Habitat restoration weakens negative environmental effects on telomere dynamics.

Habitat quality can have far-reaching effects on organismal fitness, an issue of concern given the current scale of habitat degradation. Many temperate upland streams have reduced nutrient levels due to human activity. Nutrient restoration confers benefits in terms of invertebrate food availability and subsequent fish growth rates. Here we test whether these mitigation measures also affect the rate of cellular ageing of the fish, measured in terms of the telomeres that cap the ends of eukaryotic chromosomes. We equally distributed Atlantic salmon eggs from the same 30 focal families into 10 human-impacted oligotrophic streams in northern Scotland. Nutrient levels in five of the streams were restored by simulating the deposition of a small number of adult Atlantic salmon Salmo salar carcasses at the end of the spawning period, while five reference streams were left as controls. Telomere lengths and expression of the telomerase reverse transcriptase (TERT) gene that may act to lengthen telomeres were then measured in the young fish when 15 months old. While TERT expression was unrelated to any of the measured variables, telomere lengths were shorter in salmon living at higher densities and in areas with a lower availability of the preferred substrate (cobbles and boulders). However, the adverse effects of these habitat features were much reduced in the streams receiving nutrients. These results suggest that adverse environmental pressures are weakened when nutrients are restored, presumably because the resulting increase in food supply reduces levels of both competition and stress.

Late-stage pregnancy reduces upper thermal tolerance in a live-bearing fish.

Upper thermal limits are considered a key determinant of a population's ability to persist in the face of extreme heat events. However, these limits differ considerably among individuals within a population, and the mechanisms underlying this differential sensitivity are not well understood. Upper thermal tolerance in aquatic ectotherms is thought to be determined by a mismatch between oxygen supply and the increased metabolic demands associated with warmer waters. As such, tolerance is expected to decline during reproduction given the heightened oxygen demand for gamete production and maintenance. Among live-bearing species, upper thermal tolerance of reproductive adults may decline even further after fertilization due to the cost of meeting the increasing oxygen demands of developing embryos. We examined the upper thermal tolerance of live-bearing female Trinidadian guppies at different stages of reproduction and found that critical thermal maximum was similar during the egg yolking and early embryos stage but then declined by almost 0.5 °C during late pregnancy when oxygen demands are the greatest. These results are consistent with the hypothesis that oxygen limitation sets thermal limits and show that reproduction is associated with a decline in upper thermal tolerance.

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 Rate Interacts with Resource Availability to Determine Individual Variation in Microhabitat Use in the Wild.

Ecological pressures such as competition can lead individuals within a population to partition resources or habitats, but the underlying intrinsic mechanisms that determine an individual's resource use are not well understood. Here we show that an individual's own energy demand and associated competitive ability influence its resource use, but only when food is more limiting. We tested whether intraspecific variation in metabolic rate leads to microhabitat partitioning among juvenile Atlantic salmon (Salmo salar) in natural streams subjected to manipulated nutrient levels and subsequent per capita food availability. We found that individual salmon from families with a higher baseline (standard) metabolic rate (which is associated with greater competitive ability) tended to occupy faster-flowing water, but only in streams with lower per capita food availability. Faster-flowing microhabitats yield more food, but high metabolic rate fish only benefited from faster growth in streams with high food levels, presumably because in low-food environments the cost of a high metabolism offsets the benefits of acquiring a productive microhabitat. The benefits of a given metabolic rate were thus context dependent. These results demonstrate that intraspecific variation in metabolic rate can interact with resource availability to determine the spatial structuring of wild populations.

Upper thermal limits are repeatable in Trinidadian guppies.

Measurements of thermal tolerance are critical for predicting species vulnerability to climate change. Critical thermal maximum (CTmax) is a measure of an animal's upper thermal tolerance, but there is limited evidence for how repeatable it is within individuals over time. We measured the CTmax of Trinidadian guppies (Poecilia reticulata) across six consecutive trials, each a week apart. The repeatability of CTmax over six trials was 0.43 (0.26-0.62). However, CTmax also changed over time, ranging from 39.0 to 39.6 °C and increasing by 0.6 °C across the first four trials before leveling off. This is most likely the effect of heat hardening, indicating that thermal tolerance can increase after repeated exposure to extreme heat events.

Decreased mitochondrial metabolic requirements in fasting animals carry an oxidative cost.

Many animals experience periods of food shortage in their natural environment. It has been hypothesised that the metabolic responses of animals to naturally-occurring periods of food deprivation may have long-term negative impacts on their subsequent life-history.In particular, reductions in energy requirements in response to fasting may help preserve limited resources but potentially come at a cost of increased oxidative stress. However, little is known about this trade-off since studies of energy metabolism are generally conducted separately from those of oxidative stress.Using a novel approach that combines measurements of mitochondrial function with in vivo levels of hydrogen peroxide (H2O2) in brown trout (Salmo trutta), we show here that fasting induces energy savings in a highly metabolically active organ (the liver) but at the cost of a significant increase in H2O2, an important form of reactive oxygen species (ROS).After a 2-week period of fasting, brown trout reduced their whole-liver mitochondrial respiratory capacities (state 3, state 4 and cytochrome c oxidase activity), mainly due to reductions in liver size (and hence the total mitochondrial content). This was compensated for at the level of the mitochondrion, with an increase in state 3 respiration combined with a decrease in state 4 respiration, suggesting a selective increase in the capacity to produce ATP without a concomitant increase in energy dissipated through proton leakage. However, the reduction in total hepatic metabolic capacity in fasted fish was associated with an almost two-fold increase in in vivo mitochondrial H2O2 levels (as measured by the MitoB probe).The resulting increase in mitochondrial ROS, and hence potential risk of oxidative damage, provides mechanistic insight into the trade-off between the short-term energetic benefits of reducing metabolism in response to fasting and the potential long-term costs to subsequent life-history traits.

Les restrictions alimentaires sont courantes dans le milieu naturel et peuvent impacter de nombreux animaux. Il a été émis l'hypothèse que les animaux, face à ces épisodes de restriction alimentaire, mettaient en place des réponses métaboliques pouvant affecter leurs histoires de vie future.En particulier, si une diminution des besoins énergétiques lors du jeûne peut contribuer à préserver les réserves de l'animal cela peut néanmoins entraîner une augmentation du stress oxydant. Ce type de compromis n'a toutefois pas encore été démontré car l'étude du métabolisme énergétique est généralement réalisée séparément de celle du stress oxydant.Par une nouvelle approche combinant des mesures du fonctionnement mitochondrial et des niveaux in vivo de peroxyde d'hydrogène (H2O2) chez la truite commune (Salmo trutta), nous montrons ici que le jeûne entraîne une économie d'énergie dans un tissu métaboliquement très actif tel que le foie, mais au coût d'une augmentation significative en H2O2, une forme majeure des espèces réactives de l'oxygène.Après deux semaines de jeûne, les truites communes ont réduit leurs capacités respiratoires mitochondriales (état 3, état 4 et l'activité de la cytochrome c oxydase) principalement du fait d'une réduction de la taille du foie (et donc du nombre total de mitochondries). Une compensation a été observée au niveau de la mitochondrie. Cela se traduit par une augmentation de la respiration en état 3 et une diminution concomitante de celle en état 4, suggérant une augmentation sélective des capacités de production de l'ATP sans augmentation parallèle de l'énergie dissipée par la fuite de protons. La diminution des capacités métaboliques du foie chez les poissons à jeun était associée in vivo à des niveaux quasiment doubles de H2O2 mitochondriaux (mesurés par la sonde MitoB).Cette augmentation en espèces réactives de l'oxygène dans les mitochondries, avec son risque inhérent de dommages oxydatifs, apporte une vision mécanistique du compromis entre les bénéfices énergétiques à court terme d'une réduction métabolique en réponse au jeûne et les possibles coûts à long terme sur leurs traits histoires de vie futurs. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13125/suppinfo is available for this article.

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.

Individuals exhibit consistent differences in their metabolic rates across changing thermal conditions.

Metabolic rate has been linked to growth, reproduction, and survival at the individual level and is thought to have far reaching consequences for the ecology and evolution of organisms. However, metabolic rates must be consistent (i.e. repeatable) over at least some portion of the lifetime in order to predict their longer-term effects on population dynamics and how they will respond to selection. Previous studies demonstrate that metabolic rates are repeatable under constant conditions but potentially less so in more variable environments. We measured the standard (=minimum) metabolic rate, maximum metabolic rate, and aerobic scope (=interval between standard and maximum rates) in juvenile brown trout (Salmo trutta) after 5weeks acclimation to each of three consecutive test temperatures (10, 13, and then 16°C) that simulated the warming conditions experienced throughout their first summer of growth. We found that metabolic rates are repeatable over a period of months under changing thermal conditions: individual trout exhibited consistent differences in all three metabolic traits across increasing temperatures. Initial among-individual differences in metabolism are thus likely to have significant consequences for fitness-related traits over key periods of their life history.

Nutrients from salmon parents alter selection pressures on their offspring.

Organisms can modify their surrounding environment, but whether these changes are large enough to feed back and alter their evolutionary trajectories is not well understood, particularly in wild populations. Here we show that nutrient pulses from decomposing Atlantic salmon (Salmo salar) parents alter selection pressures on their offspring with important consequences for their phenotypic and genetic diversity. We found a strong survival advantage to larger eggs and faster juvenile metabolic rates in streams lacking carcasses but not in streams containing this parental nutrient input. Differences in selection intensities led to significant phenotypic divergence in these two traits among stream types. Stronger selection in streams with low parental nutrient input also decreased the number of surviving families compared to streams with high parental nutrient levels. Observed effects of parent-derived nutrients on selection pressures provide experimental evidence for key components of eco-evolutionary feedbacks in wild populations.

The influence of dietary and whole-body nutrient content on the excretion of a vertebrate consumer.

In many contexts, nutrient excretion by consumers can impact ecosystems by altering the availability of limiting nutrients. Variation in nutrient excretion can be predicted by mass balance models, most of which are premised on two key ideas: (1) consumers maintain fixed whole-body nutrient content (i.e., %N and %P), so-called fixed homeostasis; (2) if dietary nutrients are not matched to whole-body nutrients, excesses of any nutrient are released as excretion to maintain fixed homeostasis. Mass balance models thus predict that consumer excretion should be positively correlated with diet nutrients and negatively correlated with whole-body nutrients. Recent meta-analyses and field studies, however, have often failed to find these expected patterns, potentially because of a confounding influence-flexibility in whole-body nutrient content with diet quality (flexible homeostasis). Here, we explore the impact of flexible homeostasis on nutrient excretion by comparing the N and P excretion of four genetically diverged Trinidadian guppy (Poecilia reticulata) populations when reared on diets of variable P content. As predicted by mass balance, P excretion increased on the high-P diet, but, contrary to the notion of fixed homeostasis, guppy whole-body %P also increased on the high-P diet. While there was no overall correlation between excretion nutrients and whole-body nutrients, when the effect of diet on both whole-body and excretion nutrients was included, we detected the expected negative correlation between whole-body N:P and excretion N:P. This last result suggests that mass balance can predict excretion rates within species, but only if dietary effects on whole-body nutrient content are controlled. Flexible homeostasis can obscure patterns predicted by mass balance, creating an imperative to accurately capture an organism's diet quality in predicting its excretion rate.

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 (VO 2max), 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 VO 2max 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.

Using the MitoB method to assess levels of reactive oxygen species in ecological studies of oxidative stress.

In recent years evolutionary ecologists have become increasingly interested in the effects of reactive oxygen species (ROS) on the life-histories of animals. ROS levels have mostly been inferred indirectly due to the limitations of estimating ROS from in vitro methods. However, measuring ROS (hydrogen peroxide, H2O2) content in vivo is now possible using the MitoB probe. Here, we extend and refine the MitoB method to make it suitable for ecological studies of oxidative stress using the brown trout Salmo trutta as model. The MitoB method allows an evaluation of H2O2 levels in living organisms over a timescale from hours to days. The method is flexible with regard to the duration of exposure and initial concentration of the MitoB probe, and there is no transfer of the MitoB probe between fish. H2O2 levels were consistent across subsamples of the same liver but differed between muscle subsamples and between tissues of the same animal. The MitoB method provides a convenient method for measuring ROS levels in living animals over a significant period of time. Given its wide range of possible applications, it opens the opportunity to study the role of ROS in mediating life history trade-offs in ecological settings.

Simultaneous measurement of mitochondrial respiration and ATP production in tissue homogenates and calculation of effective P/O ratios.

The use of tissue homogenate has greatly aided the study of the functioning of mitochondria. However, the amount of ATP produced per oxygen molecule consumed, that is, the effective P/O ratio, has never been measured directly in tissue homogenate. Here we combine and refine existing methods previously used in permeabilized cells and isolated mitochondria to simultaneously measure mitochondrial ATP production (JATP) and oxygen consumption (JO2) in tissue homogenate. A major improvement over existing methods is in the control of ATPases that otherwise interfere with the ATP assay: our modified technique facilitates simultaneous measurement of the rates of "uncorrected" ATP synthesis and of ATP hydrolysis, thus minimizing the amount of tissue and time needed. Finally, we develop a novel method of calculating effective P/O ratios which corrects measurements of JATP and JO2 for rates of nonmitochondrial ATP hydrolysis and respiration, respectively. Measurements of JATP and JO2 in liver homogenates from brown trout (Salmo trutta) were highly reproducible, although activity declined once homogenates were 2 h old. We compared mitochondrial properties from fed and food-deprived animals to demonstrate that the method can detect mitochondrial flexibility in P/O ratios in response to nutritional state. This method simplifies studies examining the mitochondrial bioenergetics of tissue homogenates, obviating the need for differential centrifugation or chemical permeabilization and avoiding the use of nonmitochondrial ATPase inhibitors. We conclude that our approach for characterizing effective P/O ratio opens up new possibilities in the study of mitochondrial function in very small samples, where the use of other methods is limited.

Variation in Metabolic Rate among Individuals Is Related to Tissue-Specific Differences in Mitochondrial Leak Respiration.

Standard metabolic rate (SMR) and maximum metabolic rate (MMR) typically vary two- or threefold among conspecifics, with both traits assumed to significantly impact fitness. However, the underlying mechanisms that determine such intraspecific variation are not well understood. We examined the influence of mitochondrial properties on intraspecific variation in SMR and MMR and hypothesized that if SMR supports the cost of maintaining the metabolic machinery required for MMR, then the mitochondrial properties underlying these traits should be shared. Mitochondrial respiratory capacity (leak and phosphorylating respiration) and mitochondrial content (cytochrome c oxidase activity) were determined in the liver and white muscle of brown trout Salmo trutta of similar age and maintenance conditions. SMR and MMR were uncorrelated across individuals and were not associated with the same mitochondrial properties, suggesting that they are under the control of separate physiological processes. Moreover, tissue-specific relationships between mitochondrial properties and whole-organism metabolic traits were observed. Specifically, SMR was positively associated with leak respiration in liver mitochondria, while MMR was positively associated with muscle mitochondrial leak respiration and mitochondrial content. These results suggest that a high SMR or MMR, rather than signaling a higher ability for respiration-driven ATP synthesis, may actually reflect greater dissipation of energy, driven by proton leak across the mitochondrial inner membrane. Knowledge of these links should aid interpretation of the potential fitness consequences of such variation in metabolism, given the importance of mitochondria in the utilization of resources and their allocation to performance.

Flexibility in metabolic rate and activity level determines individual variation in overwinter performance.

Energy stores are essential for the overwinter survival of many temperate and polar animals, but individuals within a species often differ in how quickly they deplete their reserves. These disparities in overwinter performance may be explained by differences in their physiological and behavioral flexibility in response to food scarcity. However, little is known about whether individuals exhibit correlated or independent changes in these traits, and how these phenotypic changes collectively affect their winter energy use. We examined individual flexibility in both standard metabolic rate and activity level in response to food scarcity and their combined consequences for depletion of lipid stores among overwintering brown trout (Salmo trutta). Metabolism and activity tended to decrease, yet individuals exhibited striking differences in their physiological and behavioral flexibility. The rate of lipid depletion was negatively related to decreases in both metabolic and activity rates, with the smallest lipid loss over the simulated winter period occurring in individuals that had the greatest reductions in metabolism and/or activity. However, changes in metabolism and activity were negatively correlated; those individuals that decreased their SMR to a greater extent tended to increase their activity rates, and vice versa, suggesting among-individual variation in strategies for coping with food scarcity.

Inadequate food intake at high temperatures is related to depressed mitochondrial respiratory capacity.

Animals, especially ectotherms, are highly sensitive to the temperature of their surrounding environment. Extremely high temperature, for example, induces a decline of average performance of conspecifics within a population, but individual heterogeneity in the ability to cope with elevating temperatures has rarely been studied. Here, we examined inter-individual variation in feeding ability and consequent growth rate of juvenile brown trout Salmo trutta acclimated to a high temperature (19°C), and investigated the relationship between these metrics of whole-animal performances and among-individual variation in mitochondrial respiration capacity. Food was provided ad libitum, yet intake varied ten-fold amongst individuals, resulting in some fish losing weight whilst others continued to grow. Almost half of the variation in food intake was related to variability in mitochondrial capacity: low intake (and hence growth failure) was associated with high leak respiration rates within liver and muscle mitochondria, and a lower coupling of muscle mitochondria. These observations, combined with the inability of fish with low food consumption to increase their intake despite ad libitum food levels, suggest a possible insufficient capacity of the mitochondria for maintaining ATP homeostasis. Individual variation in thermal performance is likely to confer variation in the upper limit of an organism's thermal niche and might affect the structure of wild populations in warming environments.

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

Metabolic rate has been linked to several 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 might 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 with 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.

Differential effects of food availability on minimum and maximum rates of metabolism.

Metabolic rates reflect the energetic cost of living but exhibit remarkable variation among conspecifics, partly as a result of the constraints imposed by environmental conditions. Metabolic rates are sensitive to changes in temperature and oxygen availability, but effects of food availability, particularly on maximum metabolic rates, are not well understood. Here, we show in brown trout (Salmo trutta) that maximum metabolic rates are immutable but minimum metabolic rates increase as a positive function of food availability. As a result, aerobic scope (i.e. the capacity to elevate metabolism above baseline requirements) declines as food availability increases. These differential changes in metabolic rates likely have important consequences for how organisms partition available metabolic power to different functions under the constraints imposed by food availability.

Aerobic scope explains individual variation in feeding capacity.

Links between metabolism and components of fitness such as growth, reproduction and survival can depend on food availability. A high standard metabolic rate (SMR; baseline energy expenditure) or aerobic scope (AS; the difference between an individual's maximum and SMR) is often beneficial when food is abundant or easily accessible but can be less important or even disadvantageous when food levels decline. While the mechanisms underlying these context-dependent associations are not well understood, they suggest that individuals with a higher SMR or AS are better able to take advantage of high food abundance. Here we show that juvenile brown trout (Salmo trutta) with a higher AS were able to consume more food per day relative to individuals with a lower AS. These results help explain why a high aerobic capacity can improve performance measures such as growth rate at high but not low levels of food availability.