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Understanding how and why metabolic rate scales allometrically with body mass in fish is of particular interest given the observed decline in the maximum body size of aquatic ectotherms under climate warming. Size-dependent oxygen limitation has been proposed to drive these body size declines, particularly at warmer temperatures. If this is true, then in warm settings and with an excess oxygen supply (hyperoxia) we would expect scaling exponents to steepen as larger fish would require comparatively more oxygen to fulfill their proposed debt. To test this, we measured the aerobic metabolic performance of Galaxias maculatus upon acute exposure to hyperoxia (150% air saturation) and following five months of acclimation to hyperoxia, at 15°C and 20°C. Despite hyperoxia acclimation, scaling exponents for standard and maximum metabolic rates did not differ from normoxia-acclimated conspecifics, implying that oxygen limitation does not regulate allometric scaling. Interestingly, the scaling exponents for standard metabolic rate in normoxia- and hyperoxia-acclimated fish at 15°C were particularly high (1.05 and 1.08, respectively). Further investigation revealed that these high exponents were likely driven by differences in the reproductive state of smaller (immature) vs. larger (mature) fish, illustrating the increased energetic demands of the reproductive process and the need to account for reproductive stage when assessing metabolic scaling in fish.

Why and how does metabolic rate scale with body size?

Effects of hyperoxia and reproductive state on the metabolic scaling of a temperate fish Galaxias maculatus

technical paper

Metabolic rate is closely related to body size (body mass), but when mass increases, metabolic rate usually increases less. This allometric scaling relationship between metabolic rate and body mass is fundamental to biology and widely applied, but also one of the most debated concepts in integrative and comparative physiology, ecology, and evolutionary biology.

Several theories and hypotheses have been put forward to explain why and how metabolic rate scales with body mass, and why there appears to be variation in how steep this scaling relationship is among different taxa and physiological or ecological conditions (e.g. 'surface area', 'cell size', 'fractal distribution network', ‘dynamic energy budget’, and ‘metabolic-level boundaries’ theories or hypotheses). However, the lack of consensus on these important topics is a major barrier to our understanding of the pace at which organisms of different sizes and species expend energy and need to consume resources such as food and oxygen from the environment. It also limits our ability to predict how metabolic rate and body size (co)evolves, and how climate change affects metabolic demands and body sizes, as metabolic scaling exponents are used to model how these key organismal traits are affected by changing temperature and oxygen levels. The importance of metabolic scaling is clear, yet it remains one of the most controversial concepts in biology.

The overall aim of this satellite meeting is to bring together researchers working with metabolic scaling, to: (1) bring the field up to speed on state-of-the-art knowledge and research on metabolic scaling; (2) help understand how the vastly different views and opinions on metabolic scaling can persist despite more than a century of research (e.g. scaling in proportion to surface area or not, universal ¾-power scaling or not, governed by ecological vs. physical or geometric factors); and (3) identify and help streamline specific areas of research on metabolic scaling that will be particularly important for our further understanding of why and how metabolic rate scales with body size.

The meeting is being organised as part of the Animal Section of the SEB and will therefore have a focus on metabolic scaling of animals, but work on other organisms is welcome where this can provide insight into metabolic scaling in general.

The overall aim of this satellite meeting is to bring together researchers working with metabolic scaling, to: bring the field up to speed on state-of-the-art knowledge and research on metabolic scaling; help understand how the vastly different views and opinions on metabolic scaling can persist despite more than a century of research (e.g. scaling in proportion to surface area or not, universal ¾-power scaling or not, governed by ecological vs. physical or geometric factors); and identify and help streamline specific areas of research on metabolic scaling that will be particularly important for our further understanding of why and how metabolic rate scales with body size.

"Various well-known growth models, including the Bertalanffy model, the models developed within the framework of the Metabolic Theory of Ecology (MTE), and the standard DEB model, will be introduced and compared, with special emphasis on the interspecific scaling of the model parameters. 
At first sight, it seems that MTE and DEB would come up with assumptions that give rise to totally contradictory predictions concerning the relationship between metabolic rate and ultimate size. Whereas DEB points to the area-specific assimilation rate as the characteristic that differs among species of different ultimate size, MTE thinks that differences in ultimate size are basically due to differences in the volume-specific maintenance. Further explorations, however, yield the surprising result that under certain conditions both theories predict a similar relationship between metabolic rate and ultimate mass.

"Various well-known growth models, including the Bertalanffy model, the models developed within the framework of the Metabolic Theory of Ecology (MTE), and the standard DEB model, will be introduced and compared, with special emphasis on the interspecific scaling of the model parameters. 
At first sight, it seems that MTE and DEB would come up with assumptions that give rise to totally contradictory predictions concerning the relationship between metabolic rate and ultimate size. Whereas DEB points to the area-specific assimilation rate as the characteristic that differs among species of different ultimate size, MTE thinks that differences in ultimate size are basically due to differences in the volume-specific maintenance. Further explorations, however, yield the surprising result that under certain conditions both theories predict a similar relationship between metabolic rate and ultimate mass.

"

The scaling of metabolic rate: a comparison between predictions based on DEB theory and other approaches

"Time integrated Field Metabolic Rate (FMR) is arguably the most ecologically relevant measure of the energetics of wild animals, but the relative complexity of determining FMR routinely means that we have relatively few large datasets describing variation in energy use in free-living wild animals, particularly for aquatic ectotherms where double labelled water methods cannot be used.
Emerging proxies based on stable isotope systematics associated with respiration allow retrospective estimation of time integrated field metabolic rate, experienced temperature and growth in any free-living marine teleost fish. Here we present FMR data obtained for 780 individuals from 10 species of fishes sampled within a one month period in the Barents Sea. The dataset includes individual Atlantic cod with a body size range over 4 orders of magnitude, dramatically increasing the body size range over which scaling of field metabolic rates have been assessed for any single species. 
The allometric exponent for body size scaling of FMR is 0.9 across all taxa. Within-species scaling exponents range from 0.7 – 1.2. Mass scaling exponents may be non-linearly related to body size, but these field data are confounded by size-dependent temperature preferences. In all but one species, FMR is relatively insensitive to temperature, implying that FMR is not strongly constrained by external temperatures for fish populations operating within environmental conditions to which the population is adapted/acclimated. 



"

Body size and temperature scaling of field metabolic rate in wild marine fishes

Adult body size in insects can be influenced by environmental conditions during larval growth. The effect of such intraspecific variation in body mass on flight performance is poorly understood. In Batocera rufomaculata, a large tree boring beetle, often engaging in dispersal flight, adults emerging from larvae that developed in a dying host tree under nutrient-deprived diet conditions, are smaller than individuals who developed in nutrient rich environments. This leads to a large variation in adult body mass (1 to 7 g) with an expected effect on flight Metabolic Rate (MR). Measuring MR poses a formidable challenge in free-flying insects who can’t breathe into masks or be trained to fly in controlled settings. Consequently, flight MR has been predominantly measured on hovering or tethered insects flying in closed systems. In this study we applied the ‘bolus injection of isotopic 13C Na-bicarbonate’ method to explore the relationship between intraspecific body size and power expended during tethered steady forward flight and free flying B. rufomaculata. While the method was previously used solely on vertebrates, we report a 90% accuracy of the method in our beetle when compared to direct respirometry. We found that free FMR and tethered FMR increased in relation to body mass as 99·𝑀0.54 and 36∙M^0.56 respectfully showing that while the free FMR is ~3 times higher than FMR recorded in tethered flight, the mass exponent remains unchanged. This mass exponent of ~0.55 is lower than the expected exponent for interspecific variation in body mass of M^0.74 to M^0.98.

The scaling of free flight energetics of the mango stem borer beetle, Batocera rufomaculata

Metabolic scaling theory: New Insights in Scaling Form, Function & Diversity

Seed plants vary tremendously in size and morphology. However, variation and covariation between plant traits may at least in part be governed by universal biophysical laws and biological constants. Metabolic Scaling Theory (MST) posits that whole-organismal metabolism and growth rate are under stabilizing selection that minimizes the scaling of hydrodynamic resistance and maximizes the scaling of resource uptake. This constrains variation in physiological traits and in the rate of biomass accumulation, so that they can be expressed as mathematical functions of plant size with near constant allometric scaling exponents across species. However, observed variation in allometric exponents questions the evolutionary drivers and the universality of scaling exponents. We have measured growth scaling and fitness traits of 451 Arabidopsis thaliana accessions with sequenced genomes. Variation among accessions around the scaling exponent predicted by MST correlated with climate, seed production and stress resistance. Genomic analyses indicate that growth allometry is affected by many genes associated with local climate and abiotic stress response. The gene with the strongest effect, PUB4, has molecular signatures of balancing selection, suggesting that intraspecific variation in growth scaling is maintained by opposing selection on two major components of fitness, seed production and abiotic stress resistance. Our findings support one of the core MST predictions and suggest that allometry contributes to local adaptation to contrasting environments. Our results help reconcile past debates on the origin of allometric scaling in biology, and begin to link adaptive variation in allometric scaling to specific genes.

Adaptive diversification of metabolic scaling in the plant Arabidopsis thaliana

How much energy an organism consumes can be used to make inferences across scales and could be a valuable tool to predict energy fluxes of populations and communities. While scaling patterns can be seen across species and systems, the mechanisms that drive variation in metabolic scaling and influence functioning at larger scales remain unclear. I will present work that explores how competition alters the metabolism of individual organisms, and how these effects change as species coevolve. I will then discuss how competition modifies energy fluxes at larger scales and the implications for predicting community functioning.

How does competition alter metabolism and what are the consequences for community functioning?

Meta-analysis of metabolic rates in three anadromous fishes reveals variation in scaling with mass, temperature and swim speed

Large-scale comparisons of mammalian basal metabolic rates often transform metabolism (using Q10 effects) to approximate a standard body temperature of 37°C. However, existing mammalian body temperature datasets do not take into account the high degree of within-species and within-individual variability in body temperatures. There is increasing recognition that rather than being fully homeothermic, most endotherms display some degree of flexibility in body temperature. Yet existing body temperature datasets use temperatures from different non-analog states (such as resting versus active or scotophase versus acrophase) or mean body temperature, which has increasingly been shown to have little biological relevance. Using a new dataset of standardised resting body temperatures in mammals, we show that the relationship between basal metabolism and body temperature is not as straightforward as has been presumed. We discuss the implications of correcting, or not, for body temperature in studies on metabolic scaling and discuss the role of thermoregulatory phenotypes in shaping mammalian basal metabolic rates.

The role of body temperature in the scaling of metabolic rate in mammals

Metabolism; Scaling; Mammals; Hibernation

Seasonal Changes in the Metabolic Scaling of Hibernating Thirteen-Lined Ground Squirrels (Ictidomys tridecemlineatus)

Metabolism is a fundamental property of organisms as it underpins all life-sustaining processes. The relationship between metabolic rate (R) and body size (M) has become a central topic in ecology, yet the mechanism(s) that underlie this mass dependence (termed scaling) and its variation, remain enigmatic. Metabolic scaling can be affected by temperature and activity, as warming and muscular work not only accelerate metabolic level (L), but often lead to changes in the scaling exponent (or slope b, in log R vs. log M). This change has been explained through a shift of resource supply from volume- to surface-area related processes. However, whether the same supply constraints apply among and within species is unclear, as the quantitative relationship between b and L remains largely untested within species. Only by understanding these patterns we will be able to address proximate and ultimate mechanisms behind metabolic scaling variation. Through a meta-analysis, we quantitatively compare the intra-specific change in b when L is increased by either temperature or locomotor activity in a wide range of ectothermic vertebrates, including fish, amphibians, and reptiles. We show that b decreases with temperature-increased L in water-breathing (fish) but not in air-breathing species (amphibians and reptiles), whereas b increases with activity-increased L in air-breathing but not in water-breathing species. Our findings in the most comprehensive intra-specific analysis of ectothermic vertebrates to date indicate that b varies differently with temperature and activity in water- and air-breathers, and suggest that oxygen availability in water may constrain metabolic scaling of fish.

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