) become disproportionately smaller. Smaller bees had more power available during flight as demonstrated by flight biomechanical formulas. Smaller body size was advantageous because of a reduced power requirement for flight with no metabolic cost.

Metabolic cost of inflammatory response of ruby-throated hummingbirds (Archilochus colubris)

Animals with a slow pace of life and high mass-specific metabolic rates are expected to invest less in innate immune responses. We measured skin inflammation and the resting metabolic rate (RMR) of ruby-throated hummingbirds ( Archilochus colubris) after their immune system was challenged with phytohemagglutinin (PHA) and compared with the response of birds injected a saline solution. The PHA test measures the inflammatory process, a component of the innate response. Ruby-throated hummingbirds belong to a group that is under-represented in avian immunological studies characterized by a slow pace of life and fast metabolic rate. Hummingbirds developed an inflammatory response that lasted <28 h. PHA injection produced a significant increment of RMR (up to ~13%) with respect to RMR values after the injection of the saline solution indicating that immune response involved a metabolic cost for hummingbirds. This increment lies within the range of values previously reported for birds injected PHA (5%–29%).

Winter mortality of young mudskipper fish: Effects of size, temperature and energy depletion

Winter mortality can be a strong selective pressure on animals living in temperate regions, especially for young and small individuals. Therefore, understanding the mechanism for its size-dependence is important to predict population dynamics of the species in a fluctuating environment. In this study, we experimentally investigated the effects of body size, temperature and energy reserve depletion on the overwinter mortality of young mudskipper fish (Boleophthalmus pectinirostris). Small size-class fish (<44 mm in SL) died earlier than large size-class fish (≥44 mm) under both heated and unheated temperature conditions. Relatively higher energy and lipid content in live compared to dead fish and suspended mortality during the warm period under unheated conditions suggested the effects of both energy depletion and low temperature on overwinter mortality. Small size-class fish showed a relatively higher energy depletion rate under both temperature conditions probably due to their higher mass-specific metabolic rate. Since small size-class fish died earlier than large size-class fish, the body size difference in energy depletion rate may be one of the factors affecting the size-dependence of overwinter mortality. The energy depletion rate gradually decreased with the progress of experiment, suggesting that fasting-induced metabolic depression occurred. The estimated relative lipid mass decreased with the progress of experiment. The lipid depletion rate with elapsed days significantly reduced in small size-class fish under unheated conditions comparing with large size-class fish, while the rate in large size-class fish was significantly larger than that of small size-class fish in heated conditions. The difference suggests the possibility that small size-class fish switches metabolic fuels at a relatively lower temperature condition to maintain a minimum activity level for unpredictable feeding opportunities during the pre-winter season.

Untangling life span and body mass discrepancies in canids: phylogenetic comparison of oxidative stress in blood from domestic dogs and wild canids.

Canids are a morphological and physiological diverse group of animals, with the most diversity found within one species, the domestic dog. Underlying observed morphological differences, there must also be differences at other levels of organization that could lead to elucidating aging rates and life span disparities between wild and domestic canids. Furthermore, small-breed dogs live significantly longer lives than large-breed dogs, while having higher mass-specific metabolic rates and faster growth rates. At the cellular level, a clear mechanism underlying whole animal traits has not been fully elucidated, although oxidative stress has been implicated as a potential culprit of the disparate life spans of domestic dogs. We used plasma and red blood cells from known aged domestic dogs and wild canids, and measured several oxidative stress variables: total antioxidant capacity (TAC), lipid damage, and enzymatic activities of catalase, superoxide dismutase, and glutathione peroxidase (GPx). We used phylogenetically informed general linear mixed models and nonphylogenetically corrected linear regression analysis. We found that lipid damage increases with age in domestic dogs, whereas TAC increases with age and TAC and GPx activity increases as a function of age/maximum life span in wild canids, which may partly explain longer potential life spans in wolves. As body mass increases, TAC and GPx activity increase in wild canids, but not domestic dogs, highlighting that artificial selection may have decreased antioxidant capacity in domestic dogs. We found that small-breed dogs have significantly higher circulating lipid damage compared with large-breed dogs, concomitant to their high mass-specific metabolism and higher growth rates, but in opposition to their long life spans.

Body-size Scaling is Related to Gut Microbial Diversity, Metabolism and Dietary Niche of Arboreal Folivorous Flying Squirrels

Thermal homeostasis of mammals is constrained by body-size scaling. Consequently, small mammals require considerable energy to maintain a high mass-specific metabolic rate (MSMR) and sustain target body temperature. In association with gut microbiota, mammalian hosts acquire absorbable molecules and fulfill their metabolic requirements. Our objective was to characterize gut microbes in wild mammals and relate those findings to host body-size scaling. Two large (Petaurista philippensis grandis and P. alborufus lena), one medium (Trogopterus xanthipes) and one small (Pteromys volans orii) species of flying squirrels (FS) were studied. Using 16S rRNA genes, 1,104 OTUs were detected from four FS, with 1.99% of OTUs shared among all FS. Although all FS gut microbiota were dominated by Firmicutes, they were constituted by different bacterial families. Moreover, Bacteroidetes accounted for up to 19% of gut microbiota in small FS, but was absent in large FS. Finally, based on metagenome predictions, carbohydrate and amino acid metabolism genes were enriched in small body-size FS. In conclusion, gut microbiota compositions and predictive metabolic functions were characteristic of body-size in FS, consistent with their adaptations to folivorous dietary niches.

The allometry of daily energy expenditure in hummingbirds: An energy budget approach.

Within-clade allometric relationships represent standard laws of scaling between energy and size, and their outliers provide new avenues for physiological and ecological research. According to the metabolic-level boundaries hypothesis, metabolic rates as a function of mass are expected to scale closer to 0.67 when driven by surface-related processes (e.g. heat or water flux), while volume-related processes (e.g. activity) generate slopes closer to one. In birds, daily energy expenditure (DEE) scales with body mass (M) in the relationship , consistent with surface-level processes driving the relationship. However, taxon-specific patterns differ from the scaling slope of all birds. Hummingbirds have the highest mass-specific metabolic rates among all vertebrates. Previous studies on a few hummingbird species, without accounting for the phylogeny, estimated that the DEE-body mass relationship for hummingbirds was . In Contrast to the theoretical expectations, this slope >1 indicates that larger hummingbirds are less metabolically efficient than smaller hummingbirds. We collected DEE and mass data for 12 hummingbird species, which, combined with published data, represented 17 hummingbird species in eight of nine hummingbird clades over a sixfold size range of body size (2.7-17.5g). After accounting for phylogenetic relatedness, we found DEE scales with body mass as . This slope of 0.95 is lower than previously estimated for hummingbirds, but much higher than the slope for all birds (0.68). The high slopes of torpor, hovering and flight potentially explain the high interspecific DEE slope for hummingbirds compared to other endotherms.

Improved mitochondrial coupling as a response to high mass-specific metabolic rate in extremely small mammals.

Mass-specific metabolic rate negatively co-varies with body mass from the whole-animal to the mitochondrial levels. Mitochondria are the mainly consumers of oxygen inspired by mammals to generate ATP or compensate energetic losses dissipated as the form of heat (proton leak) during oxidative phosphorylation. Consequently, ATP synthesis and proton leak thus compete for the same electrochemical gradient. Because proton leak co-varies negatively with body mass, it is unknown if extremely small mammals further decouple their mitochondria to maintain their body temperature or if they implement metabolic innovations to ensure cellular homeostasis. The present study investigates the impact of body mass variation on cellular and mitochondrial functioning in small mammals, comparing the two extremely small African pygmy mice (Mus mattheyi, approx. 5 g and Mus minutoides, approx. 7 g) with the larger house mouse (Mus musculus, approx. 22 g). Oxygen consumption rates were measured from the animal to the mitochondrial levels. We also measured mitochondrial ATP synthesis in order to appreciate the mitochondrial efficiency (ATP/O). At the whole-animal scale, mass- and surface-specific metabolic rates co-varied negatively with body mass, whereas this was not necessarily the case at cellular and mitochondrial levels. M. mattheyi had generally the lowest cellular and mitochondrial fluxes, depending on the tissue considered (liver or skeletal muscle), as well as having higher efficient muscle mitochondria than the other two species. M. mattheyi presents metabolic innovations to ensure its homeostasis, by generating more ATP per oxygen consumed.

Links between muscle phenotype and life history: differentiation of myosin heavy chain composition and muscle biochemistry in precocial and altricial pinniped pups.

In marine mammals, muscular development has been identified as a rate-limiting factor in achieving adult dive capacities. This study investigates the rate that myosin heavy chain (MHC) composition matures in a postural and locomotor skeletal muscle for four pinniped species with different lactation lengths: hooded seals, Cystophora cristata; harp seals, Pagophilus groenlandicus; northern fur seals, Callorhinus ursinus, and Steller sea lions, Eumetopias jubatus. The ontogeny of MHC isoform expression was compared with developmental rates of myoglobin concentrations, and aerobic (citrate synthase, β-hydroxyacyl-CoA dehydrogenase) and anaerobic (lactate dehydrogenase) enzyme activities. Within taxonomic families, species with shorter lactation periods had more mature muscles biochemically at birth, and fiber types differentiated earlier during ontogeny (Phocidae: hooded > harp seals, Otariidae: northern fur seals > Steller sea lions). Northern fur seal neonates had the most phenotypically-mature muscles in this study, with no immature MHC isoforms. The relationship between muscle biochemistry and MHC composition became more pronounced with age, and developed to reflect swimming mode and activity levels. In adults, phocids had more slow-twitch oxidative protein in their primary locomotor muscle, the Longissimus dorsi (LD), than otariids which likely reflects oxygen-sparing strategies for the phocids' longer dives. Conversely, northern fur seal muscles had higher proportions of fast-twitch MHCs in the Pectoralis and LD, likely indicative of this species' smaller size and higher mass-specific metabolic rates. Thus, muscle phenotype is linked with species life history, and a mismatch between muscle biochemistry and MHC composition at weaning has important implications for the first year of independent foraging in pinniped pups.

Components of standard metabolic rate variability in three species of gammarids

Abstract. Standard metabolic rate is a major functional trait with large inter-individual variability in many groups of aquatic species. Here we present results of an experimental study to address variation in standard metabolic rates, over different scales of organisation and environments, within a specific group of aquatic macro-invertebrates (i.e. gammarid amphipods) that represent the primary consumers in detritus food webs. The study was carried out using flow-through microrespirometric techniques on male specimens of three gammarid species from freshwater, transitional water and marine ecosystems. We examined individual metabolic rate variations at three scales: (1) at the individual level, during an 8 h period of daylight; (2) at the within-population level, along body-size and body-condition gradients; (3) at the interspecific level, across species occurring in the field in the three different categories of aquatic ecosystems, from freshwater to marine. We show that standard metabolic rates vary significantly at all three scales examined, with the highest variation observed at the within-population level. Variation in individual standard metabolic rates during the daylight hours was generally low (coefficient of variation, CV&lt;10 %) and unrelated to time. The average within-population CV ranged between 30.0 % and 35.0 %, with body size representing a significant source of overall inter-individual variation in the three species and individual body condition exerting only a marginal influence. In all species, the allometric equations were not as steep as would be expected from the 3∕4 power law, with significant variation in mass-specific metabolic rates among populations. The population from the transitional water ecosystem had the highest mass-specific metabolic rates and the lowest within-population variation. In the gammarid species studied here, body-size-independent variations in standard individual metabolic rates were higher than those explained by allometric body size scaling, and the costs of adaptation to short-term periodic variations in water salinity in the studied ecosystems also seemed to represent a major source of variation.

Metabolic Rate of Diploid and Triploid Edible Frog Pelophylax esculentus Correlates Inversely with Cell Size in Tadpoles but Not in Frogs.

In multicellular organisms, cell size may have crucial consequences for basic parameters, such as body size and whole-body metabolic rate (MR). The hypothesis predicts that animals composed of smaller cells (a higher membrane surface-to-cell volume ratio) should have a higher mass-specific MR because a large part of their energy is used to maintain cell membranes and ionic gradients. In this article, we investigated the link between cell size and MR in diploid and triploid tadpoles and froglets of the hybridogenetic frog Pelophylax esculentus. In our previous study, we showed that triploids had significantly larger cells (erythrocytes, hepatocytes, and epidermal cells were measured). Therefore, we hypothesized that triploid tadpoles and froglets would have a lower standard metabolic rate (SMR). Our study demonstrated for the first time two distinct effects of polyploidy/cell size on MR within a single species developing in both aquatic and terrestrial habitats. As we hypothesized, diploid tadpoles had a higher SMR than triploids, whereas in froglets, ploidy did not affect the SMR. We also found that the water temperatures in which tadpoles were reared had no effect on the SMR of froglets after metamorphosis. Based on our results and other reports, we suggest that cell size may have more consequences for whole-body MR in aquatic habitats than in terrestrial habitats because oxygen is less available in water and its availability in relation to oxygen demand decreases with temperature.

Swimming metabolic rates vary by sex and development stage, but not by species, in three species of Australian otariid seals.

Physiology may limit the ability for marine mammals to adapt to changing environments. Depth and duration of foraging dives are a function of total available oxygen stores, which theoretically increase as animals grow, and metabolic costs. To evaluate how physiology may influence the travelling costs for seals to foraging patches in the wild, we measured metabolic rates of a cross-section of New Zealand fur seals, Australian fur seals and Australian sea lions representing different foraging strategies, development stages, sexes and sizes. We report values for standard metabolic rate, active metabolic rate (obtained from submerged swimming), along with estimates of cost of transport (COT), measured via respirometry. We found a decline in mass-specific metabolic rate with increased duration of submerged swimming. For most seals mass-specific metabolic rate increased with speed and for all seals mass-specific COT decreased with speed. Mass-specific metabolic rate was higher for subadult than adult fur seals and sea lions, corresponding to an overall higher minimum COT. Some sex differences were also apparent, such that female Australian fur seals and Australian sea lions had higher mass-specific metabolic rates than males. There were no species differences in standard or active metabolic rates for adult males or females. The seals in our study appear to operate at their physiological optimum during submerged swimming. However, the higher metabolic rates of young and female fur seals and sea lions may limit their scope for increasing foraging effort during times of resource limitation.

Premigratory ruby-throated hummingbirds, Archilochus colubris, exhibit multiple strategies for fuelling migration

Many avian species fatten to fuel migratory flights. However, the amount of fat deposited prior to departure is variable depending on individual migration strategies. Despite their small size and high mass-specific metabolic rates, migratory hummingbirds at isolated meadows can fatten up to 44% in just 4 days prior to resuming migration, suggesting profound changes in energy acquisition. However, it remains to be seen whether hummingbirds fatten at the breeding grounds prior to initiating migration. Using feeder stations outfitted with radiofrequency identification readers and digital scales, we identified a subset of premigratory ruby-throated hummingbirds that exhibited significant mass gain in the 4 days leading up to migration (premigratory fattening) and identified others that did not (premigratory nonfattening). We further assessed foraging behaviour, monitored individual mass throughout the day and calculated rates of overnight mass loss to understand what behavioural variation allowed some premigratory birds to rapidly fatten. Premigratory fattening hummingbirds abandoned foraging restraint during the middle of the day, a behaviour thought to enhance aerial agility, and increased foraging effort during both the middle of the day and the evenings by increasing the duration but not the frequency of feeder visits. Groups did not differ in their morning foraging strategy. Premigratory fattening hummingbirds also lost mass overnight at reduced rates, implying that birds conserved energy to minimize the depletion of existing fat stores, possibly via increased nocturnal torpor use. Fattening hummingbirds used a two-pronged approach of increasing energy intake during specific daily periods and reducing overnight energy expenditure to achieve substantial premigratory mass gain over just 4 days. However, not all hummingbirds adopted this premigratory fuelling strategy; those that did were adults (>1 year old), suggesting that the use of a premigratory fuelling strategy may be age related.

Metabolic responses to chronic hypoxic incubation in embryonic American alligators (Alligator mississippiensis)

Chronic hypoxic incubation is a common tool used to study developmental changes in reduced O2 conditions, and it has been useful for identifying phenotypically plastic periods during ontogeny in laboratory settings. Reptilian embryos can be subjected to natural hypoxia due to nesting strategy, and recent studies have been important in establishing the phenotypic responses of several species to low developmental oxygen. In particular, the cardiovascular responses of American alligators (Alligator mississippiensis) to low developmental oxygen have been detailed, including a substantial cardiac enlargement that may support a higher mass specific metabolic rate. However, embryo mass-specific metabolic demands of hypoxic incubated alligator embryos have not been measured. In this study, alligator eggs were incubated in 10% O2 (H) or 21% O2 (N) environments for the entire course of embryonic development. Acute metabolic measures in 21% and 10% O2 were taken for both H and N groups. We hypothesized that acute 10% O2 exposure has no impact on metabolic rate of embryonic alligators, and that metabolic rate is unaffected by chronic hypoxic incubation when studied in embryos measured at 21% O2. Our findings suggest phenotypic changes resulting from hypoxic incubation early in incubation, in particular relative cardiac enlargement, enable embryonic alligators to sustain metabolic rate during acute hypoxic exposure.

Physiological underpinnings in life-history trade-offs in man's most popular selection experiment: the dog.

Animal life-history traits fall within a limited ecological space, a continuum referred to as a "slow-fast" life-history axis. Differences of life-history traits are thought to result from trade-offs between behavioral and physiological aspects in each species as mediated by the biotic and abiotic environment, as well as genetic mechanisms. Domestic animals tend to show inverse relationships between body size and life span. Dogs are a good example of this, with smaller dogs having higher mass-specific metabolic rates and longer lifespans compared with larger dogs. Thus, dogs provide a unique system to examine physiological consequences of life-history trade-offs. I have collected data from the literature to explore implications of these trade-offs at several levels of physiological organization including whole-animal, organ systems, and cells. Small dogs tend to have longer lifespans, fewer pups per litter, faster and shorter developmental trajectories, and higher mass-specific metabolic rates, and in general, larger metabolically active organs compared with large dogs. From work on isolated primary fibroblast cells and telomeres of dogs, I show that selection for body size may influence the attributes of cells that shape proliferative cellular rates and rates of telomere shortening. The potential links between body size, and cellular oxidative stress in dogs as they age are discussed. Furthermore, small size in dogs has been linked to concentrations of reduced insulin growth factor-1 (IGF-1) levels in plasma, a possible metabolic advantage that may provide higher resistance to oxidative stress, a parameter essential to increases in lifespan.

Benefits of being small? The scaling of flight performance in stingless bees and size‐dependent scaling of insect flight metabolism

Intra‐specific evolutionary studies usually report major advantages to large body size, yet most species in all animal clades are small. Smaller animals have some fundamental disadvantages such as reduced force production and movement speed that inhibit their abilities to effectively compete with larger species. In addition, smaller animals generally have higher mass specific metabolic rates at rest, leading to higher costs of living and per gram food requirements. What are the evolutionary advantages of being small? Across insects, birds and mammals, the cost of flight has been reported to scale hypometrically, with larger species having lower mass‐specific energy costs during hovering flight. We studied the flight physiology of stingless bee species ranging from 1–115 mg in mass using flow‐through respirometry to assess metabolic rate, high speed video and sound analysis to measure wingbeat frequency, and “grad and stab” thermocouple measurements of body temperature. We also measured the masses of head, thorax, and abdomen, wing size, and voluntary load‐lifting abilities. Thorax masses scaled isometrically, and all bees lifted similar fractions of their body mass. Bees less than 7 mg had body segment temperatures 1–3°C above air temperature, independent of mass, while larger bees had substantially elevated body temperatures. Metabolic rate during hovering scaled hypermetrically (scaling slope = 1.26) and hypermetry could not be explained by thermal effects. Wingbeat frequency was constant across size, but larger bees had relatively smaller wings. Combining our data with prior literature, we show that the scaling of flight metabolic rate changes from hypermetric to hypometric at approximately 40 mg body mass in insects. Thus a reduced absolute and per‐gram cost of flight likely provides selective advantages for the evolution of small body size in insects. This research was partially supported by NSF IOS 1122157 and a short‐term fellowship from the Smithsonian Tropical Research Institute.

Mass-Specific Metabolic Rate Influences Sperm Performance through Energy Production in Mammals.

Mass-specific metabolic rate, the rate at which organisms consume energy per gram of body weight, is negatively associated with body size in metazoans. As a consequence, small species have higher cellular metabolic rates and are able to process resources at a faster rate than large species. Since mass-specific metabolic rate has been shown to constrain evolution of sperm traits, and most of the metabolic activity of sperm cells relates to ATP production for sperm motility, we hypothesized that mass-specific metabolic rate could influence sperm energetic metabolism at the cellular level if sperm cells maintain the metabolic rate of organisms that generate them. We compared data on sperm straight-line velocity, mass-specific metabolic rate, and sperm ATP content from 40 mammalian species and found that the mass-specific metabolic rate positively influences sperm swimming velocity by (a) an indirect effect of sperm as the result of an increased sperm length, and (b) a direct effect independent of sperm length. In addition, our analyses show that species with higher mass-specific metabolic rate have higher ATP content per sperm and higher concentration of ATP per μm of sperm length, which are positively associated with sperm velocity. In conclusion, our results suggest that species with high mass-specific metabolic rate have been able to evolve both long and fast sperm. Moreover, independently of its effect on the production of larger sperm, the mass-specific metabolic rate is able to influence sperm velocity by increasing sperm ATP content in mammals.