Genetic variances and covariances of aerobic metabolic rates in laboratory mice

Estimating the relationship between fitness and metabolic rate: which rate should we use?

As physiologists seek to better understand how and why metabolism varies, they have focused on how metabolic rate covaries with fitness-that is, selection. Evolutionary biologists have developed a sophisticated framework for exploring selection, but there are particular challenges associated with estimating selection on metabolic rate owing to its allometric relationship with body mass. Most researchers estimate selection on mass and absolute metabolic rate; or selection on mass and mass-independent metabolic rate (MIMR)-the residuals generated from a nonlinear regression. These approaches are sometimes treated as synonymous: their coefficients are often interpreted in the same way. Here, we show that these approaches are not equivalent because absolute metabolic rate and MIMR are different traits. We also show that it is difficult to make sound biological inferences about selection on absolute metabolic rate because its causal relationship with mass is enigmatic. By contrast, MIMR requires less-desirable statistical practices (i.e. residuals as a predictor), but provides clearer causal pathways. Moreover, we argue that estimates of selection on MIMR have more meaningful interpretations for physiologists interested in the drivers of variation in metabolic allometry. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Differences in the range of thermoneutral zone between mouse strains: potential effects on translational research

Laboratory mice are commonly used for studies emulating human metabolism. To render human energetics, their ratio of daily (DEE) to basal (BMR) energy expenditure of 1.7-1.8 should be maintained. However, the DEE/BMR ratio strongly depends on whether a given study using a mouse model is carried out above, or below the lower critical temperature (LCT) of the thermoneutral zone, which is rarely considered in translational research. Here, we used mice artificially selected for high or low rates of BMR along with literature data to analyze the effect of ambient temperature on possible systematic bias in DEE/BMR. We demonstrated that the estimated LCTs of mice from the high and low BMR lines differ by more than 7°C. Furthermore, the range of variation of LCTs of mouse strains used in translational research spans from 23 to 33°C. Differences between LCTs in our selected mice and other mouse strains are mirrored by differences in their DEE-to-BMR ratio, on average increasing it at the rate of 0.172°C-1 at temperatures below LCT. Given the wide range of LCTs in different mouse strains, we conclude that the energetic cost of thermoregulation may differ greatly for different mouse strains with a potentially large impact on translational outcomes. Thus, the LCT of a given mouse strain is an important factor that must be considered in designing translational studies.

Is personality and its association with energetics sex-specific in yellow-necked mice Apodemus flavicollis?

For the last two decades, behavioral physiologists aimed to explain a plausible covariation between energetics and personality, predicted by the "pace-of-life syndrome" (POLS) hypothesis. However, the results of these attempts are mixed with no definitive answer as to which of the two most acknowledged models "performance" or "allocation" predicts covariation between consistent among-individual variation in metabolism and repeatable behavior (animal personality). The general conclusion is that the association between personality and energetics is rather context-dependent. Life-history, behavior, and physiology as well as its plausible covariation can be considered a part of sexual dimorphism. However, up to now, only a few studies demonstrated a sex-specific correlation between metabolism and personality. Therefore, we tested the relationships between physiological and personality traits in a single population of yellow-necked mice Apodemus flavicollis in the context of a plausible between-sexes difference in this covariation. We hypothesized that the performance model will explain proactive behavior in males and the allocation model will apply to females. Behavioral traits were determined using the latency of risk-taking and the open field tests, whereas the basal metabolic rates (BMR) was measured using indirect calorimetry. We have found a positive correlation between body mass-adjusted BMR and repeatable proactive behavior in male mice, which can support the performance model. However, the females were rather consistent mainly in avoidance of risk-taking that did not correlate with BMR, suggesting essential differences in personality between sexes. Most likely, the lack of convincing association between energetics and personality traits at the population level is caused by a different selection acting on the life histories of males and females. This may only result in weak support for the predictions of the POLS hypothesis when assuming that only a single model explaining the link between physiology and behavior operates in males and females. Thus, there is a need to consider the differences between sexes in behavioral studies to evaluate this hypothesis.

Spatial variation in the evolutionary potential and constraints of basal metabolic rate and body mass in a wild bird

An organism's energy budget is strongly related to resource consumption, performance, and fitness. Hence, understanding the evolution of key energetic traits, such as basal metabolic rate (BMR), in natural populations is central for understanding life-history evolution and ecological processes. Here we used quantitative genetic analyses to study evolutionary potential of BMR in two insular populations of the house sparrow (Passer domesticus). We obtained measurements of BMR and body mass (M_b ) from 911 house sparrows on the islands of Leka and Vega along the coast of Norway. These two populations were the source populations for translocations to create an additional third, admixed 'common garden' population in 2012. With the use of a novel genetic group animal model concomitant with a genetically determined pedigree, we differentiate genetic and environmental sources of variation, thereby providing insight into the effects of spatial population structure on evolutionary potential. We found that the evolutionary potential of BMR was similar in the two source populations, whereas the Vega population had a somewhat higher evolutionary potential of M_b than the Leka population. BMR was genetically correlated with M_b in both populations, and the conditional evolutionary potential of BMR (independent of body mass) was 41% (Leka) and 53% (Vega) lower than unconditional estimates. Overall, our results show that there is potential for BMR to evolve independently of M_b , but that selection on BMR and/or M_b may have different evolutionary consequences in different populations of the same species.

Individual response in body mass and basal metabolism to the risks of predation and starvation in passerines

Wintering energy management in small passerines has focused on the adaptive regulation of the daily acquisition of energy reserves within a starvation-predation trade-off framework. However, the possibility that the energetic cost of living, i.e. basal metabolic rate (BMR), is being modulated as part of the management energy strategy has been largely neglected. Here, we addressed this possibility by experimentally exposing captive great tits (Parus major) during winter to two consecutive treatments of increased starvation and predation risk for each individual bird. Body mass and BMR were measured prior to and after each week-long treatment. We predicted that birds should be lighter but with a higher metabolic capacity (higher BMR) as a response to increased predation risk, and that birds should increase internal reserves while reducing their cost of living (lower BMR) when exposed to increased starvation risk. Wintering great tits kept a constant body mass independently of a week-long predation or starvation treatment. However, great tits reduced the cost of living (lower BMR) when exposed to the starvation treatment, while BMR remained unaffected by the predation treatment. Energy management in wintering small birds partly relies on BMR regulation, which challenges the current theoretical framework based on body mass regulation.

Variability in energy expenditure is much greater in males than females

In mammals, trait variation is often reported to be greater among males than females. However, to date, mainly only morphological traits have been studied. Energy expenditure represents the metabolic costs of multiple physical, physiological, and behavioral traits. Energy expenditure could exhibit particularly high greater male variation through a cumulative effect if those traits mostly exhibit greater male variation, or a lack of greater male variation if many of them do not. Sex differences in energy expenditure variation have been little explored. We analyzed a large database on energy expenditure in adult humans (1494 males and 3108 females) to investigate whether humans have evolved sex differences in the degree of interindividual variation in energy expenditure. We found that, even when statistically comparing males and females of the same age, height, and body composition, there is much more variation in total, activity, and basal energy expenditure among males. However, with aging, variation in total energy expenditure decreases, and because this happens more rapidly in males, the magnitude of greater male variation, though still large, is attenuated in older age groups. Considerably greater male variation in both total and activity energy expenditure could be explained by greater male variation in levels of daily activity. The considerably greater male variation in basal energy expenditure is remarkable and may be explained, at least in part, by greater male variation in the size of energy-demanding organs. If energy expenditure is a trait that is of indirect interest to females when choosing a sexual partner, this would suggest that energy expenditure is under sexual selection. However, we present a novel energetics model demonstrating that it is also possible that females have been under stabilizing selection pressure for an intermediate basal energy expenditure to maximize energy available for reproduction.

A quantitative genetics perspective on the body-mass scaling of metabolic rate

Widely observed allometric scaling (log-log slope<1) of metabolic rate (MR) with body mass (BM) in animals has been frequently explained using functional mechanisms, but rarely studied from the perspective of multivariate quantitative genetics. This is unfortunate, given that the additive genetic slope (bA) of the MR-BM relationship represents the orientation of the 'line of least genetic resistance' along which MR and BM may most likely evolve. Here, we calculated bA in eight species. Although most bA values were within the range of metabolic scaling exponents reported in the literature, uncertainty of each bA estimate was large (only one bA was significantly lower than 3/4 and none were significantly different from 2/3). Overall, the weighted average for bA (0.667±0.098 95% CI) is consistent with the frequent observation that metabolic scaling exponents are negatively allometric in animals (b<1). Although bA was significantly positively correlated with the phenotypic scaling exponent (bP) across the sampled species, bP was usually lower than bA, as reflected in a (non-significantly) lower weighted average for bP (0.596±0.100). This apparent discrepancy between bA and bP resulted from relatively shallow MR-BM scaling of the residuals [weighted average residual scaling exponent (be)=0.503±0.128], suggesting regression dilution (owing to measurement error and within-individual variance) causing a downward bias in bP. Our study shows how the quantification of the genetic scaling exponent informs us about potential constraints on the correlated evolution of MR and BM, and by doing so has the potential to bridge the gap between micro- and macro-evolutionary studies of scaling allometry.

Low Heritability but Significant Early Environmental Effects on Resting Metabolic Rate in a Wild Passerine

AbstractPredicting the impact of climate change on biodiversity requires understanding the adaptation potential of wild organisms. Evolutionary responses depend on the additive genetic variation associated with the phenotypic traits targeted by selection. We combine 5 years of cross-fostering experiments, measurements of resting metabolic rate (RMR) on nearly 200 wild collared flycatcher (Ficedula albicollis) nestlings, and animal models using a 17-year pedigree to evaluate the potential for an evolutionary response to changing environmental conditions. Contrary to other avian studies, we find no significant heritability of whole-organism, mass-independent, or mass-specific RMR, but we report a strong effect of nest environment instead. We therefore conclude that variation in nestling RMR is explained by variation in the early-life environment provided by the parents. We discuss possible underlying specific parental effects and the importance of taking different mechanisms into account to understand how animals phenotypically adapt (or fail to adapt) to climate change.

Individual (Co)variation in Resting and Maximal Metabolic Rates in Wild Mice

AbstractBasal metabolic rate (BMR) represents the lowest level of aerobic metabolism in a resting, postabsorptive endotherm as measured within the thermoneutral zone. By contrast, maximal metabolic rate ([Formula: see text]max) reflects the upper limit of aerobic metabolism achieved during intensive exercise. As BMR and [Formula: see text]max define the boundaries of the possible levels of aerobic metabolism expressed by a normothermic individual, a key question is whether BMR and [Formula: see text]max are correlated. In the present study, we took repeated paired measurements of thermoneutral resting metabolic rate (RMR_t) and [Formula: see text]max on 165 white-footed mice (Peromyscus leucopus). Over a single summer (May-October), repeatability (R ± SE) was low but statistically significant ([Formula: see text]) for both RMR_t and [Formula: see text]max ([Formula: see text] for RMR_t; [Formula: see text] for [Formula: see text]max). Willingness to run during the forced-exercise trials was also significantly repeatable ([Formula: see text]). At the residual level (within individual), RMR_t and [Formula: see text]max tended to be positively correlated ([Formula: see text], [Formula: see text]), suggesting the presence of correlated phenotypic plasticity. By contrast, RMR_t and [Formula: see text]max were significantly negatively correlated at the among-individual level ([Formula: see text]). To the extent that variation in RMR_t reflects variation in BMR, the negative among-individual correlation does not corroborate the idea that a costly metabolic machinery is needed to support a high [Formula: see text]max. Future research should investigate the (genetic) relationship between RMR_t (and BMR) and other energetically expensive behaviors and activities to better understand how energy is allocated within individuals.

Energetic constraints on mammalian distribution areas

Energy is a universal resource essential for all life functions. The rate of transformation of energy into an organism, and the energetic investment into reproduction, determines population and ecological-level processes. Several hypotheses predicted that the ecological expansion and size of the geographic distribution of a species are shaped by, among other factors, metabolic performance. However, how organismal energetic characteristics contribute to species geographic range size is poorly understood. With phylogenetic comparative methods whether energetic maintenance costs (basal metabolic rate, BMR), aerobic capacity (maximum exercise metabolic rate, VO₂ max), summit thermoregulation (summit metabolic rate, VO₂ sum) and the ability to sustain energy provisioning (daily energy expenditure, DEE) determine the distribution of mammalian species range sizes was tested. Both basal and maximum exercise metabolic rates (accounting for body mass), but not summit thermogenic metabolic rate, were positively associated with species range sizes. Furthermore, daily energy expenditure (accounting for body mass) was positively associated with species ranges. Body mass (accounting for energetic maintenance) was negatively related to range sizes. High aerobic exercise capacity, aiding mobility such as running and dispersal, and high sustained energy provisioning, aiding reproductive effort such as pregnancy, lactation and natal dispersal, can facilitate the establishment of large mammalian geographic ranges. Consequently, the pace of organismal physiological processes can shape important ecological and biodiversity patterns by setting limits to species' range sizes.

Metabolic Rates Predict Baseline Corticosterone and Reproductive Output in a Free-Living Passerine

Organisms continuously face environmental fluctuations, and allocation of metabolic investment to meet changing energetic demands is fundamental to survival and reproductive success. Glucocorticoid (GC) hormones (e.g., corticosterone [CORT]) play an important role in energy acquisition and allocation in the face of environmental challenges, partly through mediation of energy metabolism. Although GCs and metabolic rate are expected to covary, surprisingly few empirical studies have demonstrated such relationships, especially in wild animals. Moreover, studies testing for associations between GCs and fitness generally do not account for among-individual differences in energy expenditure or energy allocation. We measured CORT (baseline and stress-induced) and metabolic traits (resting metabolic rate [RMR], cold-induced VO_2max [M_sum], and aerobic scope [the difference between M_sum and RMR]) in female tree swallows (Tachycineta bicolor) during chick-rearing, and tested for their associations with several variables of reproductive performance. We found a positive relationship between RMR and baseline CORT, but no consistent associations between stress-induced CORT (SI-CORT) and M_sum. This suggests that while baseline CORT may be a good indicator of an individual's baseline metabolic investment, SI-CORT responses are not associated with aerobic scope or the upper limits of aerobic performance. Furthermore, we found that metabolic traits were associated with reproductive performance: females with higher reproductive output showed higher M_sum, and also tended to show higher RMR. Overall, these results suggest that metabolic traits are better predictors of reproductive output in tree swallows than CORT concentrations. They further point to the maximal aerobic capacity being higher in females investing more heavily in a current reproductive event, but whether this association reflects trade-offs between current and future reproductive efforts remains to be tested.

Repeatability and heritability of resting metabolic rate in a long-lived amphibian

Resting metabolic rate (RMR), i.e. spent energy necessary to maintain basic life functions, is a basic component of energy budget in ectotherms. The evolution of RMR through natural selection rests on the premise of its non-zero repeatability and heritability, i.e. consistent variation within individual lifetimes and resemblance between parents and their offspring, respectively. Joint estimates of RMR repeatability and heritability are missing in ectotherms, however, which precludes estimations of the evolutionary potential of this trait. We examined RMR repeatability and heritability in a long-lived ectotherm, the alpine newt (Ichthyosaura alpestris). Individual RMR was repeatable over both six-month (0.28 ± 0.09 [SE]) and five-year (0.16 ± 0.07) periods. While there was no resemblance between parent and offspring RMR (0.21 ± 0.34), the trait showed similarity among offspring within families (broad-sense heritability; 0.25 ± 0.09). Similar repeatability and broad-sense heritability values in parental and offspring generations, respectively, and non-conclusive narrow-sense heritability suggest the contribution of non-additive genetic factors to total phenotypic variance in this trait. We conclude that RMR evolutionary trajectories are shaped by other processes than natural selection in this long-lived ectotherm.

Energetic mechanisms for coping with changes in resource availability

Given current anthropogenic alterations to many ecosystems and communities, it is becoming increasingly important to consider whether and how organisms can cope with changing resources. Metabolic rate, because it represents the rate of energy expenditure, may play a key role in mediating the link between resource conditions and performance and thereby how well organisms can persist in the face of environmental change. Here, we focus on the role that energy metabolism plays in determining organismal responses to changes in food availability over both short-term ecological and longer-term evolutionary timescales. Using a meta-analytical approach encompassing multiple species, we find that individuals with a higher metabolic rate grow faster under high food levels but slower once food levels decline, suggesting that the association between metabolism and life-history traits shifts along resource gradients. We also find that organisms can cope with changing resource availability through both phenotypic plasticity and genetically based evolutionary adaptation in their rates of energy metabolism. However, the metabolic rates of individuals within a population and of species within a lineage do not all respond in the same manner to changes in food availability. This diversity of responses suggests that there are benefits but also costs to changes in metabolic rate. It also underscores the need to examine not just the energy budgets of organisms within the context of metabolic rate but also how energy metabolism changes alongside other physiological and behavioural traits in variable environments.

Can Energetic Capacity Help Explain Why Physical Activity Reduces Cancer Risk?

Increased physical activity reduces cancer risk in humans, but why this whole-organism attribute reduces cancer remains unclear. Active individuals tend to have high capacity to generate energy on a sustained basis, which in turn can permit greater immune responses crucial for fighting emerging neoplasia. Thus, we suggest energetic capacity as a potential mechanism to explain the activity-cancer link, given that humans are intrinsically (not externally) energy limited. Human and rodent studies show that individuals with high energetic capacity mount greater immune responses and have lower cancer incidence; these trends persist after controlling for actual physical activity, supporting a direct role of energetic capacity. If true, exercise efforts might best target those that increase one's energetic capacity, which may be both individual and exercise specific.

Effects of Selection for Mass-Independent Maximal Metabolic Rate on Food Consumption

Metabolic rates potentially regulate the pace of important physiological and life-history traits. Natural selection has shaped the evolution of metabolic rates and the physiology that supports them, including digestibility and the rate of food consumption. Understanding the relationship between metabolic rates and energy internalization is central to understanding how resources are allocated among competing physiological functions. We investigated how artificial selection on mass-independent basal metabolic rate (BMR) and mass-independent aerobic maximal metabolic rate (MMR) affected food consumption and apparent digestibility in mice. Evolved changes in mass-corrected BMR-but not mass-corrected MMR-corresponded with changes in food consumption. This result is consistent with previous work showing that BMR constitutes a large portion of an animal's daily energy budget and thus that BMR might provide a better indicator of daily food requirements than MMR. In contrast, digestive efficiencies did not differ among selection treatments and did not evolve in these mice. This study provides insights into how evolution of metabolic rates may affect food consumption and overall energy use.

Evolution of physiological performance capacities and environmental adaptation: insights from high-elevation deer mice (Peromyscus maniculatus)

Analysis of variation in whole-animal performance can shed light on causal connections between specific traits, integrated physiological capacities, and Darwinian fitness. Here, we review and synthesize information on naturally occurring variation in physiological performance capacities and how it relates to environmental adaptation in deer mice (Peromyscus maniculatus). We discuss how evolved changes in aerobic exercise capacity and thermogenic capacity have contributed to adaptation to high elevations. Comparative work on deer mice at high and low elevations has revealed evolved differences in aerobic performance capacities in hypoxia. Highland deer mice have consistently higher aerobic performance capacities under hypoxia relative to lowland natives, consistent with the idea that it is beneficial to have a higher maximal metabolic rate (as measured by the maximal rate of O₂ consumption, VO_2max) in an environment characterized by lower air temperatures and lower O₂ availability. Observed differences in aerobic performance capacities between highland and lowland deer mice stem from changes in numerous subordinate traits that alter the flux capacity of the O₂-transport system, the oxidative capacity of tissue mitochondria, and the relationship between O₂ consumption and ATP synthesis. Many such changes in physiological phenotype are associated with hypoxia-induced changes in gene expression. Research on natural variation in whole-animal performance forms a nexus between physiological ecology and evolutionary biology that requires insight into the natural history of the study species.

Metabolic Scope as a Proximate Constraint on Individual Behavioral Variation: Effects on Personality, Plasticity, and Predictability

Behavioral ecologists have hypothesized that among-individual differences in resting metabolic rate (RMR) may predict consistent individual differences in mean values for costly behaviors or for behaviors that affect energy intake rate. This hypothesis has empirical support and presently attracts considerable attention, but, notably, it does not provide predictions for individual differences in (a) behavioral plasticity or (b) unexplained variation (residual variation from mean individual behavior, here termed predictability). We outline how consideration of aerobic maximum metabolic rate (MMR) and particularly aerobic scope (= MMR - RMR) can be used to simultaneously make predictions about mean and among- and within-individual variation in behavior. We predict that while RMR should be proportional to an individual's mean level of sustained behavioral activity (one aspect of its personality), individuals with greater aerobic scope will also have greater scope to express behavioral plasticity and/or greater unpredictability in behavior (=greater residual variation). As a first step toward testing these predictions, we analyze existing activity data from selectively bred lines of mice that differ in both daily activity and aerobic scope. We find that replicate high-scope mice are more active on average and show greater among-individual variation in activity, greater among-individual variation in plasticity, and greater unpredictability. These data provide some tentative first support for our hypothesis, suggesting that further research on this topic would be valuable.

Understanding variation in metabolic rate

Metabolic rate reflects an organism's capacity for growth, maintenance and reproduction, and is likely to be a target of selection. Physiologists have long sought to understand the causes and consequences of within-individual to among-species variation in metabolic rates – how metabolic rates relate to performance and how they should evolve. Traditionally, this has been viewed from a mechanistic perspective, relying primarily on hypothesis-driven approaches. A more agnostic, but ultimately more powerful tool for understanding the dynamics of phenotypic variation is through use of the breeder's equation, because variation in metabolic rate is likely to be a consequence of underlying microevolutionary processes. Here we show that metabolic rates are often significantly heritable, and are therefore free to evolve under selection. We note, however, that ‘metabolic rate’ is not a single trait: in addition to the obvious differences between metabolic levels (e.g. basal, resting, free-living, maximal), metabolic rate changes through ontogeny and in response to a range of extrinsic factors, and is therefore subject to multivariate constraint and selection. We emphasize three key advantages of studying metabolic rate within a quantitative genetics framework: its formalism, and its predictive and comparative power. We make several recommendations when applying a quantitative genetics framework: (i) measuring selection based on actual fitness, rather than proxies for fitness; (ii) considering the genetic covariances between metabolic rates throughout ontogeny; and (iii) estimating genetic covariances between metabolic rates and other traits. A quantitative genetics framework provides the means for quantifying the evolutionary potential of metabolic rate and why variance in metabolic rates within populations might be maintained.

Metabolic rate evolves rapidly and in parallel with the pace of life history

Metabolic rates and life history strategies are both thought to set the “pace of life”, but whether they evolve in tandem is not well understood. Here, using a common garden experiment that compares replicate paired populations, we show that Trinidadian guppy (Poecilia reticulata) populations that evolved a fast-paced life history in high-predation environments have consistently higher metabolic rates than guppies that evolved a slow-paced life history in low-predation environments. Furthermore, by transplanting guppies from high- to low-predation environments, we show that metabolic rate evolves in parallel with the pace of life history, at a rapid rate, and in the same direction as found for naturally occurring populations. Together, these multiple lines of inference provide evidence for a tight evolutionary coupling between metabolism and the pace of life history.

The ‘pace of life’ depends on both metabolic rate and life history traits; however, whether these evolve similarly in response to the environment is not clear. Here, Auer et al. show parallel evolution of metabolic rate and a suite of life history traits in response to predator environment in Trinidadian guppies.

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.

A disparity between locomotor economy and territory-holding ability in male house mice

Both economical locomotion and physical fighting are important performance traits to many species because of their direct influence on components of Darwinian fitness. Locomotion represents a substantial portion of the total daily energy budget of many animals. Fighting performance often determines individual reproductive fitness through the means of resource control, social dominance and access to mates. However, phenotypic traits that improve either locomotor economy or fighting ability may diminish performance in the other. Here, we tested for a predicted disparity between locomotor economy and competitive ability in wild-derived house mice (Mus musculus). We used 8 week social competition trials in semi-natural enclosures to directly measure male competitive ability through territorial control and female occupancy within territories. We also measured oxygen consumption during locomotion for each mouse using running trials in an enclosed treadmill and open-flow respirometry. Our results show that territory-holding males have higher absolute and mass-specific oxygen consumption when running (i.e. reduced locomotor economy) compared with males that do not control territories. This relationship was present both before and after 8 week competition trials in semi-natural enclosures. This disparity between physical competitive ability and economical locomotion may impose viability costs on males in species for which competition over mates is common and may constrain the evolution of behavioral and phenotypic diversity, particularly in natural settings with environmental and resource variability.

Comparative analysis of basal locomotor activity-related metabolic phenotypes between C57BL/6 mice and ICR mice substrains derived from three different sources

Animal model, as an indispensable tool for scientific purposes of biomedical researches and clinical application, is a commonly used in various researches. Regarding to this, it is necessary to establish the metabolic phenotype of animal model to minimize spurious interpretations and ensure a level of accuracy and reliability adequate for experimental research. However, the metabolic phenotype-related analysis within rodent strains derived from different source is nonexistent, especially in C57BL/6Korl mice and Korl:ICR mice (a domestic mouse strain). To compare the physiological and metabolic phenotypes over a period of time, we utilized the C57BL/6 mice (C57BL/6Korl, A:C57BL/6, and B:C57BL/6) and ICR mice (Korl:ICR, A:ICR, and B:ICR) derived from three different sources. Our data showed that average body weight, body temperature, food intake, and water consumption have a similar tendency among the C57BL/6 and ICR groups, except for the higher body weight of C57BL/6Korl mice over a period of time. Moreover, some significant differences was observed in adipose tissue mass and adipocyte size among the groups, with a higher tendency of C57BL/6Korl mice and Korl:ICR mice. Most importantly, resting metabolic rate (RMR) serves as an approximation of the metabolic phenotype showed no significant difference among the groups of C57BL/6 mice and ICR mice, except for the lower oxygen uptake of C57BL/6Korl mice compare to the A:C57BL/6 mice. Taken together, our data suggest that C57BL/6 mice and ICR mice derived from three different sources have an overall similar feature of physiological and metabolic phenotypes.

Resting vs. active: a meta-analysis of the intra- and inter-specific associations between minimum, sustained, and maximum metabolic rates in vertebrates

Variation in aerobic capacity has far reaching consequences for the physiology, ecology, and evolution of vertebrates. Whether at rest or active, animals are constrained to operate within the energetic bounds determined by their minimum (minMR) and sustained or maximum metabolic rates (upperMR). MinMR and upperMR can differ considerably among individuals and species but are often presumed to be mechanistically linked to one another. Specifically, minMR is thought to reflect the idling cost of the machinery needed to support upperMR. However, previous analyses based on limited datasets have come to conflicting conclusions regarding the generality and strength of their association.

Here we conduct the first comprehensive assessment of their relationship, based on a large number of published estimates of both the intra‐specific (n = 176) and inter‐specific (n = 41) phenotypic correlations between minMR and upperMR, estimated as either exercise‐induced maximum metabolic rate (VO₂max), cold‐induced summit metabolic rate (Msum), or daily energy expenditure (DEE).

Our meta‐analysis shows that there is a general positive association between minMR and upperMR that is shared among vertebrate taxonomic classes. However, there was stronger evidence for intra‐specific correlations between minMR and Msum and between minMR and DEE than there was for a correlation between minMR and VO₂max across different taxa. As expected, inter‐specific correlation estimates were consistently higher than intra‐specific estimates across all traits and vertebrate classes.

An interesting exception to this general trend was observed in mammals, which contrast with birds and exhibit no correlation between minMR and Msum. We speculate that this is due to the evolution and recruitment of brown fat as a thermogenic tissue, which illustrates how some species and lineages might circumvent this seemingly general association.

We conclude that, in spite of some variability across taxa and traits, the contention that minMR and upperMR are positively correlated generally holds true both within and across vertebrate species. Ecological and comparative studies should therefore take into consideration the possibility that variation in any one of these traits might partly reflect correlated responses to selection on other metabolic parameters.

A lay summary is available for this article.

How low can you go? An adaptive energetic framework for interpreting basal metabolic rate variation in endotherms

Adaptive explanations for both high and low body mass-independent basal metabolic rate (BMR) in endotherms are pervasive in evolutionary physiology, but arguments implying a direct adaptive benefit of high BMR are troublesome from an energetic standpoint. Here, we argue that conclusions about the adaptive benefit of BMR need to be interpreted, first and foremost, in terms of energetics, with particular attention to physiological traits on which natural selection is directly acting. We further argue from an energetic perspective that selection should always act to reduce BMR (i.e., maintenance costs) to the lowest level possible under prevailing environmental or ecological demands, so that high BMR per se is not directly adaptive. We emphasize the argument that high BMR arises as a correlated response to direct selection on other physiological traits associated with high ecological or environmental costs, such as daily energy expenditure (DEE) or capacities for activity or thermogenesis. High BMR thus represents elevated maintenance costs required to support energetically demanding lifestyles, including living in harsh environments. BMR is generally low under conditions of relaxed selection on energy demands for high metabolic capacities (e.g., thermoregulation, activity) or conditions promoting energy conservation. Under these conditions, we argue that selection can act directly to reduce BMR. We contend that, as a general rule, BMR should always be as low as environmental or ecological conditions permit, allowing energy to be allocated for other functions. Studies addressing relative reaction norms and response times to fluctuating environmental or ecological demands for BMR, DEE, and metabolic capacities and the fitness consequences of variation in BMR and other metabolic traits are needed to better delineate organismal metabolic responses to environmental or ecological selective forces.

Exosomes in mammals with greater habitat variability contain more proteins and RNAs

Factors determining habitat variability are poorly understood despite possible explanations based on genome and physiology. This is because previous studies only focused on primary measures such as genome size and body size. In this study, we hypothesize that specific gene functions determine habitat variability in order to explore new factors beyond primary measures. We comprehensively evaluate the relationship between gene functions and the climate envelope while statistically controlling for potentially confounding effects by using data on the habitat range, genome, body size and metabolism of various mammals. Our analyses show that the number of proteins and RNAs contained in exosomes is predominantly associated with the climate envelope. This finding indicates the importance of exosomes to habitat range expansion of mammals and provides a new hypothesis for the relationship between the genome and habitat variability.

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.

Evolution of basal metabolic rate in bank voles from a multidirectional selection experiment

A major theme in evolutionary and ecological physiology of terrestrial vertebrates encompasses the factors underlying the evolution of endothermy in birds and mammals and interspecific variation of basal metabolic rate (BMR). Here, we applied the experimental evolution approach and compared BMR in lines of a wild rodent, the bank vole (Myodes glareolus), selected for 11 generations for: high swim-induced aerobic metabolism (A), ability to maintain body mass on a low-quality herbivorous diet (H) and intensity of predatory behaviour towards crickets (P). Four replicate lines were maintained for each of the selection directions and an unselected control (C). In comparison to C lines, A lines achieved a 49% higher maximum rate of oxygen consumption during swimming, H lines lost 1.3 g less mass in the test with low-quality diet and P lines attacked crickets five times more frequently. BMR was significantly higher in A lines than in C or H lines (60.8, 56.6 and 54.4 ml O2 h(-1), respectively), and the values were intermediate in P lines (59.0 ml O2 h(-1)). Results of the selection experiment provide support for the hypothesis of a positive association between BMR and aerobic exercise performance, but not for the association of adaptation to herbivorous diet with either a high or low BMR.

Ecological Influences and Morphological Correlates of Resting and Maximal Metabolic Rates across Teleost Fish Species

Rates of aerobic metabolism vary considerably across evolutionary lineages, but little is known about the proximate and ultimate factors that generate and maintain this variability. Using data for 131 teleost fish species, we performed a large-scale phylogenetic comparative analysis of how interspecific variation in resting metabolic rates (RMRs) and maximum metabolic rates (MMRs) is related to several ecological and morphological variables. Mass- and temperature-adjusted RMR and MMR are highly correlated along a continuum spanning a 30- to 40-fold range. Phylogenetic generalized least squares models suggest that RMR and MMR are higher in pelagic species and that species with higher trophic levels exhibit elevated MMR. This variation is mirrored at various levels of structural organization: gill surface area, muscle protein content, and caudal fin aspect ratio (a proxy for activity) are positively related with aerobic capacity. Muscle protein content and caudal fin aspect ratio are also positively correlated with RMR. Hypoxia-tolerant lineages fall at the lower end of the metabolic continuum. Different ecological lifestyles are associated with contrasting levels of aerobic capacity, possibly reflecting the interplay between selection for increased locomotor performance on one hand and tolerance to low resource availability, particularly oxygen, on the other. These results support the aerobic capacity model of the evolution of endothermy, suggesting elevated body temperatures evolved as correlated responses to selection for high activity levels.

Speeding up Growth: Selection for Mass-Independent Maximal Metabolic Rate Alters Growth Rates

Investigations into relationships between life-history traits, such as growth rate and energy metabolism, typically focus on basal metabolic rate (BMR). In contrast, investigators rarely examine maximal metabolic rate (MMR) as a relevant metric of energy metabolism, even though it indicates the maximal capacity to metabolize energy aerobically, and hence it might also be important in trade-offs. We studied the relationship between energy metabolism and growth in mice (Mus musculus domesticus Linnaeus) selected for high mass-independent metabolic rates. Selection for high mass-independent MMR increased maximal growth rate, increased body mass at 20 weeks of age, and generally altered growth patterns in both male and female mice. In contrast, there was little evidence that the correlated response in mass-adjusted BMR altered growth patterns. The relationship between mass-adjusted MMR and growth rate indicates that MMR is an important mediator of life histories. Studies investigating associations between energy metabolism and life histories should consider MMR because it is potentially as important in understanding life history as BMR.

Fluctuating selection on basal metabolic rate

BMR (Basal metabolic rate) is an important trait in animal life history as it represents a significant part of animal energy budgets. BMR has also been shown to be positively related to sustainable work rate and maximal thermoregulatory capacity. To this date, most of the studies have focused on the causes of interspecific and intraspecific variation in BMR, and fairly little is known about the fitness consequences of different metabolic strategies. In this study, we show that winter BMR affects local survival in a population of wild blue tits (Cyanistes caeruleus), but that the selection direction differs between years. We argue that this fluctuating selection is probably a consequence of varying winter climate with a positive relation between survival and BMR during cold and harsh conditions, but a negative relation during mild winters. This fluctuating selection can not only explain the pronounced variation in BMR in wild populations, but will also give us new insights into how energy turnover rates can shape the life‐history strategies of animals. Furthermore, the study shows that the process of global warming may cause directional selection for a general reduction in BMR, affecting the general life‐history strategy on the population level.

Repeatability of metabolic rate is lower for animals living under field versus laboratory conditions

Metabolic rate has been linked to multiple components of fitness and is both heritable and repeatable to a certain extent. However, its repeatability can differ among studies, even after controlling for the time interval between measurements. Some of this variation in repeatability may be due to the relative stability of the environmental conditions in which the animals are living between measurements. We compared published repeatability estimates for basal, resting, and maximum metabolic rate from studies of endotherms living in the laboratory versus those living in the wild during the interval between measurements. We found that repeatability declines over time, as demonstrated previously, but show for the first time that estimates from free-living animals are also considerably lower than those from animals living under more stable laboratory conditions.

Does individual variation in metabolic phenotype predict fish behaviour and performance?

There is increasing interest in documenting and explaining the existence of marked intraspecific variation in metabolic rate in animals, with fishes providing some of the best-studied examples. After accounting for variation due to other factors, there can typically be a two to three-fold variation among individual fishes for both standard and maximum metabolic rate (SMR and MMR). This variation is reasonably consistent over time (provided that conditions remain stable), and its underlying causes may be influenced by both genes and developmental conditions. In this paper, current knowledge of the extent and causes of individual variation in SMR, MMR and aerobic scope (AS), collectively its metabolic phenotype, is reviewed and potential links among metabolism, behaviour and performance are described. Intraspecific variation in metabolism has been found to be related to other traits: fishes with a relatively high SMR tend to be more dominant and grow faster in high food environments, but may lose their advantage and are more prone to risk-taking when conditions deteriorate. In contrast to the wide body of research examining links between SMR and behavioural traits, very little work has been directed towards understanding the ecological consequences of individual variation in MMR and AS. Although AS can differ among populations of the same species in response to performance demands, virtually nothing is known about the effects of AS on individual behaviours such as those associated with foraging or predator avoidance. Further, while factors such as food availability, temperature, hypoxia and the fish's social environment are known to alter resting and MMRs in fishes, there is a paucity of studies examining how these effects vary among individuals, and how this variation relates to behaviour. Given the observed links between metabolism and measures of performance, understanding the metabolic responses of individuals to changing environments will be a key area for future research because the environment will have a strong influence on which animals survive predation, become dominant and ultimately have the highest reproductive success. Although current evidence suggests that variation in SMR may be maintained within populations via context-dependent fitness benefits, it is suggested that a more integrative approach is now required to fully understand how the environment can modulate individual performance via effects on metabolic phenotypes encompassing SMR, MMR and AS.

Variation in the link between oxygen consumption and ATP production, and its relevance for animal performance

It is often assumed that an animal's metabolic rate can be estimated through measuring the whole-organism oxygen consumption rate. However, oxygen consumption alone is unlikely to be a sufficient marker of energy metabolism in many situations. This is due to the inherent variability in the link between oxidation and phosphorylation; that is, the amount of adenosine triphosphate (ATP) generated per molecule of oxygen consumed by mitochondria (P/O ratio). In this article, we describe how the P/O ratio can vary within and among individuals, and in response to a number of environmental parameters, including diet and temperature. As the P/O ratio affects the efficiency of cellular energy production, its variability may have significant consequences for animal performance, such as growth rate and reproductive output. We explore the adaptive significance of such variability and hypothesize that while a reduction in the P/O ratio is energetically costly, it may be associated with advantages in terms of somatic maintenance through reduced production of reactive oxygen species. Finally, we discuss how considering variation in mitochondrial efficiency, together with whole-organism oxygen consumption, can permit a better understanding of the relationship between energy metabolism and life history for studies in evolutionary ecology.

A strong response to selection on mass-independent maximal metabolic rate without a correlated response in basal metabolic rate

Metabolic rates are correlated with many aspects of ecology, but how selection on different aspects of metabolic rates affects their mutual evolution is poorly understood. Using laboratory mice, we artificially selected for high maximal mass-independent metabolic rate (MMR) without direct selection on mass-independent basal metabolic rate (BMR). Then we tested for responses to selection in MMR and correlated responses to selection in BMR. In other lines, we antagonistically selected for mice with a combination of high mass-independent MMR and low mass-independent BMR. All selection protocols and data analyses included body mass as a covariate, so effects of selection on the metabolic rates are mass adjusted (that is, independent of effects of body mass). The selection lasted eight generations. Compared with controls, MMR was significantly higher (11.2%) in lines selected for increased MMR, and BMR was slightly, but not significantly, higher (2.5%). Compared with controls, MMR was significantly higher (5.3%) in antagonistically selected lines, and BMR was slightly, but not significantly, lower (4.2%). Analysis of breeding values revealed no positive genetic trend for elevated BMR in high-MMR lines. A weak positive genetic correlation was detected between MMR and BMR. That weak positive genetic correlation supports the aerobic capacity model for the evolution of endothermy in the sense that it fails to falsify a key model assumption. Overall, the results suggest that at least in these mice there is significant capacity for independent evolution of metabolic traits. Whether that is true in the ancestral animals that evolved endothermy remains an important but unanswered question.

Present-day genetic correlations and testing the aerobic capacity model

A recent article by Nespolo and Roff suggests that present-day genetic correlations between resting and maximal metabolic rate do not provide support for the aerobic capacity model for the evolution of endothermy. That conclusion is potentially misleading. The aerobic capacity model makes exacting predictions about genetic architecture. While the presence of a genetic correlation does not support the model per se, the absence of a correlation definitively falsifies the model. Testing for present-day correlations remains a useful endeavor, at least until the model is convincingly falsified or until many attempts to falsify the model fail.

Ecological and scaling analysis of the energy expenditure of rest, activity, flight, and evaporative water loss in Passeriformes and non-Passeriformes in relation to seasonal migrations and to the occupation of boreal stations in high and moderate latitudes

A unified system of bioenergetic parameters that describe thermal regulation and energy metabolism in many passerine and non-passerine species has been developed. These parameters have been analyzed as functions of ambient temperature, and bioenergetic models for various species have been developed. The level of maximum food energy or maximal existence metabolism (MPE) is 1.3 times higher in passerines than in non-passerines, which is consistent with the ratio of their basal metabolic rates (BMR). The optimal ambient temperature for maximizing productive processes (e.g., reproduction, molting) is lower for passerines than for non passerines, which allows passerines to have higher production rates at moderate ambient temperatures. This difference in the optimal ambient temperature may explain the variation in bioenergetic parameters along latitudinal gradients, such as the well-known ecological rule of clutch size (or mass) increase in the more northerly passerine birds. The increased potential for productive energy output in the north may also allow birds to molt faster there. This phenomenon allows passerine birds to occupy a habitat that fluctuates widely in ambient temperature compared with non-passerine birds of similar size. Passerines have a more effective system for maintaining heat balance at both high and low temperatures. The high metabolism and small body sizes of passerines are consistent with omnivore development and with ecological plasticity. Among large passerines, the unfavorable ratio of MPE to BMR should decrease the energy that is available for productive processes. This consequence limits both the reproductive output and the development of long migration (particularly in Corvus corax). The hypothesis regarding BMR increase in passerines was suggested based on an aerodynamic analysis of the flight speed and the wing characteristics. This allometric analysis shows that the flight velocity is approximately 20% lower in Passeriformes than in non-Passeriformes, which is consistent with the inverted ratio of their BMR level. The regressions for the aerodynamic characteristics of wings show that passerines do not change the morphological characteristics of their wings to decrease velocity. Passerine birds prefer forest habitats. The size range of 5-150 g for birds in forest habitats is almost exclusively occupied by passerines because of their large energetic capability.

Heritable variation in reaction norms of metabolism and activity across temperatures in a wild-derived population of white-footed mice (Peromyscus leucopus)

Heritable variation in metabolic traits is likely to affect fitness. In this study, white-footed mice from wild-derived photoresponsive [R, infertile in short day length (SD)] and non-photoresponsive (NR, fertile in SD) selection lines were maintained under short-day (SD 8Light:16Dark), sub-thermoneutral conditions (22 or 12 °C). Mice had significantly higher levels of food intake and resting metabolic rates (RMR) at low temperature. RMR differed significantly between lines (greater in NR mice). In contrast to previous work under thermoneutral conditions, there was no significant difference in overall activity or average daily metabolic rates (ADMR) of mice from the two lines. Reduced activity may reflect behavioral changes under cooler conditions (e.g., nest building) reducing the overall energetic cost of fertility (for NR mice). There was no significant difference in maximal rate of oxygen consumption ([Formula: see text]) between lines. R mice had significantly greater brown adipose tissue and white abdominal fat mass due to both line and temperature. Reaction norms for intake, resting metabolism (RMR/BMR) and level of activity from current (12 and 22 °C) and previously published data (28 °C) demonstrate independent effects of genetics (line) and environment (temperature) for resting metabolism, but a clear interaction between these for activity. The results suggest that fertility under winter conditions imposes metabolic costs that are related to the level of reproductive development. Under the coldest conditions tested, however, mice that remained fertile in SD reduced activity, ADMR and food requirements, decreasing the differential between selection lines. Heritable variation in reaction norms suggests a genetic by environment effect that could be subject to selection.

Testing the aerobic model for the evolution of endothermy: implications of using present correlations to infer past evolution

The evolution of endothermy is one of the most puzzling events in vertebrate evolution, for which several hypotheses have been proposed. The most accepted model is the aerobic model, which assumes the existence of a genetic correlation between resting metabolic rate (RMR) and maximum aerobic capacity (whose standard measure is maximum metabolic rate, MMR). This model posits that directional selection acted on maximum aerobic capacity and resting metabolic rate increased as a correlated response, in turn increasing body temperature. To test this hypothesis we implemented a simple two-trait quantitative genetic model in which RMR and MMR are initially independent of each other and subject to stabilizing selection to two separate optima. We show mutations that arise that affect both traits can lead to the evolution of a genetic correlation between the traits without any significant shifting of the two trait means. Thus, the presence of a genetic correlation between RMR and MMR in living animals provides no support in and of itself for the past elevation of metabolic rate via selection on aerobic capacity. This result calls into question the testability of the hypothesis that RMR increased as a correlated response to directional selection on MMR, in turn increasing body temperature, using quantitative genetics. Given the difficulty in studying ancient physiological processes, we suggest that approaches such as this model are a valuable alternative for analyzing possible mechanisms of endothermy evolution.

Basal metabolic rate can evolve independently of morphological and behavioural traits

Quantitative genetic analyses of basal metabolic rate (BMR) can inform us about the evolvability of the trait by providing estimates of heritability, and also of genetic correlations with other traits that may constrain the ability of BMR to respond to selection. Here, we studied a captive population of zebra finches (Taeniopygia guttata) in which selection lines for male courtship rate have been established. We measure BMR in these lines to see whether selection on male sexual activity would change BMR as a potentially correlated trait. We find that the genetic correlation between courtship rate and BMR is practically zero, indicating that the two traits can evolve independently of each other. Interestingly, we find that the heritability of BMR in our population (h(2)=0.45) is markedly higher than was previously reported for a captive zebra finch population from Norway. A comparison of the two studies shows that additive genetic variance in BMR has been largely depleted in the Norwegian population, especially the genetic variance in BMR that is independent of body mass. In our population, the slope of BMR increase with body mass differs not only between the sexes but also between the six selection lines, which we tentatively attribute to genetic drift and/or founder effects being strong in small populations. Our study therefore highlights two things. First, the evolvability of BMR may be less constrained by genetic correlations and lack of independent genetic variation than previously described. Second, genetic drift in small populations can rapidly lead to different evolvabilities across populations.