Natural selection reduces energy metabolism in the garden snail, helix aspersa (cornu aspersum)

How Hot is too Hot? Metabolic Responses to Temperature Across Life Stages of a Small Ectotherm

To understand how global warming will impact biodiversity, we need to pay attention to those species with higher vulnerability. However, to assess vulnerability, we also need to consider the thermoregulatory mechanisms, body size, and thermal tolerance of species. Studies addressing thermal tolerance on small ectotherms have mostly focused on insects, while other arthropods, such as arachnids remain understudied. Here, we quantified the physiological thermal sensitivity of the pseudoscorpion Dactylochelifer silvestris using a respirometry setup with a ramping temperature increase. Overall, we found that D. silvestris has a much lower metabolic rate than other organisms of similar size. As expected, metabolic rate increased with body size, with adults having larger metabolic rates, but the overall metabolic scaling exponent was low. Both the temperature at which metabolism peaked and the critical thermal maxima were high (>44°C) and comparable to those of other arachnids. The activation energy, which characterizes the rising portion of the thermal sensitivity curve, was 0.66 eV, consistent with predictions for insects and other taxa in general. Heat tolerances and activation energy did not differ across life stages. We conclude that D. silvestris has low metabolic rates and a high thermal tolerance, which would likely influence how all stages and sexes of this species could endure climate change.

Among-individual variation in thermal plasticity of fish metabolic rates causes profound variation in temperature-specific trait repeatability, but does not co-vary with behavioural plasticity

Conspecifics of the same age and size differ consistently in the pace with which they expend energy. This among-individual variation in metabolic rate is thought to influence behavioural variation, since differences in energy requirements should motivate behaviours that facilitate energy acquisition, such as being bold or active in foraging. While there is evidence for links between metabolic rate and behaviour in constant environments, we know little about whether metabolic rate and behaviour change together when the environment changes-that is, if metabolic and behavioural plasticity co-vary. We investigated this using a fish that becomes dormant in winter and strongly reduces its activity when the environment cools, the cunner (Tautogolabrus adspersus). We found strong and predictable among-individual variation in thermal plasticity of metabolic rates, from resting to maximum levels, but no evidence for among-individual variation in thermal plasticity of movement activity, meaning that these key physiological and behavioural traits change independently when the environment changes. The strong among-individual variation in metabolic rate plasticity resulted in much higher repeatability (among-individual consistency) of metabolic rates at warm than cold temperatures, indicating that the potential for metabolic rate to evolve under selection is temperature-dependent, as repeatability can set the upper limit to heritability. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Intergenerational plasticity aligns with temperature-dependent selection on offspring metabolic rates

Metabolic rates are linked to key life-history traits that are thought to set the pace of life and affect fitness, yet the role that parents may have in shaping the metabolism of their offspring to enhance survival remains unclear. Here, we investigated the effect of temperature (24°C or 30°C) and feeding frequency experienced by parent zebrafish (Danio rerio) on offspring phenotypes and early survival at different developmental temperatures (24°C or 30°C). We found that embryo size was larger, but survival lower, in offspring from the parental low food treatment. Parents exposed to the warmer temperature and lower food treatment also produced offspring with lower standard metabolic rates-aligning with selection on embryo metabolic rates. Lower metabolic rates were correlated with reduced developmental and growth rates, suggesting selection for a slow pace of life. Our results show that intergenerational phenotypic plasticity on offspring size and metabolic rate can be adaptive when parent and offspring temperatures are matched: the direction of selection on embryo size and metabolism aligned with intergenerational plasticity towards lower metabolism at higher temperatures, particularly in offspring from low-condition parents. These findings provide evidence for adaptive parental effects, but only when parental and offspring environments match. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Individual variation in thermally induced plasticity of metabolic rates: ecological and evolutionary implications for a warming world

Energy metabolism is a fundamental property of life providing the energy for all processes and functions within an organism. As it is temperature-dependent, it mediates the effects of changing climate on ectotherm fitness and population dynamics. Though resting metabolic rate is a highly labile trait, part of its variation is individually consistent. Recent findings show that resting metabolic rate contains consistent variation not only in the elevations (intercepts) but also in the slopes of individual thermal dependence curves, challenging the thermal dependence assumption for this trait in several ectotherm taxa. I argue that among-individual variation in thermal metabolic curves represents a previously undetected component of ectotherm response to climate change, potentially affecting their adaptive capacity and population resilience under increasing stochasticity of thermal environment. Future studies need to examine not only the amount of among-individual variation in thermal metabolic curves across phylogenetic contexts but also other aspects concerning its mechanisms and adaptive significance to improve predictions about the impact of climate change on ectotherm population dynamics. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Temperature and nutrition do not interact to shape the evolution of metabolic rate

Metabolic cold adaptation, or Krogh's rule, is the controversial hypothesis that predicts a monotonically negative relationship between metabolic rate and environmental temperature for ectotherms living along thermal clines measured at a common temperature. Macrophysiological patterns consistent with Krogh's rule are not always evident in nature, and experimentally evolved responses to temperature have failed to replicate such patterns. Hence, temperature may not be the sole driver of observed variation in metabolic rate. We tested the hypothesis that temperature, as a driver of energy demand, interacts with nutrition, a driver of energy supply, to shape the evolution of metabolic rate to produce a pattern resembling Krogh's rule. To do this, we evolved replicate lines of Drosophila melanogaster at 18, 25 or 28°C on control, low-calorie or low-protein diets. Contrary to our prediction, we observed no effect of nutrition, alone or interacting with temperature, on adult female and male metabolic rates. Moreover, support for Krogh's rule was only in females at lower temperatures. We, therefore, hypothesize that observed variation in metabolic rate along environmental clines arises from the metabolic consequences of environment-specific life-history optimization, rather than because of the direct effect of temperature on metabolic rate. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Modeling heterothermic fitness landscapes in a marsupial hibernator using changes in body composition

Hibernation is an adaptive strategy that allows animals to enter a hypometabolic state, conserving energy and enhancing their fitness by surviving harsh environmental conditions. However, addressing the adaptive value of hibernation, at the individual level and in natural populations, has been challenging. Here, we applied a non-invasive technique, body composition analysis by quantitative magnetic resonance (qMR), to calculate energy savings by hibernation in a population of hibernating marsupials (Dromiciops gliroides). Using outdoor enclosures installed in a temperate rainforest, and measuring qMR periodically, we determined the amount of fat and lean mass consumed during a whole hibernation cycle. With this information, we estimated the daily energy expenditure of hibernation (DEEH) at the individual level and related to previous fat accumulation. Using model selection approaches and phenotypic selection analysis, we calculated linear (directional, β), quadratic (stabilizing or disruptive, γ) and correlational (ρ) coefficients for DEEH and fat accumulation. We found significant, negative directional selection for DEEH (βDEEH = - 0.58 ± 0.09), a positive value for fat accumulation (βFAT = 0.34 ± 0.07), and positive correlational selection between both traits (ρDEEH × FAT = 0.24 ± 0.07). Then, individuals maximizing previous fat accumulation and minimizing DEEH were promoted by selection, which is visualized by a bi-variate selection surface estimated by generalized additive models. At the comparative level, results fall within the isometric allometry known for hibernation metabolic rate in mammals. Thus, by a combination of a non-invasive technique for body composition analysis and semi-natural enclosures, we were characterized the heterothermic fitness landscape in a semi-natural population of hibernators.

Metabolic rate and ecological traits of ectoparasites: a case study with seven flea species from the Negev Desert

We studied the relationship between fleas' metabolic rate and their ecological traits, using data on standard metabolic rate (SMR), mean abundance, host specificity, and geographic range size in males and females of seven desert flea species. SMR was measured via mass-specific CO2 emission, whereas host specificity was measured as (a) the mean number of host species used by a flea per region in regions where this flea was recorded; (b) the total number of host species a flea exploited across its geographic range; and (c) the phylogenetic diversity of the flea's hosts. To control for confounding effects of phylogeny when analysing data on multiple species, we applied the Phylogenetic Generalised Least Squares (PGLS) model. We found that the only ecological trait significantly correlating with flea SMR was the phylogenetic diversity of hosts utilized by a flea across its geographic range. The strength of the association between SMR and host phylogenetic diversity was higher in male than in female fleas. We explain the relationship between flea SMR and their host specificity by the necessity of host-opportunistic species to compensate for the high energetic cost of neutralizing multiple defences from multiple hosts by increased SMR.

The Evolution of Offspring Size: a Metabolic Scaling Perspective

Size at the start of life reflects the initial per offspring parental investment - including both the embryo and the nutrients supplied to it. Initial offspring size can vary substantially both within and among species. Within species, increasing offspring size can enhance growth, reproduction, competitive ability, and reduce susceptibility to predation and starvation later in life, that can ultimately increase fitness. Previous work has suggested that the fitness benefits of larger offspring size may be driven by energy expenditure during development - or how offspring metabolic rate scales with offspring size. Despite the importance of early life energy expenditure in shaping later life fitness trajectories, consideration of among-species scaling of metabolic rate at the time of birth as a potential source of general metabolic scaling patterns has been overlooked by theory. Here we review the patterns and processes of energy expenditure at the start of life when mortality is often greatest. We compile existing data on metabolic rate and offspring size for 191 ectotherm species spanning eight phyla and use phylogenetically-controlled methods to quantify among-species scaling patterns. Across a 109-fold mass range, we find that offspring metabolic rate scales hypometrically with size, with an overall scaling exponent of 0.66. This exponent varies across ontogenetic stage and feeding activity, but is consistently hypometric, including across environmental temperatures. Despite differences in parental investment, life history and habitat, large-offspring species use relatively less energy as a proportion of size, compared with small-offspring species. Greater residual energy can be used to fuel the next stages of life, particularly in low resource environments. Based on available evidence, we conclude that, while large knowledge gaps remain, the evolution of offspring size is likely shaped by context-dependent selection acting on correlated traits, including metabolic rates maintaining hypometric scaling, that operates within broader physical constraints.

Energetic dissociation of individual and species ranges

The use of energy is universal to all life forms and all levels of biological organization, potentially linking processes operating at variable scales. Individual and species ranges might be energetically constrained, yet divergent metabolic limitations at both scales can disassociate these individual and species traits. We analysed comparative energetic and range data to unravel the mechanistic basis of the dissociation between individual and species range sizes observed among mammalian species. Our results demonstrate that basal, or maintenance, metabolism negatively correlates with individual ranges, but, at the same time, it positively correlates with species ranges. High aerobic capacity, i.e. maximum metabolic rate, positively correlates with individual ranges, but it is weakly related to species range size. These antagonistic energetic constraints on both ranges could lead to a disassociation between individual and species traits and to a low covariation between home and species range sizes. We show that important organismal functions, such as basal and maximum metabolic rates, have the potential to unravel mechanisms operating at different levels of biological organization and to expose links between energy-dependent processes at different scales.

Geographic variation and thermal plasticity shape salamander metabolic rates under current and future climates

Predicted changes in global temperature are expected to increase extinction risk for ectotherms, primarily through increased metabolic rates. Higher metabolic rates generate increased maintenance energy costs which are a major component of energy budgets. Organisms often employ plastic or evolutionary (e.g., local adaptation) mechanisms to optimize metabolic rate with respect to their environment. We examined relationships between temperature and standard metabolic rate across four populations of a widespread amphibian species to determine if populations vary in metabolic response and if their metabolic rates are plastic to seasonal thermal cues. Populations from warmer climates lowered metabolic rates when acclimating to summer temperatures as compared to spring temperatures. This may act as an energy saving mechanism during the warmest time of the year. No such plasticity was evident in populations from cooler climates. Both juvenile and adult salamanders exhibited metabolic plasticity. Although some populations responded to historic climate thermal cues, no populations showed plastic metabolic rate responses to future climate temperatures, indicating there are constraints on plastic responses. We postulate that impacts of warming will likely impact the energy budgets of salamanders, potentially affecting key demographic rates, such as individual growth and investment in reproduction.

Meta-analysis reveals that resting metabolic rate is not consistently related to fitness and performance in animals

Explaining variation in the fitness of organisms is a fundamental goal in evolutionary ecology. Maintenance energy metabolism is the minimum energy required to sustain biological processes at rest (resting metabolic rate: RMR) and is proposed to drive or constrain fitness of animals; however, this remains debated. Hypotheses have been proposed as to why fitness might increase with RMR (the 'increased intake' or 'performance' hypothesis), decrease with RMR (the 'compensation' or 'allocation' hypothesis), or vary among species and environmental contexts (the 'context dependent' hypothesis). Here, we conduct a systematic review and meta-analysis of the literature, finding 114 studies with 355 relationships between RMR and traits that may be related to fitness. We show that individuals with relatively high RMR generally have high fitness overall, which might be supported by an increased energy intake. However, fitness proxies are not interchangeable: the nature of the RMR-fitness relationship varied substantially depending on the specific trait in question, and we found no consistent relationship between RMR and those traits most closely linked with actual fitness (i.e., lifetime reproductive success). We hypothesise that maintaining high RMR is not costly when resources are unlimited, and we propose ideas for future studies to identify mechanisms underlying RMR-fitness relationships.

The Influence of Temperature on the Larval Development of Aelurostrongylus abstrusus in the Land Snail Cornu aspersum

The metastrongyloid Aelurostrongylus abstrusus has an indirect lifecycle involving gastropod intermediate hosts. The widespread snail Cornu aspersum is an efficient intermediate host of A. abstrusus. As the temperature may influence the developmental rate of metastrongyloids from first (L1) to the third infective larval stage (L3) inside molluscs, this study evaluated the effect of two controlled temperatures on the development of A. abstrusus in C. aspersum. Overall, 300 snails were infected with 500 L1 of A. abstrusus and kept at ∼25 °C. Fifteen days post infection (D15), the overall developmental rate to L3 (0.8%) was assessed in a subset of 20 snails. The remaining gastropods were divided in 2 groups, i.e., 180 still kept at ∼25 °C (G1) and 100 hibernated at ∼4 °C (G2). On D30, the larval development was evaluated in 20 snails from each group, while another batch of 80 snails was selected random from G1 and hibernated at ∼4 °C (G3). The larval developmental rate was determined digesting 20 snails from each of the three groups on D45, D60, and D75. The higher mean developmental rate was registered in G1 (3.8%) compared to G2 (1.9%) and G3 (2.3%), indicating that the development to L3 of A. abstrusus in C. aspersum is positively influenced by the increase of temperature.

Characterization of metapopulation of Ellobium chinense through Pleistocene expansions and four covariate COI guanine-hotspots linked to G-quadruplex conformation

The land snail Ellobium chinense (L. Pfeiffer, 1855) (Eupulmonata, Ellobiida, Ellobiidae), which inhabits the salt marshes along the coastal areas of northwestern Pacific, is an endangered species on the IUCN Red List. Over recent decades, the population size of E. chinense has consistently decreased due to environmental interference caused by natural disasters and human activities. Here, we provide the first assessment of the genetic diversity and population genetic structures of northwestern Pacific E. chinense. The results analyzed with COI and microsatellites revealed that E. chinense population exhibit metapopulation characteristics, retaining under the influence of the Kuroshio warm currents through expansion of the Late-Middle and Late Pleistocene. We also found four phylogenetic groups, regardless of geographical distributions, which were easily distinguishable by four unidirectional and stepwise adenine-to-guanine transitions in COI (sites 207–282–354–420: A–A–A–A, A–A–G–A, G–A–G–A, and G–G–G–G). Additionally, the four COI hotspots were robustly connected with a high degree of covariance between them. We discuss the role of these covariate guanines which link to form four consecutive G-quadruplexes, and their possible beneficial effects under positive selection pressure.

Phenotypic plasticity is not a cline: Thermal physiology of an intertidal barnacle over 20° of latitude

Our understanding of the plastic and evolutionary potential of ectothermic organisms and their populational impacts in the face of rapid global change remains limited. Studies attempting on the relationship between the magnitude of thermal variability across latitude and the degree of phenotypic plasticity exhibited by marine ectotherms are inconclusive. We state that the latter arises from the narrow range of thermal variability captured by the limited span of the latitudinal gradients studied to date. Using a mechanistic ecophysiological approach and a satellite-based assessment of the relevant environmental variables (i.e. temperature and food availability), we studied individuals of the intertidal barnacle Jehlius cirratus from seven local populations widely spread along the Humboldt current system that spanning two biogeographic regions. At the same time, we synthesized published information on the local abundance of our study species across a total of 76 sites representing 20° of latitude, and spanning from 18 to 42°S. We examined the effects of latitude and environmental variability on metabolic rate plasticity, thermal tolerance (thermal breadth and thermal safety margins) and their impacts on the abundance of this widespread marine invertebrate. We demonstrate that the phenotypic plasticity of metabolic rate in J. cirratus populations is not related to latitude. In turn, thermal breadth is explained by the temperature variability each population experiences. Furthermore, we found clinal variation with a poleward decrease of the critical thermal minimum, suggesting that episodic extreme low temperatures represent a ubiquitous selective force on the lower thermal limit for ectotherms. Across our study gradient, plasticity patterns indicate that populations at the equatorial extreme are more vulnerable to a warming climate, while populations located in the biogeographic transitional zone (i.e. high environmental heterogeneity), on the centre of the gradient, display higher levels of phenotypic plasticity and may represent a genetic buffer for the effects of ocean warming. Together, our results suggest the existence of a fitness trade-off involving the metabolic cost of plasticity and population density that is evident only across the vast latitudinal gradient examined.

Fluctuating asymmetry, a marker of poor growth quality, is associated with adult male metabolic rate

OBJECTIVES

Life history theory, a branch of evolutionary theory, predicts the existence of trade-offs in energetic allocation between competing physiological functions. The core metabolic cost of self-maintenance, measured by resting metabolic rate (RMR), represents a large component of human daily energy expenditure. Despite strong selective pressures for energetic frugality and high observed interindividual variation in RMR, the link between RMR and energetic allocation to life-history traits remains understudied in humans.

MATERIALS

In a sample of 105 (m = 57, f = 48), we investigated the relationship between adult RMR and investment in growth quality, as measured by fluctuating asymmetry (FA).

RESULTS

Measurement of RMR and FA in university rowers revealed a significant positive correlation amongst males (n = 57, r = 0.344, p = 0.005, 1-tailed; standardized 95% CI, 0.090 to 0.598). Convincing evidence for a correlation among females was not found (n = 48, r = 0.142, p = 0.169, 1-tailed, standardized 95% CI, -0.152 to 0.435).

DISCUSSION

The data suggest that low-quality asymmetrical growth is associated with later-life metabolic inefficiencies in males. Energetic investment in processes (likely concerning the stress-response) unrelated to growth during childhood may thereby trade-off against adult metabolic efficiency. We suggest that the presence of a relationship between RMR and FA in males but not females may be explained by the additional metabolic strain associated with larger body size and increased male muscularity, which may amplify the inefficiencies arising from low-quality growth.

Temperature dependence of metabolic rate in tropical and temperate aquatic insects: Support for the Climate Variability Hypothesis in mayflies but not stoneflies

A fundamental gap in climate change vulnerability research is an understanding of the relative thermal sensitivity of ectotherms. Aquatic insects are vital to stream ecosystem function and biodiversity but insufficiently studied with respect to their thermal physiology. With global temperatures rising at an unprecedented rate, it is imperative that we know how aquatic insects respond to increasing temperature and whether these responses vary among taxa, latitudes, and elevations. We evaluated the thermal sensitivity of standard metabolic rate in stream-dwelling baetid mayflies and perlid stoneflies across a ~2,000 m elevation gradient in the temperate Rocky Mountains in Colorado, USA, and the tropical Andes in Napo, Ecuador. We used temperature-controlled water baths and microrespirometry to estimate changes in oxygen consumption. Tropical mayflies generally exhibited greater thermal sensitivity in metabolism compared to temperate mayflies; tropical mayfly metabolic rates increased more rapidly with temperature and the insects more frequently exhibited behavioral signs of thermal stress. By contrast, temperate and tropical stoneflies did not clearly differ. Varied responses to temperature among baetid mayflies and perlid stoneflies may reflect differences in evolutionary history or ecological roles as herbivores and predators, respectively. Our results show that there is physiological variation across elevations and species and that low-elevation tropical mayflies may be especially imperiled by climate warming. Given such variation among species, broad generalizations about the vulnerability of tropical ectotherms should be made more cautiously.

Quantifying selection on standard metabolic rate and body mass in Drosophila melanogaster

Standard metabolic rate (SMR), defined as the minimal energy expenditure required for self-maintenance, is a key physiological trait. Few studies have estimated its relationship with fitness, most notably in insects. This is presumably due to the difficulty of measuring SMR in a large number of very small individuals. Using high-throughput flow-through respirometry and a Drosophila melanogaster laboratory population adapted to a life cycle that facilitates fitness measures, we quantified SMR, body mass, and fitness in 515 female and 522 male adults. We used a novel multivariate approach to estimate linear and nonlinear selection differentials and gradients from the variance-covariance matrix of fitness, SMR, and body mass, allowing traits specific covariates to be accommodated within a single model. In males, linear selection differentials for mass and SMR were positive and individually significant. Selection gradients were also positive but, despite substantial sample sizes, were nonsignificant due to increased uncertainty given strong SMR-mass collinearity. In females, only nonlinear selection was detected and it appeared to act primarily on body size, although the individual gradients were again nonsignificant. Selection did not differ significantly between sexes although differences in the fitness surfaces suggest sex-specific selection as an important topic for further study.

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.

Metabolic traits in brown trout (Salmo trutta) vary in response to food restriction and intrinsic factors

Metabolic rates vary hugely within and between populations, yet we know relatively little about factors causing intraspecific variation. Since metabolic rate determines the energetic cost of life, uncovering these sources of variation is important to understand and forecast responses to environmental change. Moreover, few studies have examined factors causing intraspecific variation in metabolic flexibility. We explore how extrinsic environmental conditions and intrinsic factors contribute to variation in metabolic traits in brown trout, an iconic and polymorphic species that is threatened across much of its native range. We measured metabolic traits in offspring from two wild populations that naturally show life-history variation in migratory tactics (one anadromous, i.e. sea-migratory, one non-anadromous) that we reared under either optimal food or experimental conditions of long-term food restriction (lasting between 7 and 17 months). Both populations showed decreased standard metabolic rates (SMR-baseline energy requirements) under low food conditions. The anadromous population had higher maximum metabolic rate (MMR) than the non-anadromous population, and marginally higher SMR. The MMR difference was greater than SMR and consequently aerobic scope (AS) was higher in the anadromous population. MMR and AS were both higher in males than females. The anadromous population also had higher AS under low food compared to optimal food conditions, consistent with population-specific effects of food restriction on AS. Our results suggest different components of metabolic rate can vary in their response to environmental conditions, and according to intrinsic (population-background/sex) effects. Populations might further differ in their flexibility of metabolic traits, potentially due to intrinsic factors related to life history (e.g. migratory tactics). More comparisons of populations/individuals with divergent life histories will help to reveal this. Overall, our study suggests that incorporating an understanding of metabolic trait variation and flexibility and linking this to life history and demography will improve our ability to conserve populations experiencing global change.

The standard metabolic rate of a land snail (Cepaea hortensis) is a repeatable trait and influences winter survival

Phenotypic selection on physiological parameters is an underrepresented topic in studies of evolutionary biology. There is especially a lack of studies involving invertebrate organisms. We studied the repeatability of the standard metabolic rate (SMR) and the effect of individual variation in SMR on the subsequent winter survival in a terrestrial shell-bearing mollusc, the white-lipped snail (Cepaea hortensis) in mid-Norway. SMR was measured twice during the autumn and - after an experimental overwintering at controlled conditions - twice during the following spring. We found a significant repeatability of SMR over all three time periods tested, with a clear effect of time, with a high repeatability of 0.56 over 4 days during spring, 0.44 over 12 days in the autumn and 0.17 over 194 days from autumn to spring. That SMR is a repeatable physiological trait across the winter period during which a possible selection might occur, suggests that SMR could be a potential target of natural selection. We indeed found that the autumn SMR significantly influenced the probability of survival during the winter period, with a combination of a positive linear (P = .011) and a quadratic stabilizing (P = .001) effect on SMR. Our results hence support the view that metabolic rate is an important physiological component influencing the fitness of an organism.

The effect of the hibernation on the larval development of Troglostrongylus brevior in the land snail Cornu aspersum

Troglostrongylus brevior, a lungworm affecting wild felids, has been increasingly reported in domestic cats from Europe. Troglostrongylosis is a relevant disease that may result in a potentially life-threatening bronchopneumonia, especially in kittens. The life cycle of T. brevior is indirect with terrestrial gastropods acting as intermediate host. The widely distributed spread land snail Cornu aspersum (former Helix aspersa) is competent for T. brevior development and may transmit the nematode in natural conditions. The present study evaluated the larval development of T. brevior in C. aspersum at two different environmental temperature conditions, with a focus on the effect of hibernation. One hundred and seventy snails were infected with 500 first stage larvae (L1) of T. brevior and kept in vivaria at 25 ± 2 °C. Fifteen days post infection (p.i.), 20 specimens were digested to evaluate the overall larval developmental rate from L1 to L3 (2.5 % on days 15 p.i.) and then the snails were divided in two groups, i.e. G1 kept at 25 ± 2 °C and G2 that were hibernated at 4 ± 2 °C. The developmental rate of T. brevior was evaluated in these groups on 30 and 60 days p.i. by snail digestion at each time-point. An additional batch of 40 snails (G2-1) was hibernated on D15 and digested on D60. Larvae recovered were morphologically and morphometrically examined. The infective third larval stage (L3) was detected in the muscular foot of C. aspersum at different rates depending on the environmental temperature. In particular, T. brevior showed a higher developmental rate in hibernated snails (G2: 6.9 % and 14.1 % on days 30 and 60 p.i; G2-1: 4%; G2 + G2-1 overall mean percentage: 9%) compared to non-hibernated snails (G1: 4% and 5.2 % on days 30 and 60 p.i.), indicating that lower temperatures may positively influence the developmental in C. aspersum. These data are suggestive for a seasonal pattern of T. brevior infections under field conditions, with snails containing higher parasitic burdens after their natural hibernation occurring in winter. Studies on the larval development of T. brevior in other mollusc species in field surveys evaluating differences in developmental rates and transmission patterns in different seasons are warranted.

Ecological and evolutionary consequences of metabolic rate plasticity in response to environmental change

Basal or standard metabolic rate reflects the minimum amount of energy required to maintain body processes, while the maximum metabolic rate sets the ceiling for aerobic work. There is typically up to three-fold intraspecific variation in both minimal and maximal rates of metabolism, even after controlling for size, sex and age; these differences are consistent over time within a given context, but both minimal and maximal metabolic rates are plastic and can vary in response to changing environments. Here we explore the causes of intraspecific and phenotypic variation at the organ, tissue and mitochondrial levels. We highlight the growing evidence that individuals differ predictably in the flexibility of their metabolic rates and in the extent to which they can suppress minimal metabolism when food is limiting but increase the capacity for aerobic metabolism when a high work rate is beneficial. It is unclear why this intraspecific variation in metabolic flexibility persists-possibly because of trade-offs with the flexibility of other traits-but it has consequences for the ability of populations to respond to a changing world. It is clear that metabolic rates are targets of selection, but more research is needed on the fitness consequences of rates of metabolism and their plasticity at different life stages, especially in natural conditions. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

Chronic elevation of glucorticoids late in life generates long lasting changes in physiological state without a life history switch

Chronic stressors have profound impacts on phenotypes and life history strategies on the short term, but delayed effects of stress experienced late in life remain poorly investigated in wild populations. Here, we used a combined laboratory and field experiment to test if chronic stress late in life has immediate and delayed effects on physiological and demographic traits in the common lizard, Zootoca vivipara. We increased plasma corticosterone levels in adults and yearlings during three weeks of the post-reproductive season. We quantified immediate responses in the laboratory, delayed intra-generational effects in field enclosures one month and one year later during the next reproductive season, and delayed inter-generational effects in the first generation of offspring. Our phenotypic assays included metabolism, immune capacities, lipid metabolism and oxidative stress. Relative to placebos, lizards treated with corticosterone had higher body condition and lower oxidative damages but an increased skin swelling response directly after the manipulation. Delayed responses in field enclosures were of three types. First, we found catch-up growth for body mass such the placebos had similar body conditions one month after the laboratory manipulation. Second, we found persistent differences in oxidative damages during one month but not one year later. Third, during the next reproductive season, corticosterone-treated females had higher levels of plasma triglycerides, whereas corticosterone-treated individuals had a higher skin swelling response. We found no delayed inter-generational effects on demographic traits of offspring. Our study demonstrates the potential for long-lasting physiological consequences of chronic corticosterone enhancement despite no obvious changes in life history.

Snails in the sun: Strategies of terrestrial gastropods to cope with hot and dry conditions

Terrestrial gastropods do not only inhabit humid and cool environments but also habitat in which hot and dry conditions prevail. Snail species that are able to cope with such climatic conditions are thus expected to having developed multifaceted strategies and mechanisms to ensure their survival and reproduction under heat and desiccation stress. This review paper aims to provide an integrative overview of the numerous adaptation strategies terrestrial snails have evolved to persist in hot and dry environments as well as their mutual interconnections and feedbacks, but also to outline research gaps and questions that remained unanswered. We extracted relevant information from more than 140 publications in order to show how biochemical, cellular, physiological, morphological, ecological, thermodynamic, and evolutionary parameters contribute to provide an overall picture of this classical example in stress ecology. These mechanisms range from behavioral and metabolic adaptations, including estivation, to the induction of chaperones and antioxidant enzymes, mucocyte and digestive gland cell responses and the modification and frequency of morphological features, particularly shell pigmentation. In this context, thermodynamic constraints call for processes of complex adaptation at varying levels of biological organization that are mutually interwoven. We were able to assemble extensive, mostly narrowly focused information from the literature into a web of network parameters, showing that future work on this subject requires multicausal thinking to account for the complexity of relationships involved in snails' adaptation to insolation, heat, and drought.

Thermal physiology and thermoregulatory behaviour exhibit low heritability despite genetic divergence between lizard populations

Ectothermic species are particularly sensitive to changes in temperature and may adapt to changes in thermal environments through evolutionary shifts in thermal physiology or thermoregulatory behaviour. Nevertheless, the heritability of thermal traits, which sets a limit on evolutionary potential, remains largely unexplored. In this study, we captured brown anole lizards (Anolis sagrei) from two populations that occur in contrasting thermal environments. We raised offspring from these populations in a laboratory common garden and compared the shape of their thermal performance curves to test for genetic divergence in thermal physiology. Thermal performance curves differed between populations in a common garden in ways partially consistent with divergent patterns of natural selection experienced by the source populations, implying that they had evolved in response to selection. Next, we estimated the heritability of thermal performance curves and of several traits related to thermoregulatory behaviour. We did not detect significant heritability in most components of the thermal performance curve or in several aspects of thermoregulatory behaviour, suggesting that contemporary selection is unlikely to result in rapid evolution. Our results indicate that the response to selection may be slow in the brown anole and that evolutionary change is unlikely to keep pace with current rates of environmental change.

Mapping of quantitative trait loci for life history traits segregating within common frog populations

The evolution of complex traits is often shaped by adaptive divergence. However, very little is known about the number, effect size, and location of the genomic regions influencing the variation of these traits in natural populations. Based on a dense linkage map of the common frog, Rana temporaria, we have localized, for the first time in amphibians, three significant and nine suggestive quantitative trait loci (QTLs) for metabolic rate, growth rate, development time, and weight at metamorphosis, explaining 5.6-18.9% of the overall phenotypic variation in each trait. We also found a potential pleiotropic QTL between development time and size at metamorphosis that, if confirmed, might underlie the previously reported genetic correlation between these traits. Furthermore, we demonstrate that the genetic variation linked to fitness-related larval traits segregates within Rana temporaria populations. This study provides the first insight into the genomic regions that affect larval life history traits in anurans, providing a valuable resource to delve further into the genomic basis of evolutionary change in amphibians.

Developmental stage does not affect resting metabolic rate in the monitor lizard, Varanus salvator

We have studied resting metabolic rate (RMR) of the water monitor lizard (Varanus salvator) at different developmental stages (hatchling, juvenile and adult) to test whether individuals at different ages differ in RMR when controlling for the effects of body mass. We found that: 1) resting metabolic rates of hatchlings, juveniles and adults were all positively related to their body mass with the same coefficients and that 2) developmental stage had a non-significant influence on the resting metabolic rate when controlling for the effects of body mass. Our results suggest that variation in resting metabolic rate for V. salvator is directly caused by body mass differences, which conforms to previous findings in mammal species and birds.

Discovering divergence in the thermal physiology of intertidal crabs along latitudinal gradients using an integrated approach with machine learning

In intertidal marine crustaceans, phenotypic variation in physiological and life-history traits is pervasive along latitudinal clines. However, organisms have complex phenotypes, and their traits do not vary independently but rather interact differentially between them, effect that is caused by genetic and/or environmental forces. We evaluated the geographic variation in phenotypic integration of three marine crab species that inhabit different vertical thermal microhabitats of the intertidal zone. We studied seven populations of each species along a latitudinal gradient that spans more than 3000 km of the Chilean coast. Specifically we measured nine physiological traits that are highly related to thermal physiology. Of the nine traits, we selected four that contributed significantly to the observed geographical variation among populations; this variation was then evaluated using mixed linear models and an integrative approach employing machine learning. The results indicate that patterns of physiological variation depend on species vertical microhabitat, which may be subject to chronic or acute environmental variation. The species that inhabit the high- intertidal sites (i.e., exposed to chronic variation) better tolerated thermal stress compared with populations that inhabit the lower intertidal. While those in the low-intertidal only face conditions of acute thermal variation, using to a greater extent the plasticity to face these events. Our main results reflect that (1) species that inhabit the high-intertidal maintain a greater integration between their physiological traits and present lower plasticity than those that inhabit the low-intertidal. (2) Inverse relationship that exists between phenotypic plasticity and phenotypic integration of the physiological traits identified, which could help optimize energy resources. In general, the study of multiple physiological traits provides a more accurate picture of how the thermal traits of organisms vary along temperature gradients especially when exposed to conditions close to tolerance limits.

Natural selection on plasticity of thermal traits in a highly seasonal environment

For ectothermic species with broad geographical distributions, latitudinal/altitudinal variation in environmental temperatures (averages and extremes) is expected to shape the evolution of physiological tolerances and the acclimation capacity (i.e., degree of phenotypic plasticity) of natural populations. This can create geographical gradients of selection in which environments with greater thermal variability (e.g., seasonality) tend to favor individuals that maximize performance across a broader range of temperatures compared to more stable environments. Although thermal acclimation capacity plays a fundamental role in this context, it is unknown whether natural selection targets this trait in natural populations. Additionally, understanding whether and how selection acts on thermal physiological plasticity is also highly relevant to climate change and biological conservation. Here, we addressed such an important gap in our knowledge in the northernmost population of the four-eyed frog, Pleurodema thaul. We measured plastic responses of critical thermal limits for activity, behavioral thermal preference, and thermal sensitivity of metabolism to acclimation at 10 and 20°C. We monitored survival during three separate recapture efforts and used mark-recapture integrated into an information-theoretic approach to evaluate the relationship between survivals as a function of the plasticity of thermal traits. Overall, we found no evidence that thermal acclimation in this population is being targeted by directional selection, although there might be signals of selection on individual traits. According to the most supported models, survival increased in individuals with higher tolerance to cold when cold-acclimated, probably because daily low extremes are frequent during the cooler periods of the year. Furthermore, survival increased with body size. However, in both cases, the directional selection estimates were nonsignificant, and the constraints of our experimental design prevented us from evaluating more complex models (i.e., nonlinear selection).

Shifts in space and time: ecological transitions affect the evolution of resting metabolic rates in microteiid lizards

Ecological diversification often encompasses exposure to new thermal regimes given by the use of specific spatial (microhabitat) and temporal (activity periods) niches. Empirical evidence provides links between temperature and physiology (e.g. rates of oxygen consumption), fostering predictions of evolutionary changes in metabolic rates coupled with ecological shifts. One example of such correspondence is the evolution of fossoriality and nocturnality in vertebrate ectotherms, where changes in metabolic rates coupled with niche transitions are expected. Because most studies address single transitions (fossoriality or nocturnality), metabolic changes associated with concomitant shifts in spatial and temporal components of habitat usage are underestimated, and it remains unclear which transition plays a major role for metabolic evolution. Integrating multiple ecological aspects that affect the evolution of thermosensitive traits is essential for a proper understanding of physiological correlates in niche transitions. Here, we provide the first phylogenetic multidimensional description of effects from ecological niche transitions both in space (origin of fossorial lineages) and in time (origin of nocturnal lineages) on the evolution of microteiid lizard (Gymnophthalmidae) metabolic rates. We found that evolution of resting metabolic rates was affected by both niche transitions, but with opposite trends. Evolution of fossoriality in endemic diurnal microteiids is coupled with a less thermally sensitive metabolism and higher metabolic rates. In contrast, a reduction in metabolic rates was detected in the endemic fossorial-nocturnal lineage, although metabolic thermal sensitivity remained as high as that observed in epigeal species, a pattern that likely reduces locomotion costs at lower temperatures and also favors thermoregulation in subsuperficial sand layers.

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.

Selection on the morphology-physiology-performance nexus: Lessons from freshwater stickleback morphs

Conspecifics inhabiting divergent environments frequently differ in morphology, physiology, and performance, but the interrelationships amongst traits and with Darwinian fitness remains poorly understood. We investigated population differentiation in morphology, metabolic rate, and swimming performance in three-spined sticklebacks (Gasterosteus aculeatus L.), contrasting a marine/ancestral population with two distinct freshwater morphotypes derived from it: the "typical" low-plated morph, and a unique "small-plated" morph. We test the hypothesis that similar to plate loss in other freshwater populations, reduction in lateral plate size also evolved in response to selection. Additionally, we test how morphology, physiology, and performance have evolved in concert as a response to differences in selection between marine and freshwater environments. We raised pure-bred second-generation fish originating from three populations and quantified their lateral plate coverage, burst- and critical swimming speeds, as well as standard and active metabolic rates. Using a multivariate QST-FST framework, we detected signals of directional selection on metabolic physiology and lateral plate coverage, notably demonstrating that selection is responsible for the reduction in lateral plate coverage in a small-plated stickleback population. We also uncovered signals of multivariate selection amongst all bivariate trait combinations except the two metrics of swimming performance. Divergence between the freshwater and marine populations exceeded neutral expectation in morphology and in most physiological and performance traits, indicating that adaptation to freshwater habitats has occurred, but through different combinations of traits in different populations. These results highlight both the complex interplay between morphology, physiology and performance in local adaptation, and a framework for their investigation.

Metabolic rate covaries with fitness and the pace of the life history in the field

Metabolic rate reflects the 'pace of life' in every organism. Metabolic rate is related to an organism's capacity for essential maintenance, growth and reproduction-all of which interact to affect fitness. Although thousands of measurements of metabolic rate have been made, the microevolutionary forces that shape metabolic rate remain poorly resolved. The relationship between metabolic rate and components of fitness are often inconsistent, possibly because these fitness components incompletely map to actual fitness and often negatively covary with each other. Here we measure metabolic rate across ontogeny and monitor its effects on actual fitness (lifetime reproductive output) for a marine bryozoan in the field. We also measure key components of fitness throughout the entire life history including growth rate, longevity and age at the onset of reproduction. We found that correlational selection favours individuals with higher metabolic rates in one stage and lower metabolic rates in the other-individuals with similar metabolic rates in each developmental stage displayed the lowest fitness. Furthermore, individuals with the lowest metabolic rates lived for longer and reproduced more, but they also grew more slowly and took longer to reproduce initially. That metabolic rate is related to the pace of the life history in nature has long been suggested by macroevolutionary patterns but this study reveals the microevolutionary processes that probably generated these patterns.

Costly neighbours: Heterospecific competitive interactions increase metabolic rates in dominant species

The energy costs of self-maintenance (standard metabolic rate, SMR) vary substantially among individuals within a population. Despite the importance of SMR for understanding life history strategies, ecological sources of SMR variation remain only partially understood. Stress-mediated increases in SMR are common in subordinate individuals within a population, while the direction and magnitude of the SMR shift induced by interspecific competitive interactions is largely unknown. Using laboratory experiments, we examined the influence of con- and heterospecific pairing on SMR, spontaneous activity, and somatic growth rates in the sympatrically living juvenile newts Ichthyosaura alpestris and Lissotriton vulgaris. The experimental pairing had little influence on SMR and growth rates in the smaller species, L. vulgaris. Individuals exposed to con- and heterospecific interactions were more active than individually reared newts. In the larger species, I. alpestris, heterospecific interactions induced SMR to increase beyond values of individually reared counterparts. Individuals from heterospecific pairs and larger conspecifics grew faster than did newts in other groups. The plastic shift in SMR was independent of the variation in growth rate and activity level. These results reveal a new source of individual SMR variation and potential costs of co-occurrence in ecologically similar taxa.

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.

Phenotypic plasticity in the common garden snail: big guts and heavier mucus glands compete in snails faced with the dual challenge of poor diet and coarse substrate

Phenotypic plasticity allows animals to manage environmental challenges. Studies aimed at quantifying plasticity often focus on one challenge, such as diet, and one organ system, such the gastrointestinal tract, but this approach may not adequately reflect how plasticity could buffer multiple challenges. Thus, we investigated the outcomes of a dual challenge experiment that fed land snails either a high-fibre (low quality) or a low-fibre (high quality) diet, and simultaneously exercised them daily over 1.2 m on either a smooth surface of polyvinyl chloride (PVC) or a rough sandpaper. By the end of 20 days, snails fed the poor quality diet had a longer crop and oesophagus and a heavier intestine and rectum than those offered a low-fibre diet. Additionally, high-fibre fed snails had a smaller spermoviduct and oviduct. When also exercised on sandpaper, high-fibre fed snails had a smaller digestive gland, a main energy store, than those exercised on PVC. All snails exercised on sandpaper had a heavier pedal mucus gland, used a loping gait and used less mucus than those on PVC plastic, but there was no difference in the average speed of snails on either surface, supporting the conclusion that loping is a mucus conserving gait. Notably, snails faced with both a diet and substrate challenge had a smaller kidney, which could directly effect fecundity. This demonstrates that our dual challenge approach has potential for evaluating the costs and limits of the plasticity necessary to fully appreciate the evolutionary significance of plasticity in snails and other species.

Studying the evolutionary significance of thermal adaptation in ectotherms: The diversification of amphibians' energetics

A fundamental problem in evolutionary biology is the understanding of the factors that promote or constrain adaptive evolution, and assessing the role of natural selection in this process. Here, comparative phylogenetics, that is, using phylogenetic information and traits to infer evolutionary processes has been a major paradigm . In this study, we discuss Ornstein-Uhlenbeck models (OU) in the context of thermal adaptation in ectotherms. We specifically applied this approach to study amphibians's evolution and energy metabolism. It has been hypothesized that amphibians exploit adaptive zones characterized by low energy expenditure, which generate specific predictions in terms of the patterns of diversification in standard metabolic rate (SMR). We complied whole-animal metabolic rates for 122 species of amphibians, and adjusted several models of diversification. According to the adaptive zone hypothesis, we expected: (1) to find "accelerated evolution" in SMR (i.e., diversification above Brownian Motion expectations, BM), (2) that a model assuming evolutionary optima (i.e., an OU model) fits better than a white-noise model and (3) that a model assuming multiple optima (according to the three amphibians's orders) fits better than a model assuming a single optimum. As predicted, we found that the diversification of SMR occurred most of the time, above BM expectations. Also, we found that a model assuming an optimum explained the data in a better way than a white-noise model. However, we did not find evidence that an OU model with multiple optima fits the data better, suggesting a single optimum in SMR for Anura, Caudata and Gymnophiona. These results show how comparative phylogenetics could be applied for testing adaptive hypotheses regarding history and physiological performance in ectotherms.

Stress-induced peak (but not resting) metabolism correlates with mating display intensity in male guppies

Recent empirical and conceptual papers have highlighted the potential for metabolism to act as a proximate mechanism for behavior that could explain animal personality (consistency over time). Under this hypothesis, individuals with consistently high levels of behavioral activity should also have high resting metabolic rate (RMR) as it can reflect capacity to process food and generate energy. We tested for the predicted positive covariance between RMR and three behaviors that differ in energy demands in 30 male guppies, using multivariate mixed models; we repeatedly measured their activity (10 times each), courtship displays (nine times), voracity (10 times), and metabolism (four‐times). Resting metabolic rate (measured overnight in respirometry trials) did not consistently differ among males, whereas initial peak metabolism measured during those same trials (R = 0.42), and all behaviors were repeatable (R = 0.33–0.51). RMR declined over time suggesting habituation to the protocol, whereas peak metabolism did not. Initial peak metabolism was negatively correlated with courtship display intensity, and voracity was positively correlated with activity, but all other among‐individual correlations were not significant. We conclude that RMR does not provide a proximate explanation for consistent individual differences in behavior in male guppies, and therefore the potential for independent evolution of these physiological and behavioral traits seems possible. Finally, we identify peak metabolism as a potential measure of the stress response to confinement, which highlights the value of considering various aspects of metabolic rates recording during respirometry trials.

Associations between Resting, Activity, and Daily Metabolic Rate in Free-Living Endotherms: No Universal Rule in Birds and Mammals

Energy management models provide theories and predictions for how animals manage their energy budgets within their energetic constraints, in terms of their resting metabolic rate (RMR) and daily energy expenditure (DEE). Thus, uncovering what associations exist between DEE and RMR is key to testing these models. Accordingly, there is considerable interest in the relationship between DEE and RMR at both inter- and intraspecific levels. Interpretation of the evidence for particular energy management models is enhanced by also considering the energy spent specifically on costly activities (activity energy expenditure [AEE] = DEE - RMR). However, to date there have been few intraspecific studies investigating such patterns. Our aim was to determine whether there is a generality of intraspecific relationships among RMR, DEE, and AEE using long-term data sets for bird and mammal species. For mammals, we use minimum heart rate (fH), mean fH, and activity fH as qualitative proxies for RMR, DEE, and AEE, respectively. For the birds, we take advantage of calibration equations to convert fH into rate of oxygen consumption in order to provide quantitative proxies for RMR, DEE, and AEE. For all 11 species, the DEE proxy was significantly positively correlated with the RMR proxy. There was also evidence of a significant positive correlation between AEE and RMR in all four mammal species but only in some of the bird species. Our results indicate there is no universal rule for birds and mammals governing the relationships among RMR, AEE, and DEE. Furthermore, they suggest that birds tend to have a different strategy for managing their energy budgets from those of mammals and that there are also differences in strategy between bird species. Future work in laboratory settings or highly controlled field settings can tease out the environmental and physiological processes contributing to variation in energy management strategies exhibited by different species.

Measuring Selection on Physiology in the Wild and Manipulating Phenotypes (in Terrestrial Nonhuman Vertebrates)

To understand why organisms function the way that they do, we must understand how evolution shapes physiology. This requires knowledge of how selection acts on physiological traits in nature. Selection studies in the wild allow us to determine how variation in physiology causes variation in fitness, revealing how evolution molds physiology over evolutionary time. Manipulating phenotypes experimentally in a selection study shifts the distribution of trait variation in a population to better explore potential constraints and the adaptive value of physiological traits. There is a large database of selection studies in the wild on a variety of traits, but very few of those are physiological traits. Nevertheless, data available so far suggest that physiological traits, including metabolic rate, thermal physiology, whole-organism performance, and hormone levels, are commonly subjected to directional selection in nature, with stabilizing and disruptive selection less common than predicted if physiological traits are optimized to an environment. Selection studies on manipulated phenotypes, including circulating testosterone and glucocorticoid levels, reinforce this notion, but reveal that trade-offs between survival and reproduction or correlational selection can constrain the evolution of physiology. More studies of selection on physiological traits in nature that quantify multiple traits are necessary to better determine the manner in which physiological traits evolve and whether different types of traits (dynamic performance vs. regulatory) evolve differently.

Thermal tolerance and climate warming sensitivity in tropical snails

Tropical ectotherms are predicted to be especially vulnerable to climate change because their thermal tolerance limits generally lie close to current maximum air temperatures. This prediction derives primarily from studies on insects and lizards and remains untested for other taxa with contrasting ecologies. We studied the HCT (heat coma temperatures) and ULT (upper lethal temperatures) of 40 species of tropical eulittoral snails (Littorinidae and Neritidae) inhabiting exposed rocky shores and shaded mangrove forests in Oceania, Africa, Asia and North America. We also estimated extremes in animal body temperature at each site using a simple heat budget model and historical (20 years) air temperature and solar radiation data. Phylogenetic analyses suggest that HCT and ULT exhibit limited adaptive variation across habitats (mangroves vs. rocky shores) or geographic locations despite their contrasting thermal regimes. Instead, the elevated heat tolerance of these species (HCT = 44.5 ± 1.8°C and ULT = 52.1 ± 2.2°C) seems to reflect the extreme temperature variability of intertidal systems. Sensitivity to climate warming, which was quantified as the difference between HCT or ULT and maximum body temperature, differed greatly between snails from sunny (rocky shore; Thermal Safety Margin, TSM = -14.8 ± 3.3°C and -6.2 ± 4.4°C for HCT and ULT, respectively) and shaded (mangrove) habitats (TSM = 5.1 ± 3.6°C and 12.5 ± 3.6°C). Negative TSMs in rocky shore animals suggest that mortality is likely ameliorated during extreme climatic events by behavioral thermoregulation. Given the low variability in heat tolerance across species, habitat and geographic location account for most of the variation in TSM and may adequately predict the vulnerability to climate change. These findings caution against generalizations on the impact of global warming across ectothermic taxa and highlight how the consideration of nonmodel animals, ecological transitions, and behavioral responses may alter predictions of studies that ignore these biological details.

Physiological effects of compensatory growth during the larval stage of the ladybird, Cryptolaemus montrouzieri

The growth rate of insects may vary in response to shifty environments. They may achieve compensatory growth after a period of food restriction followed by ad libitum food, which may further affect the reproductive performance and lifespan of the resulting phenotypes. However, little is known about the physiological mechanisms associated with such growth acceleration in insects. The present study examined the metabolic rate, the antioxidant enzyme activity and the gene expression of adult Cryptolaemus montrouzieri (Coleoptera: Coccinellidae) after experiencing compensatory growth during its larval stages. Starved C. montrouzieri individuals achieved a similar developmental time and adult body mass as those supplied with ad libitum food during their entire larval stage, indicating that compensatory growth occurred as a result of the switch in larval food regime. Further, the compensatory growth was found to exert effects on the physiological functions of C. montrouzieri, in terms of its metabolic rates and enzyme activities. The adults undergoing compensatory growth were characterized by a higher metabolic rate, a lower activity of the antioxidant enzymes glutathione reductase, catalase, and superoxide dismutase and a lower gene expression of P450 and trehalase. Taken together, the results indicate that although compensatory growth following food restriction in early larval life prevents developmental delay and body mass loss, the resulting adults may encounter physiological challenges affecting their fitness.

Quantification of correlational selection on thermal physiology, thermoregulatory behavior, and energy metabolism in lizards

Phenotypic selection is widely accepted as the primary cause of adaptive evolution in natural populations, but selection on complex functional properties linking physiology, behavior, and morphology has been rarely quantified. In ectotherms, correlational selection on thermal physiology, thermoregulatory behavior, and energy metabolism is of special interest because of their potential coadaptation. We quantified phenotypic selection on thermal sensitivity of locomotor performance (sprint speed), thermal preferences, and resting metabolic rate in captive populations of an ectothermic vertebrate, the common lizard, Zootoca vivipara. No correlational selection between thermal sensitivity of performance, thermoregulatory behavior, and energy metabolism was found. A combination of high body mass and resting metabolic rate was positively correlated with survival and negatively correlated with fecundity. Thus, different mechanisms underlie selection on metabolism in lizards with small body mass than in lizards with high body mass. In addition, lizards that selected the near average preferred body temperature grew faster that their congeners. This is one of the few studies that quantifies significant correlational selection on a proxy of energy expenditure and stabilizing selection on thermoregulatory behavior.