Balancing the competing requirements of saltatorial and fossorial specialisation: burrowing costs in the spinifex hopping mouse,Notomys alexis

How and Why Does Metabolism Scale with Body Mass?

Most explanations for the relationship between body size and metabolism invoke physical constraints; such explanations are evolutionarily inert, limiting their predictive capacity. Contemporary approaches to metabolic rate and life history lack the pluralism of foundational work. Here, we call for reforging of the lost links between optimization approaches and physiology.

Variations in cost of transport and their ecological consequences: a review

Movement is essential in the ecology of most animals, and it typically consumes a large proportion of individual energy budgets. Environmental conditions modulate the energetic cost of movement (cost of transport, COT), and there are pronounced differences in COT between individuals within species and across species. Differences in morphology affect COT, but the physiological mechanisms underlying variation in COT remain unresolved. Candidates include mitochondrial efficiency and the efficiency of muscle contraction-relaxation dynamics. Animals can offset increased COT behaviourally by adjusting movement rate and habitat selection. Here, we review the theory underlying COT and the impact of environmental changes on COT. Increasing temperatures, in particular, increase COT and its variability between individuals. Thermal acclimation and exercise can affect COT, but this is not consistent across taxa. Anthropogenic pollutants can increase COT, although few chemical pollutants have been investigated. Ecologically, COT may modify the allocation of energy to different fitness-related functions, and thereby influence fitness of individuals, and the dynamics of animal groups and communities. Future research should consider the effects of multiple stressors on COT, including a broader range of pollutants, the underlying mechanisms of COT and experimental quantifications of potential COT-induced allocation trade-offs.

Three-dimensional geometric morphometric analysis of the humerus: Comparative postweaning ontogeny between fossorial and semiaquatic water voles (Arvicola)

Different types of locomotion in phylogenetically close rodent species can lead to significantly different growth patterns of certain skeletal structures. In the present study, we compared the allometric and phenotypic trajectories of the humerus in semiaquatic (Arvicola sapidus) and fossorial (Arvicola scherman) water vole taxa, using three-dimensional geometric morphometrics, to investigate the relationships between functional and ontogenetic differences. Results revealed shared humerus traits between A. sapidus and A. scherman, specifically an expansion of the epicondylar and deltopectoral crests along postnatal ontogeny. In both species, the humerus of young specimens is more robust than in adults, possibly as a compensatory response for lower bone stiffness. However, significant interspecific differences were detected in all components of allometric and phenotypic trajectories. Noticeably divergent allometric trajectories were observed, probably as a result of different functional pressures exerted on this bone. Important differences in the form of the adult humerus between taxa were also found, particularly in features located in muscle insertion zones. Furthermore, the allometric regression revealed certain shape variation not associated with size in A. scherman, suggesting mechanical stress produced by the persistent digging activity during adulthood. A. scherman is a chisel-tooth digger that shares several traits in the humerus morphology with scratch-digger rodent species. Nevertheless, these shared characteristics are less pronounced in fossorial water voles, which is congruent with the different implications of the forelimb in the digging activity in these two types of diggers.

Comparative analyses of basal rate of metabolism in mammals: data selection does matter

Basal rate of metabolism (BMR) is a physiological parameter that should be measured under strictly defined experimental conditions. In comparative analyses among mammals BMR is widely used as an index of the intensity of the metabolic machinery or as a proxy for energy expenditure. Many databases with BMR values for mammals are available, but the criteria used to select metabolic data as BMR estimates have often varied and the potential effect of this variability has rarely been questioned. We provide a new, expanded BMR database reflecting compliance with standard criteria (resting, postabsorptive state; thermal neutrality; adult, non-reproductive status for females) and examine potential effects of differential selectivity on the results of comparative analyses. The database includes 1739 different entries for 817 species of mammals, compiled from the original sources. It provides information permitting assessment of the validity of each estimate and presents the value closest to a proper BMR for each entry. Using different selection criteria, several alternative data sets were extracted and used in comparative analyses of (i) the scaling of BMR to body mass and (ii) the relationship between brain mass and BMR. It was expected that results would be especially dependent on selection criteria with small sample sizes and with relatively weak relationships. Phylogenetically informed regression (phylogenetic generalized least squares, PGLS) was applied to the alternative data sets for several different clades (Mammalia, Eutheria, Metatheria, or individual orders). For Mammalia, a 'subsampling procedure' was also applied, in which random subsamples of different sample sizes were taken from each original data set and successively analysed. In each case, two data sets with identical sample size and species, but comprising BMR data with different degrees of reliability, were compared. Selection criteria had minor effects on scaling equations computed for large clades (Mammalia, Eutheria, Metatheria), although less-reliable estimates of BMR were generally about 12-20% larger than more-reliable ones. Larger effects were found with more-limited clades, such as sciuromorph rodents. For the relationship between BMR and brain mass the results of comparative analyses were found to depend strongly on the data set used, especially with more-limited, order-level clades. In fact, with small sample sizes (e.g. <100) results often appeared erratic. Subsampling revealed that sample size has a non-linear effect on the probability of a zero slope for a given relationship. Depending on the species included, results could differ dramatically, especially with small sample sizes. Overall, our findings indicate a need for due diligence when selecting BMR estimates and caution regarding results (even if seemingly significant) with small sample sizes.

Phylogenetic Analysis Supports the Aerobic-Capacity Model for the Evolution of Endothermy

The evolution of endothermy is a controversial topic in evolutionary biology, although several hypotheses have been proposed to explain it. To a great extent, the debate has centered on the aerobic-capacity model (AC model), an adaptive hypothesis involving maximum and resting rates of metabolism (MMR and RMR, respectively; hereafter "metabolic traits"). The AC model posits that MMR, a proxy of aerobic capacity and sustained activity, is the target of directional selection and that RMR is also influenced as a correlated response. Associated with this reasoning are the assumptions that (1) factorial aerobic scope (FAS; MMR/RMR) and net aerobic scope (NAS; MMR - RMR), two commonly used indexes of aerobic capacity, show different evolutionary optima and (2) the functional link between MMR and RMR is a basic design feature of vertebrates. To test these assumptions, we performed a comparative phylogenetic analysis in 176 vertebrate species, ranging from fish and amphibians to birds and mammals. Using disparity-through-time analysis, we also explored trait diversification and fitted different evolutionary models to study the evolution of metabolic traits. As predicted, we found (1) a positive phylogenetic correlation between RMR and MMR, (2) diversification of metabolic traits exceeding that of random-walk expectations, (3) that a model assuming selection fits the data better than alternative models, and (4) that a single evolutionary optimum best fits FAS data, whereas a model involving two optima (one for ectotherms and another for endotherms) is the best explanatory model for NAS. These results support the AC model and give novel information concerning the mode and tempo of physiological evolution of vertebrates.

Crouching to fit in: the energetic cost of locomotion in tunnels

Animals that are specialized for a particular habitat or mode of locomotion often demonstrate locomotor efficiency in a focal environment when compared to a generalist species. However, measurements of these focal habitats or behaviors are often difficult or impossible to do in the field. In this study, the energetics and kinematics of simulated tunnel locomotion by two unrelated semi-fossorial mammals, the ferret and degu, were analyzed using open-flow respirometry and digital video. Animals were trained to move inside of normal (unconstrained, overground locomotion) and height-decreased (simulated tunnel, adjusted to tolerance limits for each species) Plexiglas chambers that were mounted flush onto a treadmill. Both absolute and relative tunnel performance differed between the species; ferrets tolerated a tunnel height that forced them to crouch at nearly 25% lower hip height than in an unconstrained condition, while degus would not perform on the treadmill past a ∼9% reduction in hip height. Both ferrets and degus exhibited significantly higher metabolic rates and cost of transport (CoT) values when moving in the tunnel condition relative to overgound locomotion. When comparing CoT values across small (&lt;10kg) mammals, ferrets demonstrated a lower than predicted metabolic cost during both tunnel and terrestrial locomotion, whereas degus were very close to line of best fit. Although tunnel locomotion requires a more striking change in posture for ferrets, ferrets are more efficient locomotors in both conditions than mammals of similar mass.

Morphology and burrowing energetics of semi-fossorial skinks (Liopholis)

Burrowing is an important form of locomotion in reptiles, but no study has examined the energetic cost of burrowing for reptiles. This is significant since burrowing is the most energetically expensive mode of locomotion undertaken by animals, and many burrowing species therefore show specialisations for their subterranean lifestyle. We examined the effect of temperature and substrate characteristics (coarse sand or fine sand) on the net energetic cost of burrowing (NCOB) and burrowing rate in two species of the Egernia group of skinks (Liopholis striata and Liopholis inornata) and compared it with those of other burrowing animals. We further tested for morphological specialisations among burrowing species by comparing the relationship between body shape and retreat preference in Egernia skinks. For L. striata and L. inornata, NCOB is 350 times more expensive than the predicted cost of pedestrian terrestrial locomotion. Temperature had a positive effect on burrowing rate for both species, and a negative effect on NCOB for L. striata but not L. inornata. Both NCOB and burrowing rate were independent of substrate type. Burrows constructed by skinks had smaller cross-sectional area than those constructed by mammals of comparable mass, and NCOB of skinks was lower than that of mammals of similar mass. After accounting for body size, retreat preference was significantly correlated with body shape in Egernia group skinks. Species of Egernia group skinks that use burrows for retreats have narrower bodies and shorter front limbs than other species. We conclude that the morphological specialisations of burrowing skinks allow them to construct relatively narrow burrows, thereby reducing NCOB and the total cost of constructing their burrow retreats.

Metabolic scaling in animals: methods, empirical results, and theoretical explanations

Life on earth spans a size range of around 21 orders of magnitude across species and can span a range of more than 6 orders of magnitude within species of animal. The effect of size on physiology is, therefore, enormous and is typically expressed by how physiological phenomena scale with mass(b). When b ≠ 1 a trait does not vary in direct proportion to mass and is said to scale allometrically. The study of allometric scaling goes back to at least the time of Galileo Galilei, and published scaling relationships are now available for hundreds of traits. Here, the methods of scaling analysis are reviewed, using examples for a range of traits with an emphasis on those related to metabolism in animals. Where necessary, new relationships have been generated from published data using modern phylogenetically informed techniques. During recent decades one of the most controversial scaling relationships has been that between metabolic rate and body mass and a number of explanations have been proposed for the scaling of this trait. Examples of these mechanistic explanations for metabolic scaling are reviewed, and suggestions made for comparing between them. Finally, the conceptual links between metabolic scaling and ecological patterns are examined, emphasizing the distinction between (1) the hypothesis that size- and temperature-dependent variation among species and individuals in metabolic rate influences ecological processes at levels of organization from individuals to the biosphere and (2) mechanistic explanations for metabolic rate that may explain the size- and temperature-dependence of this trait.

Phylogenetic analysis of mammalian maximal oxygen consumption during exercise

We compiled published values of mammalian maximum oxygen consumption during exercise (VO2max) and supplemented these data with new measurements of VO2max for the largest rodent (capybara), 20 species of smaller-bodied rodents, two species of weasels, and one small marsupial. Many of the new data were obtained with running-wheel respirometers instead of the treadmill systems used in most previous measurements of mammalian VO2max. We used both conventional and phylogenetically informed allometric regression models to analyze VO2max of 77 ‘species’ (including subspecies or separate populations within species) in relation to body size, phylogeny, diet, and measurement method. Both body mass and allometrically mass-corrected VO2max showed highly significant phylogenetic signal (i.e., related species tended to resemble each other). The Akaike Information Criterion corrected for sample size was used to compare 27 candidate models predicting VO2max (all of which included body mass). In addition to mass, the two best-fitting models (cumulative Akaike weight = 0.93) included dummy variables coding for three species previously shown to have high VO2max (pronghorn, horse, and a bat), and incorporated a transformation of the phylogenetic branch lengths under an Ornstein-Uhlenbeck model of residual variation (thus indicating phylogenetic signal in the residuals). We found no statistical difference between wheel- and treadmill-elicited values, and diet had no predictive ability for VO2max. Averaged across all models, the allometric scaling exponent was 0.839, with 95% confidence limits of 0.795 and 0.883, which does not provide support for a scaling exponent of 0.67, 0.75 or unity.

Energetics of burrowing by the cirratulid polychaete Cirriformia moorei

Burrowing through marine sediments has been considered to be much more energetically expensive than other forms of locomotion, but previous studies were based solely on external work calculations and lacked an understanding of the mechanical responses of sediments to forces applied by burrowers. Muddy sediments are elastic solids through which worms extend crack-shaped burrows by fracture. Here we present data on energetics of burrowing by Cirriformia moorei. We calculated the external energy per distance traveled from the sum of the work to extend the burrow by fracture and the elastic work done to displace sediment as a worm moves into the newly formed burrow to be 9.7 J kg–1 m–1 in gelatin and 64 J kg–1 m–1 in sediment, much higher than for running or walking. However, because burrowing worms travel at slow speeds, the increase in metabolic rate due to burrowing is predicted to be small. We tested this prediction by measuring aerobic metabolism (oxygen consumption rates) and anaerobic metabolism (concentrations of the anaerobic metabolite tauropine and the energy-storage molecule phosphocreatine) of C. moorei. None of these components was significantly different between burrowing and resting worms, and the low increases in oxygen consumption rates or tauropine concentrations predicted from external work calculations were within the variability observed across individuals. This result suggests that the energy to burrow, which could come from aerobic or anaerobic sources, is not a substantial component of the total metabolic energy of a worm. Burrowing incurs a low cost per unit of time.