Peak Metabolic Rate Research Articles

TROPICAL BIRDS DOWNSIZED ORGANS

![][1] Life in the tropics tends to have a more relaxed pace than at more extreme latitudes. According to Joe Williams and his colleagues from Ohio State University, USA, the basal and peak metabolic rates of tropical birds are significantly lower than those of their more temperate

Effect of meal size on postprandial metabolic response and excess post-exercise oxygen consumption in juvenile rock carp (Procypris rabaudi)

To investigate the effect of meal size on postprandial metabolic response and excess post-exercise oxygen consumption(EPOC) in juvenile rock carp(Procypris rabaudi)[(13.05±0.17) g,n=44],experimental fish were lightly anaesthetized and force-fed with compound feed(meal sizes: 0%,0.5%,1% and 2% body weight) at(25 ±0.5)℃.After diet offered,oxygen consumption rates at different intervals were measured.Variables of postprandial metabolic and EPOC responses were calculated.The results of present study showed that resting metabolic rates(RMR) were not significantly different among all groups.The postprandial metabolic rate first increased and then slowly decreased to pre-fed level.The time to peak metabolic rate and the energy expended on Specific dynamic action(SDA) in 0.5% and 1% meal size groups were lower than that of 2% meal size group(P0.05).The peak metabolic rates(PMR) and the SDA coefficients were not significantly different among 0.5%,1% and 2% meal size groups.Pre-exercise metabolic rate in feeding group was significantly higher than that of fasting group(P0.05).Peak post-exercise metabolic rate,metabolic rate increment and excess post-exercise oxygen consumption magnitude were not significantly different between feeding group and fasting group.The maximum metabolic rate(MMR) and MMR/RMR were significantly higher than PMR and PMR/RMR(P0.05).The fish fulfilled the increased energy demand during digestive process by means of prolonged SDA duration with the increase of meal size.EPOC was not influenced by feeding in the fish.

A long life in the fast lane: positive association between peak metabolic rate and lifespan in a butterfly

High peak metabolic rates may provide a performance advantage, but it may also entail a physiological cost. A long-held assumption is that high mass-specific energy expenditure is associated with short lifespan. To examine the relationship between energy expenditure and lifespan we asked two questions. First, do individuals have a consistent rate of metabolism throughout their life? Second, is metabolic rate correlated with lifespan? We analysed the repeatability of measurements of resting (RMR) and peak flight metabolic rate (MR(peak)) throughout the life of the Glanville fritillary butterfly (Melitaea cinxia). Measurements of MR(peak) showed significant repeatability. Senescence occurred only shortly before death. RMR showed a U-shaped relationship with age and very low repeatability. Intraspecific association between metabolic rates and lifespan was tested under three conditions: in the laboratory, under field conditions and in a laboratory experiment with repeated flight treatments. There was a significant correlation between MR(peak) and lifespan in all three experiments, but the correlation was positive, not negative. RMR was not correlated with lifespan. Both MR(peak) and lifespan may reflect physiological condition and therefore be positively correlated. Individuals with a large resource pool may be able to invest in mechanisms that slow down ageing. Individuals with high metabolic capacity may also possess adaptations against ageing. Molecular polymorphism in the gene phosphoglucose isomerase (Pgi) was significantly associated with both MR(peak) and lifespan, and may have coevolved with defence mechanisms against senescence. Generalisations such as 'live fast, die young' may be too simple to explain the complex processes affecting ageing and lifespan.

Developmental stress has sex-specific effects on nestling growth and adult metabolic rates but no effect on adult body size or body composition in song sparrows

Variation in the prenatal and postnatal environments can have long-term effects on adult phenotype. In humans and other animals, exposure to stressors can lead to long-term changes in physiology. These changes may predispose individuals to disease, especially disorders involving energy metabolism. In addition, by permanently altering metabolic rates and energy requirements, such effects could have important fitness consequences. We determined the effects of early-life food restriction and corticosterone (CORT) treatment on growth and adult body size, body composition (assessed via quantitative magnetic resonance) and metabolic rates in the song sparrow, Melospiza melodia. Nestlings were hand-raised in captivity from 3 days of age. Treatments (ad libitum food, food restriction or CORT treatment) lasted from day 7 to day 60. Both experimental treatments had sex-specific effects on growth. In the nestling period, CORT-treated males weighed more than controls, whereas CORT-treated females weighed less than controls. Food-restricted males weighed the same as controls, whereas food-restricted females weighed less than controls. Both experimental treatments also had sex-specific effects on standard metabolic rate (SMR). Females exposed to food restriction or CORT treatment during development had higher SMRs in adulthood than control females, but neither stressor affected SMR in males. There were no effects of either treatment on adult body size, body composition (lean or fat mass) or peak metabolic rate. Therefore, early-life stress may have sex-specific programming effects on metabolic rates and energy expenditure in song sparrows. In addition, both treatments affected nestling growth in a manner that exaggerated the typical sex difference in nestling mass, which could provide male nestlings with a competitive advantage over their sisters when developing in a poor-quality environment.

Excess posthypoxic oxygen consumption in rainbow trout (Oncorhynchus mykiss): recovery in normoxia and hypoxia

Under certain conditions, a number of fish species may perform brief excursions into severe hypoxia and return to water with a higher oxygen content. The term severe hypoxia describes oxygen conditions that are below the critical oxygen saturation (Scrit), defined here as the oxygen threshold at which the standard metabolic rate becomes dependent upon the ambient oxygen content. Using rainbow trout ( Oncorhynchus mykiss (Walbaum, 1792), this study quantified the excess posthypoxic oxygen consumption (EPHOC) occurring after exposure to oxygen availability below Scrit. Tests showed that Scritwas 13.5% air saturation (O2sat). Fish were exposed to 10% O2satfor 0.97 h, and the EPHOC was quantified in normoxia (≥95% O2sat) and hypoxia (30% O2sat) to test the hypothesis that reduced oxygen availability would decrease the peak metabolic rate (MO2peak) and prolong the duration of the metabolic recovery. Results showed that MO2peakduring the recovery was reduced from 253 to 127 mg O2·kg–1·h–1in hypoxia compared with normoxia. Metabolic recovery lasted 5.2 h in normoxia and 9.8 h in hypoxia. The EPHOC, however, did not differ between the two treatments. Impeded metabolic recovery in hypoxia may have implications for fish recovering from exposure to oxygen availability below Scrit.

Effect of meal size on the specific dynamic action of the juvenile snakehead (Channa argus)

The effect of meal size on the specific dynamic action (SDA) of the juvenile snakehead (Channa argus) was assessed at 25°C. The fish were fed with test diets at meal sizes of 0.5, 1, 2, 3, 4, and 5% body mass and the postprandial oxygen consumption rate was determined at 1-h intervals until it returned to the pre-prandial level. The peak metabolic rate increased from 237.4 to 283.2mgO2kg−1h−1 as the relative meal size increased from 0.5% to 3% and leveled off at 4% and 5%. Factorial metabolic scope increased from 1.53 to 1.99 and SDA duration increased from 11.7 to 32.3h as the relative meal size increased from 0.5% to 5%. The relationship between SDA duration (D) and relative meal size (M) was described as: D=4.28M+10.62 (r2=0.752, P<0.05, n=50). The energy expended on SDA increased while the SDA coefficient decreased with increasing meal size. The results of the present study suggest that the snakehead may adopt different feeding strategies when taking in different amounts of food.

Effects of oxygen availability on maximum aerobic performance inMus musculusselected for basal metabolic rate or aerobic capacity

Maximum aerobic metabolism cannot increase indefinitely in response to demands for ATP production and, therefore, must be constrained by one (or many) of the steps of the oxygen transport and utilization pathways. To elucidate those constraints we compared peak metabolic rate elicited by running (V(.)(O₂,run)) in hypoxia (14% O₂), normoxia (21% O₂) and hyperoxia (30% O₂) of laboratory mice divergently selected for low and high basal metabolic rate (L-BMR and H-BMR, respectively), mice selected for maximum metabolic rate elicited by swimming (V(.)(O₂,swim)) and mice from unselected lines. In all line types (V(.)(O₂,run)) was lowest in hypoxia, intermediate in normoxia and highest in hyperoxia, which suggests a 'central' limitation of oxygen uptake or delivery instead of a limit set by cellular oxidative capacity. However, the existence of a common central limitation is not in agreement with our earlier studies showing that selection on high V(.)(O₂,swim) (in contrast to selection on high BMR) resulted in considerably higher oxygen consumption during cold exposure in a He-O₂ atmosphere than V(.)(O₂,run). Likewise, between-line-type differences in heart mass and blood parameters are inconsistent with the notion of central limitation. Although responses of V(.)(O₂,run) to hypoxia were similar across different selection regimens, the selection lines showed contrasting responses under hyperoxic conditions. V(.)(O₂,run) in the H-BMR line type was highest, suggesting that selection on high BMR led to increased cellular oxidative capacity. Overall, between-line-type differences in the effect of the oxygen partial pressure on V(.)(O₂,run) and in the components of O₂ flux pathways are incompatible with the notion of symmorphosis. Our results suggest that constraints on V(.)(O₂,max) are context dependent and determined by interactions between the central and peripheral organs and tissues involved in O₂ delivery.

Circulation and metabolic rates in a natural hibernator: an integrative physiological model.

Small hibernating mammals show regular oscillations in their heart rate and body temperature throughout the winter. Long periods of torpor are abruptly interrupted by arousals with heart rates that rapidly increase from 5 beats/min to over 400 beats/min and body temperatures that increase by ∼30°C only to drop back into the hypothermic torpid state within hours. Surgically implanted transmitters were used to obtain high-resolution electrocardiogram and body temperature data from hibernating thirteen-lined ground squirrels (Spermophilus tridecemlineatus). These data were used to construct a model of the circulatory system to gain greater understanding of these rapid and extreme changes in physiology. Our model provides estimates of metabolic rates during the torpor-arousal cycles in different model compartments that would be difficult to measure directly. In the compartment that models the more metabolically active tissues and organs (heart, brain, liver, and brown adipose tissue) the peak metabolic rate occurs at a core body temperature of 19°C approximately midway through an arousal. The peak metabolic rate of the active tissues is nine times the normothermic rate after the arousal is complete. For the overall metabolic rate in all tissues, the peak-to-resting ratio is five. This value is high for a rodent, which provides evidence for the hypothesis that the arousal from torpor is limited by the capabilities of the cardiovascular system.

Genotype by temperature interactions in the metabolic rate of the Glanville fritillary butterfly

Metabolic rate is a highly plastic trait. Here I examine factors that influence the metabolic rate of the Glanville fritillary butterfly (Melitaea cinxia) in pupae and resting and flying adults. Body mass and temperature had consistent positive effects on metabolic rate in pupae and resting adults but not in flying adults. There was also a consistent nonlinear effect of the time of the day, which was strongest in pupae and weakest in flying adults. Flight metabolic rate was strongly affected by an interaction between the phosphoglucose isomerase (Pgi) genotype and temperature. Over a broad range of measurement temperatures, heterozygous individuals at a single nucleotide polymorphism (SNP) in Pgi had higher peak metabolic rate in flight, but at high temperatures homozygous individuals performed better. The two genotypes did not differ in resting metabolic rate, suggesting that the heterozygotes do not pay an additional energetic cost for their higher flight capacity. Mass-independent resting and flight metabolic rates were at best weakly correlated at the individual level, and therefore, unlike in many vertebrates, resting metabolic rate does not serve as a useful surrogate of the metabolic capacity of this butterfly.

The Relationship of Metabolic Performance and Distribution in Black‐Capped and Carolina Chickadees

In endotherms, metabolic performance is associated with a wide array of ecological traits, including species distribution. Researchers have suggested that the northern boundaries of North American passerines are limited by their ability to sustain the high metabolic rates required for thermoregulation. Black-capped chickadees (Poecile atricapillus; BC) are year-round residents in most of Canada and the northern half of the United States, whereas Carolina chickadees (Poecile carolinensis; CA) are found exclusively in the southeastern United States. These species hybridize along a narrow contact zone that has been moving northward at a rate of about 1.6 km per decade, coincident with warming temperatures in Ohio. The location of the chickadee hybrid zone in Ohio closely matches air temperature isotherms, further suggesting that metabolic rate may correlate with distribution in these species. We tested the hypothesis that distribution patterns of chickadees are linked with their rate of metabolism. For populations of BC and CA chickadees, we measured basal metabolic rates (BMRs) and cold-induced peak metabolic rates from areas that differ in winter temperatures and supplemented this information with data from other studies. Although our findings suggest a general relationship between lower air temperatures and higher metabolic rate among black-capped chickadee populations, this trend was not robust across all locations. There was no significant relationship between lower air temperatures and metabolism in Carolina chickadees. Within Ohio, hybrids had a significantly higher mass-corrected BMR than either parental species. We suggest that the mtDNA-nDNA mismatch of hybrids may produce less efficient mitochondrial protein complexes, which in turn affects the efficiency of ATP production, thereby increasing rate of oxygen consumption to meet ATP demands.

Citrate synthase activity does not account for age-related differences in maximum aerobic performance in House Sparrows (Passer domesticus)

We measured basal (BMR) and peak metabolic rates (PMR) in juvenile and adult House Sparrows. Juvenile birds had significantly higher BMR, but lower PMR than adult birds, despite having statistically indistinguishable body masses. We then evaluated the relation between PMR and masses of central and peripheral organs and found that pectoral muscle mass best correlated with PMR in both groups, accounting for about 35% of the variation in PMR. Because citrate synthase (CS) has such major importance in affecting the first committed step in the tricarboxylic acid cycle, we characterized CS activity levels in extracted muscles to see if this better explained age-related differences in peak aerobic performance. Surprisingly, juvenile sparrows had significantly higher CS activity levels than adults (197.8 vs. 179.0 µM g -1 min -1 , respectively). This higher enzyme activity in juveniles was completely offset by their significantly smaller proportion of flight musculature relative to body mass (17.7 % in adults vs. 15.3% in juveniles). Consequently, ontogenetic changes in relative sizes of organs best accounts for agerelated differences in peak metabolic rate.

Postprandial metabolic increment of southern bluefin tuna Thunnus maccoyii ingesting high or low‐lipid sardines Sardinops sagax

This study examined the postprandial metabolism and swimming speed of southern bluefin tuna Thunnus maccoyii when fed sardines Sardinops sagax of either high-lipid and high-energy content or low-lipid and low-energy content. Five groups of two or three T. maccoyii (mean+/-s.e. mass=19.8+/-0.5 kg, n=14) were fed either low [2.2% lipid, 5.5 MJ kg(-1) gross energy (GE)] or high-lipid (12.9%, 9.2 MJ kg(-1) GE) S. sagax. Before feeding, T. maccoyii swam at 0.74+/-0.03 body lengths s(-1) (n=5) and their routine metabolic rate was 305+/-15 mg kg(-1) h(-1). Swimming speed and metabolic rate of T. maccoyii increased following feeding. Thunnus maccoyii swam 1.3 and 1.8 times faster during digestion of low and high-lipid S. sagax, respectively. Postprandial peak metabolic rate, duration of elevated metabolism and total postprandial metabolic increment were all greater for T. maccoyii that ingested high-lipid S. sagax. When total postprandial increment is represented as a proportion of ingested energy, there was no difference between high and low-lipid meals, equating to between 30 and 35% of ingested energy. It was estimated that increased postprandial swimming costs account for 25 and 46% of the total postprandial metabolic response for low and high-lipid S. sagax meals, respectively. Specific dynamic action (SDA) accounts for c. 20% of ingested energy regardless of S. sagax lipid level. These results confirm that the postprandial metabolic increment of T. maccoyii is greater than most other fish species. Much of the high cost of postprandial metabolic increment can be attributed to increased postprandial swimming costs. For T. maccoyii, it appears that activity and SDA are not independent, which complicates bioenergetic evaluation. High postprandial metabolic costs accentuate the great energetic requirements of T. maccoyii.

Effects of body lipid content on the resting metabolic rate and postprandial metabolic response in the southern catfish Silurus meridionalis

We assessed the effects of body lipid content on the resting metabolic rate and specific dynamic action (SDA) of the southern catfish Silurus meridionalis. Obese and lean fish were obtained by feeding the fish with two different feeds at 27.5 °C for 4 weeks prior to the experiment. The fish were fed with experimental diets with a meal size of 4% by body mass. A continuous-flow respirometer was used to determine the oxygen consumption rate at 2-h intervals until the postprandial oxygen consumption rate had returned to the preprandial level. The body lipid content of the obese fish was significantly greater than that of the lean fish. The metabolic parameters evaluated (resting metabolic rate, peak metabolic rate (R peak), factorial ratio, time to peak, duration, energy expended on SDA (SDA E), or SDA coefficient) were not significantly affected by body fat content in terms of the whole-body or mass-specific values. Increased body fat content did not decrease the resting metabolic rate in the southern catfish, which might be due to the higher levels of highly unsaturated fatty acids in these fish. The results also suggest that the body composition does not appear to affect the SDA response.

The effect of muscle phospholipid fatty acid composition on exercise performance: a direct test in the migratory white-throated sparrow (Zonotrichia albicollis)

Dietary polyunsaturated fatty acids (PUFA) can have various effects on animal physiology through their roles as energy, structural, regulatory, and signaling molecules. Of recent interest has been the incorporation of dietary PUFA into muscle membranes as phospholipids, thereby potentially affecting exercise performance by mechanisms such as altered mitochondrial proton leak and membrane-bound protein activity. We first studied the effects of a high-omega6 PUFA diet vs. a high-omega3 PUFA diet on peak metabolic rate (PMR) in white-throated sparrows, and additionally measured mRNA expression of fatty acid transporters and the activity of major oxidative enzymes. Our experiment, thus, allowed a test of the "natural doping" hypothesis. With a simple diet manipulation, the two groups of sparrows diverged significantly in both muscle phospholipid composition and adipose triacylglycerol composition. The high-omega6 sparrows achieved higher PMR without a change in enzyme activity or transporter expression. We then fed sparrows the 2 diets, followed by a food restriction (H omega3RI and H omega6RI treatments). When their adipose stores were exhausted, we fed both groups a common diet of intermediate fatty acid composition. This protocol resulted in the H omega6RI and H omega3RI groups diverging significantly in muscle phospholipid composition, but they had substantially similar adipose stores. PMR did not differ between the H omega6RI and H omega3RI groups. We conclude that muscle phospholipids do not play a major role in affecting exercise performance. The fatty acid composition of stored triacylglycerol may instead affect exercise via the preferential use of particular fatty acids by muscles.

The effect of temperature and body weight on the routine metabolic rate and postprandial metabolic response in mulloway, Argyrosomus japonicus

Specific dynamic action (SDA) is the energy expended on the physiological processes associated with meal digestion and is strongly influenced by the characteristics of the meal and the body weight (BW) and temperature of the organism. This study assessed the effects of temperature and body weight on the routine metabolic rate (RMR) and postprandial metabolic response in mulloway, Argyrosomus japonicus. RMR and SDA were established at 3 temperatures (14, 20 and 26 °C). 5 size classes of mulloway ranging from 60 g to 1.14 kg were used to establish RMR with 3 of the 5 size classes (60, 120 and 240 g) used to establish SDA. The effect of body size on the mass-specific RMR (mg O 2 kg − 1 h − 1 ) varied significantly depending on the temperature; there was a greater relative increase in the mass-specific RMR for smaller mulloway with increasing temperature. No statistical differences were found between the mass exponent ( b) values at each temperature when tested against H 0: b = 0.8. The gross RMR of mulloway (mg O 2 fish − 1 h − 1 ) can be described as function of temperature ( T; 14–26 °C) as: (0.0195 T–0.0454)BW(g) 0.8 and the mass-specific RMR (mg O 2 kg − 1 h − 1 ) can be described as: (21.042 T–74.867)BW(g) − 0.2 . Both SDA duration and time to peak SDA were influenced by temperature and body weight; SDA duration occurred within 41–89 h and peak time occurred within 17–38 h of feeding. The effect of body size on peak metabolic rate varied significantly depending on temperature, generally increasing with temperature and decreasing with increasing body size. Peak gross oxygen consumption ( MO 2: mg O 2 fish − 1 h − 1 ) scaled allometrically with BW. Temperature, but not body size, significantly affected SDA scope, although the difference was numerically small. There was a trend for MO 2 above RMR over the SDA period to increase with temperature; however, this was not statistically significant. The average proportion of energy expended over the SDA period (SDA coefficient) ranged from approximately 7–13% of the total DE intake while the proportion of total energy expended on SDA above RMR ranged from approximately 16–27%.

The effect of acute fenitrothion exposure on a variety of physiological indices, including avian aerobic metabolism during exercise and cold exposure

The effect of fenitrothion exposure on birds was examined by measuring aerobic metabolism, blood hemoglobin content, plasma cholinesterases, and body weight for up to 21 d postdose. Peak metabolic rate was measured in a flight chamber in three-dose groups of house sparrows (Passer domesticus; 100 mg/kg = high, 60 mg/kg = medium, 30 mg/kg = low) and one-dose groups of zebra finches (Taeniopygia guttata; 3 mg/kg) and king quails (Coturnix chinensis; 26 mg/kg). Aerobic metabolism was measured during 1 h of exposure to subfreezing thermal conditions in low-dose house sparrows and king quails (26 mg/kg). Fenitrothion had no effect on metabolic rate during cold exposure or on blood hemoglobin at any time. By contrast, aerobic performance during exercise in sparrows was reduced by 58% (high), 18% (medium), and 20% (low), respectively, 2 d postdose. House sparrows (high) had the longest recovery period for peak metabolic rate (21 d) and plasma cholinesterase activity (14 d). House sparrows (high) and treated king quails had significantly lower myoglobin at 48 h postdose, whereas myoglobin was invariant in zebra finches and house sparrows (medium and low). Cholinesterase was maximally inhibited at 6 h postdose, and had recovered within 24 h, in house sparrows (low), king quails, and zebra finches. Exercise peak metabolic rate in zebra finches and king quails was reduced by 23% at 2 d and 3 d, respectively, despite these birds being asymptomatic in both behavior and plasma cholinesterase activities.

Energetics of tropical hibernation

In this field study, the energetic properties of tropical hibernation were investigated by measuring oxygen consumption and body temperature of the Malagasy primate Cheirogaleus medius in their natural hibernacula. These lemurs use tree holes with extremely varying insulation capacities as hibernacula. In poorly insulated tree holes, tree hole temperature and body temperature fluctuated strongly each day (between 12.8 and 34.4 degrees C). The metabolic rate under these conditions also showed large daily fluctuations between about 29.0 ml O(2)/h and 97.9 ml O(2)/h in parallel with changes in body temperature. In well insulated tree holes in very large trees on the other hand, tree hole temperature and body temperature remained relatively constant at about 25 degrees C. Lemurs hibernating in these tree holes showed a more constant metabolic rate at an intermediate level, but hibernation was interrupted by repeated arousals with peak metabolic rates up to 350 ml O(2)/h. The occurrence of these spontaneous arousals proved that the ability for thermoregulation persists during hibernation. Arousals were energetically costly, but much less so than in temperate and arctic hibernators. Despite the decisive influence of tree hole properties on the pattern of body temperature and metabolic rate during hibernation, the choice of the hibernaculum does not seem to be of energetic importance. The overall energetic savings by tropical hibernation amounted to about 70% as compared to the active season (31.5 vs. 114.3 kJ/d). Therefore, tropical hibernation in C. medius is an effective, well-regulated adaptive response to survive unfavourable seasons.

Effects of temperature on the specific dynamic action of the southern catfish, Silurus meridionalis

Specific dynamic action (SDA), the energy expended on all physiological processes that is associated with meal digestion and assimilation, is strongly affected by temperature. We assessed the effects of temperature on the postprandial metabolic response and calculated SDA of the southern catfish, Silurus meridionalis. The fish was fed with experimental diets at a meal size of 4% body mass, and by using an 8-chamber, continuous-flow respirometer the oxygen consumption rate was determined at a 2 h interval until the postprandial oxygen consumption rate returning to the preprandial level, at four different temperatures. The energy expended on SDA (SDA(E)) were 2.71, 3.07, 3.16, and 3.62 kJ, the SDA(coefficients) (energy expended on SDA quantified as a percentage of the digestible energy content of the meal) were 7.70, 9.44, 10.36, and 11.12%, and the peak metabolic rates (R(peak)) of SDA were 3.48, 4.31, 5.96, and 7.30 mg O2 h(-1), at 17.5, 22.5, 27.5, and 32.5 degrees C respectively. The relationships between those parameters and temperature were: SDA(E)=1.74+0.0559T (n=26, r(2)=0.676), SDA(coefficient)=4.10+0.223T (n=26, r(2)=0.726), and R(peak)=-1.34+0.264T (n=26, r(2)=0.896). The SDA durations showed a slow-fast-slow tendency of decrease with increasing temperature, and were 88.00, 85.71, 67.71, and 66.50 h at 17.5, 22.5, 27.5 and 32.5 degrees C respectively. Two separate peaks appeared during the SDA response at 17.5 degrees C, and it might be due to a rapid startup of the process with a lag of the process, which suggested that the peaks of mechanical component and biochemical component of SDA might be separated when temperature was low enough.

Fenitrothion, an organophosphate, affects running endurance but not aerobic capacity in fat-tailed dunnarts ( Sminthopsis crassicaudata)

We measured aerobic metabolism during cold exposure and exercise performance (run duration and oxygen consumption while running at 1 m s −1) in the fat-tailed dunnart Sminthopsis crassicaudata, a dasyurid marsupial, before and after ingestion of 30 mg kg −1 of fenitrothion, an organophosphate (OP) pesticide. Running endurance of OP-exposed animals was less than half that of control animals over the first 3 days after dosing and 55% of control animal endurance on day 5 post-dose. Despite these declines, peak metabolic rate at this running speed (9.3 times basal metabolic rate; BMR) was unaffected by OP exposure. Peak metabolic rate (PMR) and cumulative oxygen consumption during a 1-h exposure to conditions equivalent to −20 °C did not differ between OP-treated and control dunnarts, with PMR averaging 11 times BMR. We conclude that fenitrothion-induced exercise fatigue is not due to limitations in oxygen or substrate delivery to muscle or in their uptake per se, but more likely relates to decreased ability to sustain high-frequency neuromuscular function. The persistence of locomotor impairment following OP exposure in otherwise asymptomatic animals emphasizes the importance of using performance-based measures when characterising sublethal effects of pesticide exposure in an ecological context.

Cold- and exercise-induced peak metabolic rates in tropical birds.

Compared with temperate birds, tropical birds have low reproductive rates, slow development as nestlings, and long lifespans. These "slow" life history traits are thought to be associated with reduced energy expenditure, or a slow "pace of life." To test predictions from this hypothesis, we measured exercise-induced peak metabolic rates (PMR(E)) in 45 species of tropical lowland forest birds and compared these data with PMR(E) for three temperate species. We also compared cold-induced PMR (PMR(C)) with PMR(E) in the same individuals of 19 tropical species. Tropical birds had a 39% lower PMR(E) than did the temperate species. In tropical birds, PMR(C) and PMR(E) scaled similarly with body mass (M(b)), but PMR(E) was 47% higher than PMR(C). PMR(E) averaged 6.44 x basal metabolic rate (BMR) and PMR(C) averaged 4.52 x BMR. The slope of the equation relating PMR(E) to M(b) exceeded the slope for the equation for BMR vs. M(b), whereas slopes for the equations of PMR(C) and BMR vs. M(b) did not differ. M(b)-adjusted residuals of PMR(E) were positively correlated with residual BMR, whereas residual PMR(C) and residual BMR were not correlated. PMR(E) and PMR(C) were not correlated after we corrected for M(b). Temperate birds maintained their body temperature at an 8.6 degrees C lower average air temperature than did tropical species. The lower PMR(E) values in tropical species suggest that their suite of life history traits on the slow end of the life history continuum are associated with reduced metabolic rates.