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Basal metabolic rate is often likened to the idling speed of mammals. Measured during rest in non-reproductive, mature, fasted animals, it has become a gold standard for use in interspecific studies. Recent evolutionary analyses have shown a decoupling between basal metabolism and body temperature in endotherms. However, these studies were based on body temperature datasets that ignored the inherent variability in this trait and consisted of non-comparable temperatures (for example comparing that of a resting animal during the active phase of its activity cycle to an active animal in its rest phase). Using a new dataset of standardised resting body temperatures in mammals, we explore the relationship between basal metabolism and body temperature as well as the evolutionary history of these two traits. 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 energetics.

SEB Annual Conference 2022

The evolution of resting body temperatures in mammals

Discover the art of creating extraordinary science at SEB’s Annual Conference at Le Corum Conference Center     in Montpellier, France from 5 to  8 July 2022.

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2022 will find over 1500 experimental biologists from across the globe converging on the third-largest French city near the Mediterranean coast and the fastest-growing city in the country over the past 25 years.
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2022 will find over 1500 experimental biologists from across the globe converging on the third-largest French city near the Mediterranean coast and the fastest-growing city in the country over the past 25 years - Montpellier, France

technical paper

Mitochondria are not merely the powerhouses of cells, responsible for ATP synthesis, but are also the engines of biosynthesis, as the Krebs cycle provides the precursors for amino-acid, sugar, fatty-acid and nucleotide synthesis, shaping all aspects of growth and development. Flux through the Krebs cycle is contingent on NADH oxida(on, which in turn depends on respiratory function, particularly at complex I. This depends on two genomes, mitochondrial and nuclear, which have radically different modes and tempi of evolution. Even trivial incompatibilities between these genomes can have large effects on fitness, especially when subjected to metabolic or redox stress. Complex I is especially prone to dysfunction as it is the largest complex, with the most mitochondrially encoded subunits, and is a major source of physiological reactive oxygen species. Our work using the Drosophila mitonuclear model shows that beOer complex I function in third-instar larvae and young adults corresponds to faster development, greater fer(lity, higher survival, longer lifespan and better physical performance. By fixing an isogenic nuclear background and varying mtDNA in D. melanogaster, we can dissect the mitochondrial contribution to each phenotype using tissue-specific fluorespirometry and metabolomics. Yet the molecular basis for these unexpected differences remains unclear. Our results suggest that complex I function is not shaped directly by mitonuclear coevolution, nor by adaptive selection on mitochondrial DNA alone, but may depend on epistatic effects linked with macromolecular organization, perhaps cristae membrane structure and supercomplex assembly, which we will explore in future work.

The Determinator: How does respiratory complex I control metabolic flux, fertility and longevity?

The challenge of coping with climate changes will depend in part on individual's ability to sustain energy production. Mitochondria play an important role by being plastic and supporting 90% of ATP production, which is essential for organism performance. However, although mitochondria are well studied, less is known about the evolution of mitochondria over time in the same individual. The aims of this study were to evaluate mitochondrial metabolic traits from the same individuals over time and especially their response after the exposition of animals to a reduction in oxygen level. Using a non-lethal biopsy, mitochondria from the red muscle of European sea bass (Dicentrarchus labrax, n=40) were measured monthly in three trials. Following the second trial, oxygen level in fish tank dropped from 100% to 50% saturation. Metabolic traits of red muscle mitochondria were assessed as respiration rates (OXPHOS respiration, LEAK respiration), ATP production and cytochrome c oxidase (COX) activity, an estimate of mitochondrial density. For each mitochondrial metabolic traits, no difference was found at one month interval when fish were maintained at 100% oxygen saturation. COX activity responded to the oxygen drop: COX activity increased after a 4 week exposition of the fish to 50% oxygen saturation. Our results provide evidence of acclimatisation of individuals to changing environmental conditions. This study offers an insight into the potential of using longitudinal studies to understand mitochondrial plasticity by measuring the same individuals over time.

Longitudinal study and mitochondrial metabolic traits: a way to measure mitochondria plasticity over time

In previous studies on Arctica islandica, a bivalve that can reach more than 500 years of age, we observed that mitochondria from this mollusk have particularly robust membranes that resist to peroxidation. We also observed a particularly lower rate of mitochondrial hydrogen peroxide efflux, when compared to shorter-lived bivalves. To delineate the structural and functional causes likely related to this lower efflux, we investigated the organization of super-complexes and documented the strength of control of respiration by the different complexes (complexes I, II, III and IV). Compared to short-lived species, A islandica have stronger electron flux control at the level of complex IV, relative to complexes I and II, as well as a higher level of organization of super-compexes.

The sophisticated Electron Transport System of the longest-lived animal

The Octopus vulgaris arm is a ‘one of a kind’ soft limb, with a high maneuverability, vast range of motions and extraordinary sensing abilities. It is composed of an array of muscles and intramuscular connective tissue, allowing force and shape production, controlled by a rather autonomous peripheral nervous system. Two rows of suckers are disposed in the ventral arm surface and works as both mechanical and chemical ‘devices’ in various tasks. Suckers allow the arm to probe the environment but also to reconfigure in vertebrate-like structures through the integration of sensory feedback with central motor commands.
These features are particularly relevant in the bio-robotics field as an increasing number of researchers are currently aiming at designing and constructing bio-inspired soft-robotic manipulators capable of semi-autonomous functions.
We will show how the octopus arm motor performances emerge from the tight interaction of at least three players: the muscle control and biomechanics, the arm structural design and the sucker sensory feedback. Each of this player may offer solutions for an efficient and energetic advantageous artificial control strategy. The physiology and biomechanics of the Octopus vulgaris arm can be a valid source of bioinspiration and biomimetics as it allows for the implementation not only of the structural design but also of material and control properties underlying soft limb performance.

How to grasp and sense with no joints: the octopus as a model for soft-robotics

Ammonia is produced via protein catabolism of ingested food in the gastrointestinal tract (GIT), resulting in total ammonia (Tamm) levels in the chyme up to 50-fold higher than blood ammonia levels. Chyme ammonia also far exceeds the water ammonia levels considered toxic for fish. Thus, the high luminal ammonia concentration must impose a challenge if ammonia is transported across the gastrointestinal epithelium into the bloostream, but relatively little is known about this possibility. To address this, we monitored Tamm of blood supplying and leaving the GIT in freshwater rainbow trout (Oncorhynchus mykiss) over 48 h following feeding. We also investigated possible NH3 transport across GIT epithelia by measuring Rhesus glycoprotein mRNA expression levels (as possible NH3 transporters) and ammonia flux rates in in vitro in response to experimentally varied PNH3 gradients. We found that the subintestinal venous blood Tamm levels near the posterior intestine remain undisturbed after a meal and differ markedly from those in the lumen. Although Rhesus glycoproteins appear to be present selectively in different regions, PNH3 may not be a significant driver for ammonia transport into the bloodstream. However, high rates of ammonia absorption occurred in the stomach in vitro and Tamm in the hepatic portal vein, which collects blood from the whole GIT, increased following feeding. Overall, we found that in addition to the intestine, the stomach is involved in ammonia assimilation and that each GIT region may have distinct roles in ammonia metabolism and transport (NSERC Discovery).

In And Out Of The Gastrointestinal Tract of Trout: Selective Transport Of High Luminal Ammonia Into The Venous Drainage

This study asked whether interindividual variation in maximum and standard aerobic metabolic rates of the Gulf killifish, Fundulus grandis, held under common garden conditions correlate with gill morphology and cardiac mitochondrial bioenergetics, traits reflecting critical steps in the oxygen-supply cascade from the environment to the tissues. Maximum metabolic rate (MMR) was positively related to body mass, total gill filament length, and myocardial oxygen consumption during maximum oxidative phosphorylation (multiple R2 = 0.82). Standard metabolic rate (SMR) was positively related to body mass, total gill filament length, and myocardial oxygen consumption during maximum electron transport system activity (multiple R2 = 0.69). After controlling for body mass, individuals with longer gill filaments, summed over all filaments, or greater cardiac respiratory capacity had higher whole-animal metabolic rates. The overall model fit and the explanatory power of individual predictor variables were better for MMR than for SMR, suggesting that gill morphology and myocardial bioenergetics are more important in determining maximum rather than resting metabolism. Heart ventricle mass was not related to mass-independent variation in MMR or SMR, indicating that the quality of the heart (i.e., the capacity for mitochondrial metabolism) was more influential than heart size. Finally, myocardial oxygen consumption required to offset the dissipation of the transmembrane proton gradient in the absence of ATP synthesis was not correlated with either MMR or SMR. The results support the idea that interindividual variation in aerobic metabolism, particularly maximum metabolic rate, is explained, in part, by variation in specific steps in the oxygen-supply cascade.

Interindividual variation in maximum aerobic metabolism varies with gill morphology and myocardial bioenergetics

Among-species comparisons have been a mainstay of comparative, ecological, and evolutionary physiology for many decades. The ongoing collation of trait databases coupled with the widespread adoption of phylogenetic comparative methods has created fertile ground for the demonstration of new patterns, and for the re-examination of well-known patterns. However, the increases in data richness and analytical sophistication have not changed the fundamental limitations of correlational approaches. Here I explore these limitations, focussing on the important differences between congruence, correlation, and causation. I then use multivariate phylogenetic mixed models to demonstrate the limited extent to which among-species patterns can be used to infer the within-species processes that generate them.

What meta-analyses and comparative analyses can, and can't, tell us about biological phenomena

Life history theory typically considers the costs of reproduction in terms of energy content to released offspring (often measured as clutch mass at birth), yet direct reproductive costs are actually accrued by the mother during the offspring production and provisioning phase. While female body size is well recognised as a strong predictor of clutch mass, remarkably, there is no consensus on how the total energetic costs of reproduction should relate to adult size, nor whether the type of offspring produced (egg or live-young) can modify these relationships. We compiled estimates of body size and metabolism (i.e., reproductive costs) across gravid and non-gravid females, as well as species-specific estimates of clutch mass, for ~80 ectotherm species spanning ~10 orders of magnitude in body size, from rotifer to python. Reproductive cost scaling was hypoallometric – larger species had less reproductive costs relative to their body size than smaller species. Furthermore, species that produced heavier clutches also had relatively less costs than species that produced lighter clutches. Surprisingly, these patterns were congruent across reproductive modes implying that metabolic demands of viviparous and oviparous offspring may change dramatically once released from the mother.

Reproductive metabolism scaling across ectotherms

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. We have estimated FMR in over 1000 individual fish, across nearly 100 species. Here we draw on our data compilation to explore predictions for two contentious topics in fish ecophysiology:
(1) Metabolic cold adaptation: We show that polar species have higher FMR than temperate but not tropical species at equivalent body sizes and temperatures, and that, within species, populations at the cold edge of the range express higher FMR but lower growth rates than populations at the warm edge of the species range.
(2) Gill oxygen limitation theory: FMR data covering 4 orders of magnitude body size within single species show no strong evidence for reduction in metabolic capacity at large body sizes as predicted by the GOLT.











Field metabolic rates of wild marine fishes: reviewing old questions with new data.

The physiological benefits of exercise can be profound. Across the animal kingdom, exercise has been shown to improve locomotor performance and growth, increase brain plasticity and cognition, enhance resistance to disease, and even heighten tolerance to various environmental stressors. In aquaculture particularly, forced exercise is seen as tool to promote the productivity and welfare of fish. Yet, physiological changes following periods of exercise training can be small and context dependent. Experimental evidence has failed to reach a consensus on the mechanisms driving the physiological benefits of forced exercise in fish, particularly because the use of varying of species, life stages, and training regimes can muddy interpretations and because of the limited statistical power that any one individual study holds. We shed light on the physiological benefits of forced exercise in fish by compiling data from published literature. We focus on common physiological traits, such as metabolism, swimming performance, and growth, that are beneficial to wild and captive fishes. Using meta-analytical tools, we capture between-study heterogeneity that allows for stronger predictions on the physiological benefits of exercise training for aquaculture. Exercise training remains an exciting management prospect for the culture of ‘fitter’ fish that is strengthened by this new holistic understanding.

The evolution of resting body temperatures in mammals

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The Determinator: How does respiratory complex I control metabolic flux, fertility and longevity?

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