Higher Mass-specific Metabolic Rates Research Articles

Age and natural metabolically-intensive behavior affect oxidative stress and antioxidant mechanisms

Flying honey bees have among the highest mass-specific metabolic rates ever measured, suggesting that their flight muscles may experience high levels of oxidative stress during normal daily activities. We measured parameters of oxidative stress and antioxidant capacity in highly metabolic flight muscle and less active head tissue in cohorts of age-matched nurse bees, which rarely fly, and foragers, which fly several hours per a day. Naturally occurring foraging flight elicited an increase in flight muscle Hsp70 content in both young and old foragers; however catalase and total antioxidant capacity increased only in young flight muscle. Surprisingly, young nurse bees also showed a modest daily increase in Hsp70, catalase levels and antioxidant capacity, and these effects were likely due to collecting the young nurses soon after orientation flights. There were no differences in flight muscle carbonyl content over the course of daily activity and few differences in Hsp70, catalase, total antioxidant capacity and protein carbonyl levels in head tissue regardless of age or activity. In summary, honey bee flight likely produces high levels of reactive oxygen species in flight muscle that, when coupled with age-related decreases in antioxidant activity may be responsible for behavioral senescence and reduced longevity.

Nectar‐feeding bats fuel their high metabolism directly with exogenous carbohydrates

Summary 1Mammals usually derive energy from metabolizing fat and glycogen stores combined with exogenous food. Nectarivorous bats mostly consume a diet low in both fat and proteins but rich in simple carbohydrates. Metabolizing exogenous carbohydrates directly to fuel their high mass-specific metabolic rate would save the energetic costs of lipogenesis and gluconeogenesis for nectarivorous bats. Therefore, we expected nectarivorous bats to switch to exogenous carbohydrates rapidly when available and use them predominantly instead of fat or glycogen. 2We first investigated the rate of fractional incorporation of dietary sugars into the pool of metabolized substrates in Glossophaga soricina by measuring the change in 13C enrichment of exhaled CO2 (δ13Cbreath) when animals were fed glucose, fructose or sucrose that was isotopically distinct from their normal diet. Second, we performed a diet-switch experiment to estimate the turnover rate of fat tissue. 3When fed with sugars, the δ13Cbreath converged quickly on the isotope signature of the ingested sugars, indicating an almost exclusive use of dietary carbohydrates. The time for a 50% carbon isotope exchange in exhaled CO2 equalled 9, 13 and 14 min for fructose, glucose and sucrose, respectively. Nectarivorous bats fuelled 82% of their metabolism with exogenous carbohydrate when fed with fructose, 95% when fed with glucose and 77% when fed with sucrose. Bats depleted 50% of their fat stores each day. 4Although nectarivorous bats consumed most of their body fat each day, this was still barely enough to sustain their diurnal metabolism. The fractional incorporation rates of dietary sugars into the pool of metabolized substrates in G. soricina are the fastest rates ever found in a mammal.

Periodic Cooling of Bird Eggs Reduces Embryonic Growth Efficiency

For many bird embryos, periodic cooling occurs when the incubating adult leaves the nest to forage, but the effects of periodic cooling on embryo growth, yolk use, and metabolism are poorly known. To address this question, we conducted incubation experiments on eggs of zebra finches (Taeniopygia guttata) that were frequently cooled and then rewarmed or were allowed to develop at a constant temperature. After 12 d of incubation, embryo mass and yolk reserves were less in eggs that experienced periodic cooling than in controls incubated constantly at 37.5 degrees Celsius. Embryos that regularly cooled to 20 degrees Celsius had higher mass-specific metabolic rates than embryos incubated constantly at 37.5 degrees Celsius. Periodic cooling delayed development and increased metabolic costs, reducing the efficiency with which egg nutrients were converted into embryo tissue. Avian embryos can tolerate periodic cooling, possibly by adjusting their physiology to variable thermal conditions, but at a cost to growth efficiency as well as rate of development. This reduction in embryo growth efficiency adds a new dimension to the fitness consequences of variation in adult nest attentiveness.

SEXUAL DIMORPHISM IN THE METABOLIC RATE OF TWO SPECIES OF WOLF SPIDER (ARANEAE, LYCOSIDAE)

Abstract Spiders have long been noted as classic examples of extreme sexual dimorphism and adaptations to the lifestyle of a sit-and-wait predator. We examined sex-based differences in the metabolic rate of two species of wolf spider that differ in their degree of sexual dimorphism and predatory strategy. Pardosa milvina (Hentz 1877) is a small active wolf spider that does not exhibit a large degree of sexual dimorphism in body size. Hogna helluo (Walckenaer 1837) is a large, strongly sexually dimorphic wolf spider with large, sedentary females and smaller, active males. We found that P. milvina had a higher mass-specific metabolic rate than H. helluo. Also, P. milvina males had a higher metabolic rate than P. milvina females but there was no difference in mass-specific metabolic rate between H. helluo males and females. Our data demonstrate that an actively foraging species, P. milvina, exhibits a higher metabolic rate than species with a sit-and-wait strategy, H. helluo. This suggests that activity leve...

Phytoplankton co-existence: Results from an individual-based simulation model

We have investigated phytoplankton competition in a 1D, coupled physical-biological, individual-based model, designed to simulate a size-structured phytoplankton community, whose members belong to the same functional group but differ in size (20, 40 and 60 μm ESD), and compete for two resources (light and nutrient-nitrogen) in the frame of a food-chain plankton ecosystem, forced by astronomical and climatological conditions of a subtropical site. Allometric relationships established ranked performance: small-sized individuals have higher mass-specific metabolic rates (photosynthesis, nutrient uptake and respiration) and sink more slowly than do individuals in the larger size-classes. No a priori form of niche diversification was considered. The simulation reproduced the seasonal pattern of the environmental variables and phytoplankton biomass, displayed seasonality in relative demography and sustained multi-year co-existence. Phytoplankton biomass rose during the spring bloom until nutrient depletion, decreasing afterwards due to zooplankton grazing. In the light-controlled phase of the spring bloom, the dominance ranking in the mixed layer was consistent with the allometric ranking of energetics; small, middle and large-sized phytoplankton accounted for 77.2%, 22.4% and 0.4% of total biomass (ca. 5 gC m −2). Vernal subduction into the seasonal thermocline shaped a summer nutricline at ca. 30 m depth, below which reproduction generated a deep chlorophyll maximum. During summer, zooplankton diel vertical migration, foraging and excretion, and microbial remineralisation of detritus produced a feeble and declining source of ammonium in the oligotrophic layer. Differential subduction into the seasonal thermocline and nutrient stress promoted the dominance of small phytoplankton in this layer. By the end of summer, the survivors of the three size-classes lay at different depths, which provides a mechanism to relax competition: small cells survived in the mixed layer, the middle-sized in the seasonal thermocline, and the largest in both the seasonal and permanent thermoclines. Large phytoplankton survived longer in the eutrophic but poorly lit environment due to their lower mass-specific respiration. Oligotrophy lasted until the mixed layer reached the nutricline in autumn. Those cells in the seasonal and permanent thermoclines were entrained into the mixed layer as it deepened, seeding the growing season next year. The numbers of plankton in the three seed populations depended critically on their reproduction during summer. In winter, growth was accelerated by the re-establishment of the diurnal thermocline. From year-to-year, the relative demographic success (the annual competitive advantage, ACA) of the competing populations depends critically of their relative energetics and the biomasses in the seed populations. Taken together, these two factors yielded negligible ranking among the size-classes, and thus co-existence was achieved over three simulated years despite substantial seasonal variation in competitive advantage.

The influence of body size on the diving behaviour and physiology of the bimodally respiring turtle, Elseya albagula

In aquatic vertebrates that acquire oxygen aerially dive duration scales positively with body mass, i.e. larger animals can dive for longer periods, however in bimodally respiring animals the relationship between dive duration and body mass is unclear. In this study we investigated the relationships between body size, aquatic respiration, and dive duration in the bimodally respiring turtle, Elseya albagula. Under normoxic conditions, dive duration was found to be independent of body mass. The dive durations of smaller turtles were equivalent to that of larger individuals despite their relatively smaller oxygen stores and higher mass specific metabolic rates. Smaller turtles were able to increase their dive duration through the use of aquatic respiration. Smaller turtles had a relatively higher cloacal bursae surface area than larger turtles, which allowed them to extract a relatively larger amount of oxygen from the water. By removing the ability to respire aquatically (hypoxic conditions), the dive duration of the smaller turtles significantly decreased restoring the normal positive relationship between body size and dive duration that is seen in other air-breathing vertebrates.

Body residues as dose for sublethal responses in alevins of landlocked salmon (Salmo salar m. sebago): A direct calorimetry study

Questions of organism-specific factors, toxicity endpoints, and their relation to mode-of-action all are related to consistency and applicability of body residue-based approaches. To address this issue, direct calorimetry was used to evaluate metabolic responses of alevins of landlocked salmon (Salmo salar m. sebago) to pentachlorophenol (PCP) exposure ranging from 0 to 1.04 microM for 24 h or 24, 48, and 72 h (0, 0.26, and 0.55 microM, respectively). The body residues were used as a dose metric for sublethal responses. The body size, rapid ontogenetic development, and exposure to a specific pollutant all were heat output-modifying factors. The acute exposure (24-72 h) to PCP led to a heat output-enhancing effect, which directly was related to an internal concentration of PCP in the range of 0.01 to 0.15 micromol/g. Within the treatments, body size per level of metabolic rate and magnitude of physiological response were not correlated, thus the alevins with higher mass-specific metabolic rate were not more sensitive to PCP. Primarily, increasing metabolic rate during posthatch development controls PCP toxicity only by affecting bioaccumulation kinetics, not the toxic potency of the chemical. New information of a relationship between observed natural variation in measured physiological trait of fish and PCP-induced response and its body residue-based level is of ecotoxicological importance.

Ontogeny of total body oxygen stores and aerobic dive potential in Steller sea lions (Eumetopias jubatus)

Two key factors influence the diving and hence foraging ability of marine mammals: increased oxygen stores prolong aerobic metabolism and decreased metabolism slows rate of fuel consumption. In young animals, foraging ability may be physiologically limited due to low total body oxygen stores and high mass specific metabolic rates. To examine the development of dive physiology in Steller sea lions, total body oxygen stores were measured in animals from 1 to 29 months of age and used to estimate aerobic dive limit (ADL). Blood oxygen stores were determined by measuring hematocrit, hemoglobin, and plasma volume, while muscle oxygen stores were determined by measuring myoglobin concentration and total muscle mass. Around 2 years of age, juveniles attained mass specific total body oxygen stores that were similar to those of adult females; however, their estimated ADL remained less than that of adults, most likely due to their smaller size and higher mass specific metabolic rates. These findings indicate that juvenile Steller sea lion oxygen stores remain immature for more than a year, and therefore may constrain dive behavior during the transition to nutritional independence.

Adipose energy stores, physical work, and the metabolic syndrome: lessons from hummingbirds

Hummingbirds and other nectar-feeding, migratory birds possess unusual adaptive traits that offer important lessons concerning obesity, diabetes and the metabolic syndrome. Hummingbirds consume a high sugar diet and have fasting glucose levels that would be severely hyperglycemic in humans, yet these nectar-fed birds recover most glucose that is filtered into the urine. Hummingbirds accumulate over 40% body fat shortly before migrations in the spring and autumn. Despite hyperglycemia and seasonally elevated body fat, the birds are not known to become diabetic in the sense of developing polyuria (glucosuria), polydipsia and polyphagia. The tiny (3–4 g) Ruby-throated hummingbird has among the highest mass-specific metabolic rates known, and loses most of its stored fat in 20 h by flying up to 600 miles across the Gulf of Mexico. During the breeding season, it becomes lean and maintains an extremely accurate energy balance. In addition, hummingbirds can quickly enter torpor and reduce resting metabolic rates by 10-fold. Thus, hummingbirds are wonderful examples of the adaptive nature of fat tissue, and may offer lessons concerning prevention of metabolic syndrome in humans.

Metabolic profile of long–distance migratory flight and stopover in a shorebird

Migrating birds often complete long non-stop flights during which body energy stores exclusively support energetic demands. The metabolic correlates of such long-distance travel in free-living migrants are as yet poorly studied. Bar-tailed godwits, Limosa lapponica taymyrensis, undertake a 4500 km flight to their single spring stopover site and thus provide an excellent model in which to determine the energy fuels associated with endurance travel. To this end, we evaluated plasma concentrations of six key metabolites in arriving godwits caught immediately upon landing near their stopover site. Initial metabolite levels were compared with levels after 5 h of inactive rest to determine how flight per se affects energy metabolism. Birds refuelling on the stopover site were also examined. Arriving godwits displayed elevated plasma free fatty acids, glycerol and butyrate, confirming the importance of lipid fuel in the support of extended migratory activity. Further-more, elevated plasma triglycerides in these birds suggest that fatty acid provisioning is facilitated through hepatic synthesis and release of neutral lipids, as previously hypothesized for small migrants with high mass-specific metabolic rates. Finally, elevations in plasma uric acid suggest that protein breakdown contributes to the support of long-distance movement, to possibly maintain citric acid cycle intermediates, gluconeogenesis and/or water balance.

Effect of the bee glue (propolis) on the calorimetrically measured metabolic rate and metamorphosis of the greater wax moth Galleria mellonella

Among the moth pests of the honeybee, the greater wax moth Galleria mellonella (Lepidoptera: Pyralidae) causes the greatest damage, unless controlled at an early stage, because it feeds on wax, pollen, and cocoon of the bee larvae. This leads to the destruction of honeycomb and subsequent deterioration of weakened colonies. For controlling the pest, natural products are second to none, not least because the use of synthetic substances carries with it the problem of residues, which remain in the beehive to affect the bee products. This paper reports the results of calorimetric investigations on the effects of the bee natural insecticidal glue, propolis, on pupal metamorphosis and the metabolic rate of different larval instars.Experiments were performed by batch calorimetry to record the heat flow rate of individual larvae/pupae before and after the treatment, which consisted of dipping L5, L6, and L7 instars in a graded series of different concentrations of ethanol-dissolved propolis for 30s before blotting them. The heat production rates were then recorded for 6–7h (short period experiment) or during the entire pupal metamorphosis (long period experiment).The fifth larval instar (L5) showed higher sensitivity to propolis treatment than L6 and L7 whereby total mortality was obtained by 4% propolis for L5 and 8–10% for the latter. The higher sensitivity of L5 can be accounted for by the very high mass-specific metabolic rate and the thinner and more fragile cuticle, typical of early larval stages, allowing the free transit of nonpolar toxic substances from the surroundings after being easily disrupted by components of propolis.The treatment of the late L7 stage with nonlethal doses of propolis shortened the duration of pupal metamorphosis significantly. An untreated larva required 6.8±0.8 days (mean±S.E.,n=5) between larval–pupal and pupal–adult ecdysis, whereas this time was shortened to 5.4±0.9 and 4.8±0.5 days after treatment with 1 and 2% propolis, respectively. Though all treated larvae went through larval–pupal ecdysis, 40 and 100% of those treated with 2 and 4% propolis, respectively, displayed abortion of pupal metamorphosis and died.These results indicate that propolis is toxic at higher concentrations and an insect growth regulator at lower ones. The use of propolis in the control of G. mellonella and its subsequent occurrence in honeybee products such as honey and wax may not cause the problem of a toxic residue, as it is the natural component in the beehive.

The Influence of Foraging Mode and Arid Adaptation on the Basal Metabolic Rates of Burrowing Mammals

Two competing but nonexclusive hypotheses to explain the reduced basal metabolic rate (BMR) of mammals that live and forage underground (fossorial species) are examined by comparing this group with burrowing mammals that forage on the surface (semifossorial species). These hypotheses suggest that the low BMR of fossorial species either compensates for the enormous energetic demands of subterranean foraging (the cost-of-burrowing hypothesis) or prevents overheating in closed burrow systems (the thermal-stress hypothesis). Because phylogentically informed allometric analysis showed that arid burrowing mammals have a significantly lower BMR than mesic ones, fossorial and semifossorial species were compared within these groups. The BMRs of mesic fossorial and semifossorial mammals could not be reliably distinguished, nor could the BMRs of large (>77 g) arid fossorial and semifossorial mammals. This finding favours the thermal-stress hypothesis, because the groups appear to have similar BMRs despite differences in foraging costs. However, in support of the cost-of-burrowing hypothesis, small (<77 g) arid fossorial mammals were found to have a significantly lower BMR than semifossorial mammals of the similar size. Given the high mass-specific metabolic rates of small animals, they are expected to be under severe energy and water stress in arid environments. Under such conditions, the greatly reduced BMR of small fossorial species may compensate for the enormous energetic demands of subterranean foraging.

Energy Metabolism of European (Apis Mellifera Carnica) and Egyptian (A. M. Lamarckii) Honeybees

Two geographical subspecies of the honeybee Apis mellifera, the European bee A. m. carnica and the Egyptian bee A. m. lamarckii, were investigated by direct calorimetry. Maximum, mean and minimum heat production rates were determined for groups of 6 bees as a function of temperature and daytime. Smaller Egyptian subspecies showed significantly higher mass specific metabolic rates than the European one. Maximum and mean heat production rates decreased exponentially with growing temperatures while the minimum values remained constant.

Mitochondrial function in flying honeybees (Apis mellifera): respiratory chain enzymes and electron flow from complex III to oxygen.

The biochemical bases for the high mass-specific metabolic rates of flying insects remain poorly understood. To gain insights into mitochondrial function during flight, metabolic rates of individual flying honeybees were measured using respirometry, and their thoracic muscles were fixed for electron microscopy. Mitochondrial volume densities and cristae surface densities, combined with biochemical data concerning cytochrome content per unit mass, were used to estimate respiratory chain enzyme densities per unit cristae surface area. Despite the high content of respiratory enzymes per unit muscle mass, these are accommodated by abundant mitochondria and high cristae surface densities such that enzyme densities per unit cristae surface area are similar to those found in mammalian muscle and liver. These results support the idea that a unit area of mitochondrial inner membrane constitutes an invariant structural unit. Rates of O(2) consumption per unit cristae surface area are much higher than those estimated in mammals as a consequence of higher enzyme turnover rates (electron transfer rates per enzyme molecule) during flight. Cytochrome c oxidase, in particular, operates close to its maximum catalytic capacity (k(cat)). Thus, high flux rates are achieved via (i) high respiratory enzyme content per unit muscle mass and (ii) the operation of these enzymes at high fractional velocities.

Energetics of hummingbird foraging at low ambient temperature

Because of their small size and the high energetic costs of hovering and forward flight, hummingbirds achieve the highest mass-specific metabolic rates known among vertebrates. Rufous hummingbirds (Selasphorus rufus) stop to refuel on floral nectar in subalpine meadows as they migrate south from British Columbia to Mexico. In such habitats they face the challenge of achieving daily net energy gain despite the high energetic costs of flight and thermoregulation at near-freezing morning temperatures. Hummingbirds provided with 15 or 20% sucrose while subjected to these conditions for 4 h in the laboratory did not remain in energy balance and lost mass. However, they achieved energy balance or net energy gain on 30% sucrose. Because these sucrose concentrations are within the range observed in the nectar of hummingbird-visited flowers, the results suggest that the energetic cost of thermoregulation may influence the coevolution of hummingbirds and flowers. Hummingbirds maintaining energy balance at low ambient temperature via high foraging frequencies and high rates of energy intake can sustain average metabolic rates of about 250 W/kg over a 4-h period. These are the highest metabolic rates known among vertebrates at which rates of dietary energy intake equal rates of energy expenditure.

Lipid Metabolism in Hibernators: The Importance of Essential Fatty Acids

SYNoPSIS. Two polyunsaturated essential fatty acids, linoleic acid and linolenic acid, are important for their inherent energy during lipid oxidation. In addition, they influence the length of hibernation bouts and the metabolic rates of mammals that hibernate. Hibernators that lack linoleic acid in their diet or that are fed a diet high in saturated fatty acids have significantly shorter bouts of hibernation and have a higher mass specific metabolic rate. The decrease in the length of a bout of hibernation is significant because the animal arouses from hibernation more frequently, using more of its energy stores. This could result in a decreased chance of survival. How the essential fatty acids exert their actions in hibernators is just beginning to be elucidated. Essential fatty acids are the sole precursors for the eicosanoids that influence thermoregulation. Thus, studies of eicosanoid function during hibernation are warranted. The recent discovery and characterization of the protein leptin, which can regulate energy balance and may be regulated by polyunsaturated fatty acids, may prove to be important to hibernation and the regulation of body mass. Future investigations of the regulation of body mass during hibernation should consider the fatty acid composition of the diet and the effect of the essential fatty acids on gene transcription.

Gastrointestinal Morphology and Motility in American Kestrels Receiving High or Low Fat Diets

Fifteen yearling, male American Kestrels (Falco sparverius) were fed either intact day-old chicks (Gallus domesticus) (high-fat diet) or chicks with the yolk removed plus a piece of lean turkey meat approximately equivalent in volume to the yolk (low-fat diet). This was done for 33-37 days to determine the influence of dietary fat on the gastrointestinal gross anatomy and contractile activity of the kestrels. Fat content of the diet appeared to have little effect on either anatomy or contractions. Observations of contractile activity, via Image Intensification Radiology, revealed a unique reflux of duodenal contents associated with every duodenal flux in every gastroduodenal contraction cycle (approximately 3-4 min-1). This presumably improved mixing of ingesta with digestive secretions and hastened digestion of ingested nutrients which would be advantageous to a small bird with a high mass-specific metabolic rate. The duodenum was enlarged relative to the size of this organ in other raptors, possibly to accommodate to this process. The colon also was relatively larger in this species, perhaps to permit final processing of the diet and water absorption from the greater quantity of digesta passed from the small intestine.

Nest Morphology and Body Size of Ross' Geese and Lesser Snow Geese

--Arctic-nesting geese build large, insulated nests to protect developing embryos from cold ambient temperatures. Ross' Geese (Chen rossii) are about two-thirds the mass of Lesser Snow Geese (C. caerulescens caerulescens), have higher mass-specific metabolic rate, and maintain lower nest attentiveness, yet they hatch goslings with more functionally mature gizzards and more protein for their size than do Lesser Snow Geese. We compared nest size (a reflection of nest insulation) in four distinct habitats in a mixed breeding colony of Ross' Geese and Lesser Snow Geese at Karrak Lake, Northwest Territories, Canada. After adjusting measurements for nest-specific egg size and clutch size, we found that overall nest morphology differed between species and among habitats. Nest size increased progressively among heath, rock, mixed, and moss habitats. When nesting materials were not limiting, nests were smaller in habitats that provided cover from wind and precipitation than in habitats that did not provide cover Ross' Geese constructed relatively larger, more insulated nests than did Lesser Snow Geese, which may hasten embryonic development, minimize energy expenditure during incubation, and minimize embryonic ooling during recesses. We suggest hat relative differences in nest morphology reflect greater selection for Ross' Geese to improve nest insulation because of their smaller size (adults and embryos), higher massspecific metabolic rate, and lower incubation constancy. Received 13 May 1996, accepted 18 March 1997. IN BIRDS, HEAT IS SUPPLIED to developing embryos primarily by body warmth of the brooding parent (Afton and Paulus 1992). Heat loss from developing embryos and metabolic costs to parents can be minimized by increasing nest insulation. Nest insulation affects growth rates of nestlings (Winkler 1993), and probably affects embryonic growth as well, particularly in precocial species. Nest morphology and insulative properties of nests probably are influenced by body size, metabolic rates, incubation constancy, ambient thermal conditions, availability of nesting materials, and risk of predation (Moller 1984). Mass-specific metabolic rate and heat-transfer rate generally increase with declining body mass (Brody 1945, Lasiewski and Dawson 1967, Templeton 1970, Hill and Wyse 1989). Consequently, smaller species must assimilate nutrients at greater rates than larger species to maintain high metabolic rates. In addition, smaller species have less capacity to store endogenous nutrients than do larger 4 E-mail: kmccrac@unixl.sncc.lsu.edu species. Thus, body size is of profound consequence to incubating parents, which must fast during incubation unless they are fed by mates or auxiliaries (von Haartman 1958, Skutch 1962, Afton and Paulus 1992). If endogenous nutrients are not sufficient to meet energy requirements during incubation sessions, parents may recess from incubation in order to feed, thereby decreasing incubation constancy and exposing nests to predators and heat loss. Afton and Paulus (1992) reported that incubation constancy is positively related to body mass among all species of waterfowl (Anatidae), particularly among geese and swans. Within species, nest-building behavior may be adapted to maximize efficiency of nest insulation in response to cumulative embryonic metabolic rates, which are products of clutch size and embryo body size. Because of the relationship between surface area and volume, small eggs and clutches have greater rates of heat transfer than larger eggs and clutches, all else being equal. These factors are particularly important under the windy conditions that pre-

Sustaining the Highest Mass-Specific Metabolic Rates in the Animal Kingdom

Sustaining the Highest Mass-Specific Metabolic Rates in the Animal Kingdom

Fiber types and myosin heavy chain composition in muscles of common shrew (Sorex araneus)

Red-toothed shrews of subfamily Soricinae are small mammals with very high mass-specific metabolic rate. Owing to their high aerobic power they are interesting objects for studies concerning the limits and constraints of skeletal muscle adaptation. In order to clarify the correlation between metabolic rate and muscle properties, we have analyzed fiber types, fiber size, and myosin heavy chain composition of the common shrew (Sorex araneus) and compared them to those of rat (Rattus norvegicus). Three distinct differences between shrew and rat muscles were noted. 1) The fibers of shrew muscles are exceptionally small in comparison to rat myofibers. 2) Electrophoretic and histochemical analysis showed that shrew muscles are composed of only fast fibers (fiber types IIB and IID), the slow type I fibers being totally absent. 3) The shrew muscles are much more homogenous than rat muscles in regard to myosin heavy chain and fiber type composition. The shrew diaphragm consists exclusively myosin heavy chain type IId (MHCIId), while masseter and soleus are composed 95% and 87% of MHCIId, respectively. Other four studied muscles contain MHCIIb and MHCIId approximately in equal proportions. The present findings show that shrew muscles are composed of small, highly aerobic, fast type II fibers, which may be sufficiently fatigue-resistant to function both as postural muscles and to power fast and high frequency movements of limbs and diaphragm.