Intraspecific Variability in the Basal Metabolic Rate: Testing the Food Habits Hypothesis

Predation can shape the cascade interplay between heterothermy, exploration and maintenance metabolism under high food availability

Maintenance metabolism as the minimum energy expenditure needed to maintain homeothermy (a high and stable body temperature, T b), reflects the magnitude of metabolic machinery and the associated costs of self-maintenance in endotherms (organisms able to produce heat endogenously). Therefore, it can interact with most, if not all, organismal functions, including the behavior-fitness linkage. Many endothermic animals can avoid the costs of maintaining homeothermy and temporally reduce T b and metabolism by entering heterothermic states like torpor, the most effective energy-saving strategy. Variations in BMR, behavior, and torpor use are considered to be shaped by food resources, but those conclusions are based on research studying these traits in isolation. We tested the effect of ecological contexts (food availability and predation risk) on the interplay between the maintenance costs of homeothermy, heterothermy, and exploration in a wild mammal-the yellow-necked mouse. We measured maintenance metabolism as basal metabolic rate (BMR) using respirometry, distance moved (exploration) in the open-field test, and variation in T b (heterothermy) during short-term fasting in animals captured at different locations of known natural food availability and predator presence, and with or without supplementary food resources. We found that in winter, heterothermy and exploration (but not BMR) negatively correlated with natural food availability (determined in autumn). Supplementary feeding increased mouse density, predation risk and finally had a positive effect on heterothermy (but not on BMR or exploration). The path analysis testing plausible causal relationships between the studied traits indicated that elevated predation risk increased heterothermy, which in turn negatively affected exploration, which positively correlated with BMR. Our study indicates that adaptive heterothermy is a compensation strategy for balancing the energy budget in endothermic animals experiencing low natural food availability. This study also suggests that under environmental challenges like increased predation risk, the use of an effective energy-saving strategy predicts behavioral expression better than self-maintenance costs under homeothermy.

The fire of evolution: energy expenditure and ecology in primates and other endotherms

Total energy expenditure (TEE) represents the total energy allocated to growth, reproduction and body maintenance, as well as the energy expended on physical activity. Early experimental work in animal energetics focused on the costs of specific tasks (basal metabolic rate, locomotion, reproduction), while determination of TEE was limited to estimates from activity budgets or measurements of subjects confined to metabolic chambers. Advances in recent decades have enabled measures of TEE in free-living animals, challenging traditional additive approaches to understanding animal energy budgets. Variation in lifestyle and activity level can impact individuals' TEE on short time scales, but interspecific differences in TEE are largely shaped by evolution. Here, we review work on energy expenditure across the animal kingdom, with a particular focus on endotherms, and examine recent advances in primate energetics. Relative to other placental mammals, primates have low TEE, which may drive their slow pace of life and be an evolved response to the challenges presented by their ecologies and environments. TEE variation among hominoid primates appears to reflect adaptive shifts in energy throughput and allocation in response to ecological pressures. As the taxonomic breadth and depth of TEE data expand, we will be able to test additional hypotheses about how energy budgets are shaped by environmental pressures and explore the more proximal mechanisms that drive intra-specific variation in energy expenditure.

Evolved high aerobic capacity has context-specific effects on gut microbiota

Gut microbiota is expected to coevolve with the host's physiology and may play a role in adjusting the host's energy metabolism to suit the host's environment. To evaluate the effects of both evolved host metabolism and the environmental context in shaping the gut microbiota, we used a unique combination of (1) experimental evolution to create selection lines for a fast metabolism and (2) a laboratory-to-field translocation study. Mature bank voles Myodes glareolus from lines selected for high aerobic capacity (A lines) and from unselected control (C lines) were released into large (0.2 ha) outdoor enclosures for longitudinal monitoring. To examine whether the natural environment elicited a similar or more pronounced impact on the gut microbiota of the next generation, we also sampled the field-reared offspring. The gut microbiota were characterized using 16S rRNA amplicon sequencing of fecal samples. The artificial selection for fast metabolism had minimal impact on the gut microbiota in laboratory conditions but in field conditions, there were differences between the selection lines (A lines vs. C lines) in the diversity, community, and resilience of the gut microbiota. Notably, the selection lines differed in the less abundant bacteria throughout the experiment. The lab-to-field transition resulted in an increase in alpha diversity and an altered community composition in the gut microbiota, characterized by a significant increase in the relative abundance of Actinobacteria and a decrease of Patescibacteria. Also, the selection lines showed different temporal patterns in changes in microbiota composition, as the average gut microbiota alpha diversity of the C lines, but not A lines, was temporarily reduced during the initial transition to the field. In surviving young voles, the alpha diversity of gut microbiota was significantly higher in A-line than C-line voles. These results indicate that the association of host metabolism and gut microbiota is context-specific, likely mediated by behavioral or physiological modifications in response to the environment.

Testing the Influence of Incomplete DNA Barcode Libraries on Ecological Status Assessment of Mediterranean Transitional Waters

Simple Summary

The biodiversity and ecological status assessment of transitional water ecosystems by benthic macroinvertebrates investigation could use DNA barcode tools for more rapid and efficient outputs. The principal limits of this application are the incompleteness of DNA barcode databases, the identification of optimal primers set, and the gap in the species sequences. The influence of the incompleteness of DNA barcode libraries on species diversity indices, ecological indicators, and ecological status assessment in transitional waters of the southeast Mediterranean were analysed, underlying the importance to implement DNA barcode libraries and to put an effort toward specific species at a local level.

Abstract

The ecological assessment of European aquatic ecosystems is regulated under the framework directives on strategy for water and marine environments. Benthic macroinvertebrates are the most used biological quality element for ecological assessment of rivers, coastal-marines, and transitional waters. The morphological identification of benthic macroinvertebrates is the current tool for their assessment. Recently, DNA-based tools have been proposed as effective alternatives. The main current limits of DNA-based applications include the incompleteness of species recorded in the DNA barcode reference libraries and the primers bias. Here, we analysed the influence of the incompleteness of DNA barcode databases on species diversity indices, ecological indicators, and ecological assessment in transitional waters of the southeast Mediterranean, taking into account the availability of commonly sequenced and deposited genomic regions for listed species. The ecological quality status assigned through the potential application of both approaches to the analysed transitional water ecosystems was different in 27% of sites. We also analysed the inter-specific genetic distances to evaluate the potential application of the DNA metabarcoding method. Overall, this work highlights the importance to expand the barcode databases and to analyse, at the regional level, the gaps in the DNA barcodes.

Spatial and temporal variation of compositional, functional, and phylogenetic diversity in ectoparasite infracommunities harboured by small mammals

We studied patterns of compositional, functional, and phylogenetic α- and β-diversity in flea and gamasid mite infracommunities of small Siberian mammals, taking into account host-associated (species) and environmental (biome or sampling period) factors. We asked: (a) How do these factors and their interactions affect infracommunity diversity? (b) Does infracommunity composition, in terms of species, traits, and phylogenetic lineages, deviate from random? (c) Are species, traits, and phylogenetic lineages in infracommunities clustered or overdispersed?, and (d) Do patterns of diversity differ between the three diversity facets and/or the two ectoparasite taxa? We found that the α-diversity of infracommunities was strongly affected by host species, biome, and sampling period. The highest proportion of infracommunity diversity in both taxa was associated with the interaction between either host species and biome or host species and sampling period. Infracommunities of both taxa within, as well as between, host species, biomes, and sampling periods were characterized by the clustering of species, traits and lineages. The patterns of the effects of host species, biome, and sampling period on infracommunity diversity were congruent among the three diversity facets in both fleas and mites. We conclude that the assembly patterns in ectoparasite infracommunities mirror those characteristics of component and compound communities.

Metabolic and behavioral adaptations of greater white-toothed shrews to urban conditions

The global trend of urbanization is creating novel challenges for many animal species. Studies investigating behavioral differences between rural and urban populations often report a general increase in risk-taking behaviors in urban populations. According to the most common energy management model (the performance model), behaviors that increase access to resources, such as aggression and boldness, and behaviors that consume net energy, like locomotion and stress responses, are both positively correlated to resting metabolic rate (RMR). Thus, we expect urban populations to not only exhibit a higher level of risk-taking behavior but also a higher RMR. However, these interactions remain poorly investigated. Our main goal was to analyze the relationship between RMR and risk-taking behaviors in the greater white-toothed shrew (Crocidura russula) in rural versus urban populations. Trapped shrews were brought to captivity where we measured RMR, boldness, and exploration rate three times in each individual. Our findings revealed that urban shrews were indeed bolder and more exploratory, but contrary to our expectations, their RMR was lower than that of rural shrews. This is likely explained by differences in the environmental conditions of these two habitats, such as higher ambient temperatures and/or lower prey availability in cities. When looking at each population separately, this relationship remained similar: urban shrews with a higher RMR were less bold, and rural shrews with a higher RMR showed a lower exploration rate. We conclude that the energetic strategy of C. russula is dependent on the environmental and observational context and cannot be explained by the performance model.

Continuous growth through winter correlates with increased resting metabolic rate but does not affect daily energy budgets due to torpor use

Small mammals that are specialists in homeothermic thermoregulation reduce their self-maintenance costs of normothermy to survive the winter. By contrast, heterothermic ones that are considered generalists in thermoregulation can lower energy expenditure by entering torpor. It is well known that different species vary the use of their strategies to cope with harsh winters in temperate zones; however, little is still known about the intraspecific variation within populations and the associated external and internal factors. We hypothesized that yellow-necked mice Apodemus flavicollis decrease their resting metabolic rate (RMR) from autumn to winter, and then increase it during spring. However, since the alternative for seasonal reduction of RMR could be the development of heterothermy, we also considered the use of this strategy. We measured body mass (m_b), RMR, and body temperature (T_b) of mice during 2 consecutive years. In the 1st year, mice decreased whole animal RMR in winter, but did not do so in the 2nd year. All mice entered torpor during the 2nd winter, whereas only a few did so during the first one. Mice showed a continuous increase of m_b, which was steepest during the 2nd year. The relationship between RMR and m_b varied among seasons and years most likely due to different mouse development stages. The m_b gain at the individual level was correlated positively with RMR and heterothermy. This indicates that high metabolism in winter supports the growth of smaller animals, which use torpor as a compensatory mechanism. Isotope composition of mice hair suggests that in the 1st year they fed mainly on seeds, while in the 2nd, they likely consumed significant amounts of less digestible herbs. The study suggests that the use of specialist or generalist thermoregulatory strategies can differ with environmental variation and associated differences in developmental processes.

Life History and Immune Challenge Influence Metabolic Plasticity to Food Availability and Acclimation Temperature

Animals vary in their rates of energy expenditure for self-maintenance (standard metabolic rate [SMR]). Yet we still lack a thorough understanding of the determinants of SMR, potentially because of complex interactions among environmental, life-history, and physiological factors. Thus, we used a factorial design in female sand field crickets (Gryllus firmus) to investigate the independent and interactive effects of food availability (unlimited or limited access), acclimation temperature (control or simulated heat wave), life-history strategy (flight-capable or flight-incapable wing morphology), and immune status (control or chronic immune activation) on SMR (CO₂ production rate) measured at 28°C. Both environmental factors independently affected SMR where heat wave and food limitation reduced SMR. Furthermore, wing morphology and immune status mediated the plasticity of SMR to food and temperature. For example, the hypermetabolic effect of food availability was greater in flight-capable crickets and reduced in immune-challenged crickets. Therefore, although SMR was directly affected by food availability and acclimation temperature, interactive effects on SMR were more common, meaning several factors (e.g., life history and immune status) influenced metabolic plasticity to food and temperature. We encourage continued use of factorial experiments to reveal interaction dynamics, which are critical to understanding emergent physiological processes.

Energetics and thermal adaptation in semifossorial pine-voles Microtus lusitanicus and Microtus duodecimcostatus

Rodents colonising subterranean environments have developed several morphological, physiological and behaviour traits that promote the success of individuals in such demanding conditions. Resting metabolic rate, thermoregulation capacity and daily energy expenditure were analysed in two semi-fossorial pine-vole species Microtus lusitanicus and Microtus duodecimcostatus inhabiting distinct areas of the Iberian Peninsula. Individuals capture location varied in habitat and soil features, allowing the comparison of energetic parameters with ecological characteristics, that can help explain the use of the subterranean environment and dependence of the burrow system. Results showed that M. duodecimcostatus has lower mass independent resting metabolic rate when compared with M. lusitanicus, which may be a response to environmental features of their habitat, such as dryer soils and lower water availability. Thermal conductance increased with body mass and was dependent on the ambient temperature. No significant differences were observed in the daily energy expenditure, but water economy data demonstrated the influence of the water available in the habitat on the energetics of voles. These species may rely on behavioural adaptations and seasonal use of burrows to cope with thermal challenges of subterranean activity and soil constraints. We found strong evidence that M. lusitanicus is able to use torpor as a response to low ambient temperatures which is a new observation among Arvicolines.

Food Availability Is the Main Driver of Seasonal Changes in Resting Metabolic Rate in African Striped Mice (Rhabdomys pumilio)

Resting metabolic rate (RMR) influences energy allocation to survival, growth, and reproduction, and significant seasonal changes in RMR have been reported. According to one hypothesis, seasonal changes in RMR are mainly attributable to seasonal changes in ambient temperature (T_a) and food availability. Studies on species from the temperate zone indicated that food availability is the main driver. However, whether this is generally true is unknown, because studies from the tropics and subtropics, where most species live, are rare. We studied the African striped mouse (Rhabdomys pumilio) inhabiting a seasonal environment with hot dry seasons with low food availability and cold moist seasons with high food availability. Using 603 RMR measurements of 277 individuals, we investigated the relative importance of food availability and T_a on RMR during selected periods, in which one extrinsic factor varied while the other factor was relatively constant. At similar T_a, residual RMR increased with increasing levels of food availability. In contrast, different T_a did not influence residual RMR at similar levels of food availability. Thus, our study on a subtropical species gives support to the hypothesis, derived from temperate zone species, that food availability mainly drives seasonal changes in RMR.

Does Basal Metabolism Set the Limit for Metabolic Downregulation during Torpor?

The evolution of endothermic thermoregulation is rooted in the processes involving high metabolism, which allows the maintenance of high and stable body temperatures (T_b). In turn, selection for high endothermic metabolism correlates with increased size of metabolically active organs and thus with high basal metabolic rate (BMR). Endothermic animals are characterized by an MR several times that of similar-sized ectotherms. However, many small mammals are temporally heterothermic and are able to temporally decrease T_b and MR by entering daily torpor or hibernation. Both BMR and minimum MR during torpor (TMR_min) likely result from oxidative respiration in mitochondria of the same tissues. It should be expected that these two MRs are positively correlated, suggesting that the evolution of endothermy and higher BMR set the limit for the ability to reduce MR while entering torpor. Using published data for 96 mammal species, we tested the hypothesis that, among heterothermic mammals, the processes leading to the evolution of higher BMR limit the ability to downregulate metabolism during torpor. We found that body mass (m_b)-adjusted BMR was positively correlated with m_b- and T_b-adjusted TMR_min, in a phylogenetically corrected analysis. Phylogenetic path modeling indicated that the mechanisms underlying the evolutionary increase of BMR in endotherms most likely constrain their ability to reduce MR during torpor. Given that heterothermy is considered an ancestral state in mammals, these results suggest an increase in BMR during the evolution of endothermy in homeothermic animals, which leads to the loss of their ability to enter torpor.

Leptin resistance was involved in susceptibility to overweight in the striped hamster re-fed with high fat diet

Food restriction (FR) is the most commonly used intervention to prevent the overweight. However, the lost weight is usually followed by “compensatory growth” when FR ends, resulting in overweight. The present study was aimed to examining the behavior patterns and hormones mechanisms underpinning the over-weight. Energy budget and body fat content, and several endocrine markers related to leptin signals were examined in the striped hamsters under 20% FR refed by either low-fat diet (LF group) or high-fat diet (HF group). Body mass and fat content significantly regained when FR ended, and the hamsters in HF group showed 49.1% more body fat than in LF group (P < 0.01). Digestive energy intake was higher by 20.1% in HF than LF group, while metabolic thermogenesis and behavior patterns did not differed between the two groups. Gene expression of leptin receptor and anorexigenic peptides of pro-opiomelanocortin and cocaine- and amphetamine-regulated transcript in hypothalamus were significantly up-regulated in LF group, but down-regulated in HF group. It suggests that effective leptin signals to the brain were involved in attenuation of hyperphagia in hamsters refed with LF. However, “leptin resistance” probably occurred in hamsters refed with HF, which impaired the control of hyperphagia, resulting in development of over-weight.

Daily energy expenditure in the face of predation: hedgehog energetics in rural landscapes

Failure to balance daily energy expenditure (DEE) with energy intake can have an impact on survival and reproduction, and therefore on the persistence of populations. Here we study the DEE of the European hedgehog (Erinaceus europaeus), which is declining in the UK. We hypothesise that there is a gradient of suitable habitat for hedgehogs in rural areas, which is a result of fewer food resources, a higher risk from predation by badgers (Meles meles) and colder ambient temperatures, as distance to the nearest building increases. We used the doubly labelled water method to obtain 44 measurements of DEE from hedgehogs on four predominately arable sites, to determine the energetic costs associated with proximity to buildings, on sites with and without badgers. The mean±s.e.m. DEE was 508.9±34.8 kJ day^-1 DEE increased the further a hedgehog was from buildings during the study, possibly as they ranged larger distances on arable land, supporting the hypothesis that hedgehogs select villages owing to the lower energy demands in comparison to arable farmland. Hedgehogs had an approximately 30% lower DEE on sites with badgers. We speculate that on badger-occupied sites, hedgehogs may restrict movement and foraging in response to a threat from predation and thus have reduced DEE. Therefore, hedgehogs may also seek refuge in villages where the perceived threat of predation is lower and foraging is unrestricted. In a broader context, we demonstrate that individual differences in DEE can aid in understanding habitat selection in a patchily distributed species.

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

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

Is Maximum Food Intake in Endotherms Constrained by Net or Factorial Aerobic Scope? Lessons from the Leaf-Eared Mouse

Food availability varies substantially throughout animals' lifespans, thus the ability to profit from high food levels may directly influence animal fitness. Studies exploring the link between basal metabolic rate (BMR), growth, reproduction, and other fitness traits have shown varying relationships in terms of both magnitude and direction. The diversity of results has led to the hypothesis that these relationships are modulated by environmental conditions (e.g., food availability), suggesting that the fitness consequences of a given BMR may be context-dependent. In turn, there is indirect evidence that individuals with an increased capacity for aerobic work also have a high capacity for acquiring energy from food. Surprisingly, very few studies have explored the correlation between maximum rates of energy acquisition and BMR in endotherms, and to the best of our knowledge, none have attempted to elucidate relationships between the former and aerobic capacity [e.g., maximum metabolic rate (MMR), aerobic scope (Factorial aerobic scope, FAS; Net aerobic scope, NAS)]. In this study, we measured BMR, MMR, maximum food intake (recorded under low ambient temperature and ad libitum food conditions; MFI), and estimated aerobic scope in the leaf-eared mouse (Phyllotis darwini). We, then, examined correlations among these variables to determine whether metabolic rates and aerobic scope are functionally correlated, and whether an increased aerobic capacity is related to a higher MFI. We found that aerobic capacity measured as NAS is positively correlated with MFI in endotherms, but with neither FAS nor BMR. Therefore, it appears plausible that the capacity for assimilating energy under conditions of abundant resources is determined adaptively by NAS, as animals with higher NAS would be promoted by selection. In theory, FAS is an invariant measurement of the extreme capacity for energy turnover in relation to resting expenditure, whereas NAS represents the maximum capacity for simultaneous aerobic processes above maintenance levels. Accordingly, in our study, FAS and NAS represent different biological variables; FAS, in contrast to NAS, may not constrain food intake. The explanations for these differences are discussed in biological and mathematical terms; further, we encourage the use of NAS rather than FAS when analyzing the aerobic capacity of animals.

Nutritional content of bat-consumed fruits in a forest fragment in Southern Brazil

The State Park Mata dos Godoy has 42 identified bat species, among which 21 are frugivorous and belong to the Phyllostomidae family. Current study investigated the concentrations of carbohydrate, lipid and protein in fruits consumed by Artibeus lituratus, Carollia perspicillata and Sturnira lilium to understand their potential differentiated food preferences. The Sampling effort it was from April/2013 to March/2014. Animals captured in mist nets were identified in the field and maintained in cotton bags for 30 minutes to defecate. The diet was inferred from fecal samples analyzed in the laboratory, the fruits whose seeds were found in the feces had the determinations of the nutrients accomplished by centesimal composition method. Kruskal-Wallis test and Dunn's multiple comparison method verified the most abundant nutrients for each species of bat. Whereas Carollia perspicillata and Sturnira lilium feeds mainly in food with carbohydrates that provide them with more energy, the diet of Artibeus lituratus is richer in carbohydrates and lipids. Only fruits considered scarcely represent a complete nutrition for the three bat species, however, the complementarity with other food items such as leaves, pollen, and insects, can provide a complete nutrient diet for these animals.

Use of erythrocyte indicators of health and condition in vertebrate ecophysiology: a review and appraisal

We review evidence for and against the use of erythrocyte indicators of health status and condition, parasite infection level and physiological stress in free-living vertebrates. The use of indicators that are measured directly from the blood, such as haemoglobin concentration, haematocrit and erythrocyte sedimentation rate, and parameters that are calculated from multiple measured metrics, such as mean cell volume, mean cell haemoglobin content or mean cell haemoglobin concentration is evaluated. The evidence for or against the use of any given metric is equivocal when the relevant research is considered in total, although there is sometimes strong support for using a particular metric in a particular taxon. Possibly the usefulness of these metrics is taxon, environment or condition specific. Alternatively, in an uncontrolled environment where multiple factors are influencing a metric, its response to environmental change will sometimes, but not always, be predictable. We suggest that (i) researchers should validate a metric's utility before use, (ii) multiple metrics should be used to construct an overall erythrocyte profile for an individual or population, (iii) there is a need for researchers to compile reference ranges for free-living species, and (iv) some metrics which are useful under controlled, clinical conditions may not have the same utility or applicability for free-living vertebrates. Erythrocyte metrics provide useful information about health and condition that can be meaningfully interpreted in free-living vertebrates, but their use requires careful forethought about confounding factors.

About rats and jackfruit trees: modeling the carrying capacity of a Brazilian Atlantic Forest spiny-rat Trinomys dimidiatus (Günther, 1877) - Rodentia, Echimyidae - population with varying jackfruit tree (Artocarpus heterophyllus L.) abundances

We carried out a six-year study aimed at evaluating if and how a Brazilian Atlantic Forest small mammal community responded to the presence of the invasive exotic species Artocarpus heterophyllus, the jackfruit tree. In the surroundings of Vila Dois Rios, Ilha Grande, RJ, 18 grids were established, 10 where the jackfruit tree was present and eight were it was absent. Previous results indicated that the composition and abundance of this small mammal community were altered by the presence and density of A. heterophyllus. One observed effect was the increased population size of the spiny-rat Trinomys dimidiatus within the grids where the jackfruit trees were present. Therefore we decided to create a mathematical model for this species, based on the Verhulst-Pearl logistic equation. Our objectives were i) to calculate the carrying capacity K based on real data of the involved species and the environment; ii) propose and evaluate a mathematical model to estimate the population size of T. dimidiatus based on the monthly seed production of jackfruit tree, Artocarpus heterophyllus and iii) determinate the minimum jackfruit tree seed production to maintain at least two T. dimidiatus individuals in one study grid. Our results indicated that the predicted values by the model for the carrying capacity K were significantly correlated with real data. The best fit was found considering 20~35% energy transfer efficiency between trophic levels. Within the scope of assumed premises, our model showed itself to be an adequate simulator for Trinomys dimidiatus populations where the invasive jackfruit tree is present.

Environmental metabolomics reveal geographic variation in aerobic metabolism and metabolic substrates in Mongolian gerbils (Meriones unguiculatus)

Mongolian gerbils (Meriones unguiculatus) have a large-scale distribution in northern China. Geographic physiological variations which related to energy and water metabolism are critical to animals' local adaptation and distribution. However, the underlying biochemical mechanism of such variation and its role in adaptation remains largely unknown. We used GC-MS metabolomics approach to investigate the biochemical adaptation of Mongolian gerbils from xeric (desert), transition (desert steppe) and mesic (typical steppe) environments. Gerbils in desert population had lower resting metabolic rate (RMR) and total evaporative water loss (TEWL) than mesic population. Serum metabolomics revealed that concentrations of five tricarboxylic acid cycle intermediates (citrate, cis-aconitate, α-ketoglutarate, fumarate and malate) were lower in desert population than mesic population. Gastrocnemius metabolomics and citrate synthase activity analysis showed a lower concentration of citrate and lower citrate synthase activity in desert population. These findings suggest that desert dwelling gerbils decrease RMR and TEWL via down-regulation of aerobic respiration. Gastrocnemius metabolomics also revealed that there were higher concentrations of glucose and glycolytic intermediates, but lower concentrations of lipids, amino acids and urea in desert population than mesic population. This geographic variation in metabolic substrates may enhance metabolic water production per oxygen molecule for desert population while constraining aerobic respiration to reduce RMR and TEWL.

Reaction norms in natural conditions: how does metabolic performance respond to weather variations in a small endotherm facing cold environments?

Reaction norms reflect an organisms' capacity to adjust its phenotype to the environment and allows for identifying trait values associated with physiological limits. However, reaction norms of physiological parameters are mostly unknown for endotherms living in natural conditions. Black-capped chickadees (Poecile atricapillus) increase their metabolic performance during winter acclimatization and are thus good model to measure reaction norms in the wild. We repeatedly measured basal (BMR) and summit (Msum) metabolism in chickadees to characterize, for the first time in a free-living endotherm, reaction norms of these parameters across the natural range of weather variation. BMR varied between individuals and was weakly and negatively related to minimal temperature. Msum varied with minimal temperature following a Z-shape curve, increasing linearly between 24°C and −10°C, and changed with absolute humidity following a U-shape relationship. These results suggest that thermal exchanges with the environment have minimal effects on maintenance costs, which may be individual-dependent, while thermogenic capacity is responding to body heat loss. Our results suggest also that BMR and Msum respond to different and likely independent constraints.

Behavioral and ecological factors account for variation in the mass-independent energy expenditures of endotherms

A persistent controversy has concerned the identification of the factors that influence the quantitative variation in the physiological characters of species, an example of which is the basal rate of metabolism of endotherms. The most important factor accounting for its variation is body mass as long as the range in mass is appreciable. But mass never accounts for all of the variation and none if species have the same mass. Most of the residual variation around the mass curve is associated with behavioral characters, ecological factors, and phylogeny, i.e., history. These agents influence energy expenditure by different means and at different stages in the life history of species. Phylogeny describes the historic origin, evolution, and distribution of character states in contemporary species. However, the level of energy expenditure is quantitatively determined by the collective of realized states in combination with conditions in the environment. Therefore, two stages determine energy expenditure: (1) the evolution of character states and (2) their impact in conjunction with conditions on the environment. Behavioral characters and ecological factors, when coupled with log10 mass, usually account for >94 % of the variation in the log10 basal rates of birds and mammals, a capacity not found in phylogenetic analyses. The difficulty of determining a direct impact of phylogeny on physiological characters results from its correlation with behavioral characters. When appropriate, the passerine/non-passerine dichotomy in birds and the sub/infraclass dichotomy in mammals combine with behavioral characters, ecological factors, and log10 mass to increase r (2) to account for 96-99 % of the variation in log10 basal rate. This occurs because dichotomies incorporate factors other than those already in the analyses. The clearest demonstration of the direct impact of character states is the equality of energy expenditure in species convergent with species from other clades without any evidence of the clade to which the species belong. A multifactorial approach depends on the inclusion of life history characteristics of species, but this analysis applies only to continuous, quantitative relationships, which are often modified by discontinuous agents. Distinctive character states distinguish species and therefore must be included in any attempt to account for differences in their level of energy expenditure. No analysis that ignores life history characteristics can account for the quantitative variation in energy expenditure beyond that associated with body mass.

Context-dependent correlation between resting metabolic rate and daily energy expenditure in wild chipmunks

Several empirical studies have shown that variation in daily energy expenditure (DEE) and resting metabolic rate (RMR) is influenced by environmental and individual factors, but whether these shared influences are responsible for, or independent of, relationships between DEE and RMR remains unknown. The objectives of this study were to (i) simultaneously evaluate the effects of environmental and individual variables on DEE and RMR in free-ranging eastern chipmunks (Tamias striatus) and (ii) quantify the correlation between DEE and RMR before and after controlling for common sources of variation. We found that the influence of individual factors on DEE and RMR is most often shared, whereas the influence of environmental factors tends to be distinct. Both raw and mass-adjusted DEE and RMR were significantly correlated, but this correlation vanished after accounting for the shared effect of reproduction on both traits. However, within reproductive individuals, DEE and RMR remained positively correlated after accounting for all other significant covariates. The ratio of DEE to RMR was significantly higher during reproduction than at other times of the year and was negatively correlated with ambient temperature. DEE and RMR appear to be inherently correlated during reproduction, but this correlation does not persist during other, less energy-demanding periods of the annual cycle.

Developmental plasticity in metabolic rates reinforces morphological plasticity in response to social cues of sexual selection

Developmental plasticity allows individuals to match their phenotype to the competitive environment they are most likely to encounter. Although there are numerous studies that demonstrate adaptive shifts in life-history and metric traits, we still have a poor understanding of whether internal physiological processes demonstrate plasticity and whether this plasticity supports life-history and metric traits. Here we use the Australian redback spider (Latrodectus hasselti), a species that demonstrates adaptive developmental shifts in response to the availability of females and the density of males, to examine the relationship between the routine metabolic rate (RMR) and the expression of size, body-condition and development rate. We reared immature males in three diet treatments, and in social environments that varied the presence/absence of females and the density of males and measured their RMR, weight, size and developmental rate at maturity. We show that although RMR decreased with decreasing resource abundance, RMR was positively correlated with the density of rivals. Moreover, RMR was not correlated with size or body-condition at maturity. Our results demonstrate that plasticity in the RMR supports plasticity in metric and life-history traits to create an integrated phenotype that matches the competitive environment.

Variation of basal EROD activities in ten passerine bird species--relationships with diet and migration status

Inter-specific differences in animal defence mechanisms against toxic substances are currently poorly understood. The ethoxyresorufin-O-deethylase (EROD) enzyme plays an important role in defence against toxic chemicals in a wide variety of animals, and it is an important biomarker for environmental contamination. We compared basal hepatic EROD activity levels among ten passerine species to see if there is inter-specific variation in enzyme activity, especially in relation to their diet and migration status. Migratory insectivores showed higher EROD activity compared to granivores. We hypothesize that the variable invertebrate diet of migratory insectivores contains a wider range of natural toxins than the narrower diet of granivores. This may have affected the evolution of mixed function oxidases (MFO) system and enzyme activities. We further tested whether metabolic rates or relative liver size were associated with the variation in detoxification capacity. We found no association between EROD activity and relative (per mass unit) basal metabolic rate (BMR). Instead, EROD activity and relative liver mass (% of body mass) correlated positively, suggesting that a proportionally large liver also functions efficiently. Our results suggest that granivores and non-migratory birds may be more vulnerable to environmental contaminants than insectivores and migratory birds. The diet and migration status, however, are phylogenetically strongly connected to each other, and their roles cannot be fully separated in our analysis with only ten passerine species.

Phenotypic flexibility in basal metabolic rate is associated with rainfall variability among populations of rufous-collared sparrow

Phenotypic flexibility in metabolic rates allows organisms to reversibly adjust their energy flow to meet challenges imposed by a variable environment. In turn, the food habits hypothesis (FHH) predicts that species or populations adjust their basal metabolic rate (BMR) according to the diet attributes such as food abundance or predictability. Desert ecosystems represent a temporally heterogeneous environment because of low rain pulse predictability, which is also associated with temporal variation in food resources. In the present study, we investigated the relationship between the magnitude of BMR flexibility in response to dietary acclimation and the inter-annual rainfall variability in three populations of rufous-collared sparrows. Specifically we addressed the question of whether birds from a desert environment are more flexible in BMR than those from non-desert habitats. We found a positive trend between BMR flexibility and the inter-annual rainfall variability. In fact, dietary treatments had a significant effect only in desert birds, a result that also supported the FHH. Our study confirms the existence of phenotypic variation in response to environmental conditions among populations, and also highlights the importance of considering the circumstances in which phenotypic flexibility evolves and the specific environmental cues that induce their expression.

Is there a linkage between metabolism and personality in small mammals? The root vole (Microtus oeconomus) example

Significant inter-individual variation in the rate of animal metabolism is a widespread phenomenon that has started to accumulate general interest. Here we follow recent calls to focus on linkage between the variation in energy metabolism and animal personality. By using wild caught root voles as a study species, we examined the relationship between the behavioral patterns (assessed in open field test) and resting metabolic rate (RMR), both of which are known to show large individual differences and intra-individual consistency in voles. Our results showed only a weak relationship between personality traits and metabolism, since the most parsimonious model (according to AICc) explaining RMR included only body mass and season as factors (explaining 84.8% of variation in RMR). However, the next two alternative models (within ΔAICc=2) also included the personality trait reflecting proactive behaviors (PC1) in addition to body mass, sex and season (85.2 and 85.8% of RMR variance explained, respectively). In all, our study does not provide compelling support for recent ideas of close linkage between behavior and metabolism. Still, our study highlights that even in the case of wild caught individuals, when behavior and metabolism often carry effects of both intrinsic and extrinsic conditions, the potential metabolic effects of varying energetically costly behaviors cannot be neglected.

Comparative basal metabolic rate among passerines and the food habit hypothesis

The food habit hypothesis (FHH) predicts that mass-independent BMR is associated with dietary traits. In spite of decades of research, the FHH remains controversial, in part because ambiguities surrounding diet categorization can potentially confound interpretations from interspecific analyses of the relative importance of diet quality. In this study, we investigated possible relationships between BMR and the percentage of nitrogen, and the percentage of various prey items in the gut content of 19 passerine species under an explicit phylogenetic context. There were no significant effects of the percentage of dietary nitrogen, invertebrates, or plants on mass-independent BMR using either conventional or phylogenetically corrected regressions. Consequently, our results do not support the FHH in passerine birds. The lack of a significant correlation between diet and BMR could be explained by generally similar levels of activity and similar body composition among species. In this sense, it is possible that the impact of food habits could depend mainly on body mass, which is also associated to the high cost of the endothermy at small sizes.

Causes and significance of variation in mammalian basal metabolism

Mammalian basal metabolic rates (BMR) increase with body mass, whichs explains approximately 95% of the variation in BMR. However, at a given mass, there remains a large amount of variation in BMR. While many researchers suggest that the overall scaling of BMR with body mass is due to physiological constraints, variation at a given body mass may provide clues as to how selection acts on BMR. Here, we examine this variation in BMR in a broad sample of mammals and we test the hypothesis that, across mammals, body composition explains differences in BMR at a given body mass. Variation in BMR is strongly correlated with variation in muscle mass, and both of these variables are correlated with latitude and ambient temperature. These results suggest that selection alters BMR in response to thermoregulatory pressures, and that selection uses muscle mass as a means to generate this variation.

Measuring behavior in mice with chronic stress depression paradigm

Many studies with chronic stress, a common depression paradigm, lead to inconsistent behavioral results. We are introducing a new model of stress-induced anhedonia, which provides more reproducible induction and behavioral measuring of depressive-like phenotype in mice. First, a 4-week stress procedure induces anhedonia, defined by decreased sucrose preference, in the majority of but not all C57BL/6 mice. The remaining 30-50% non-anhedonic animals are used as an internal control for stress effects that are unrelated to anhedonia. Next, a modified sucrose test enables the detection of inter-individual differences in mice. Moreover, testing under dimmed lighting precludes behavioral artifacts caused by hyperlocomotion, a major confounding factor in stressed mice. Finally, moderation of the stress load increases the reproducibility of anhedonia induction, which otherwise is difficult to provide because of inter-batch variability in laboratory mice. We believe that our new mouse model overcomes some major difficulties in measuring behavior with chronic stress depression models.

Presence of a conspecific causes divergent changes in resting metabolism, depending on its relative size

It is well established that the average metabolism of many species of fish varies with group size. However, it is not clear whether all individuals respond in the same way. Here, we use a newly calibrated method of measuring the metabolic rate of fish from opercular (ventilatory) movements that allows for the first-time estimation of changes in resting metabolic rate (RMR) of each individual within different social groups and when alone. The presence of a conspecific had divergent effects on the RMR of juvenile Atlantic salmon Salmo salar, depending on its relative body size: the presence of a smaller fish caused a 40 per cent reduction, whereas the presence of a slightly larger fish approximately doubled RMR. These effects occurred in the absence of activity and were sustained at lower magnitude in the case of the relatively smaller conspecific even if a transparent barrier prevented any physical interactions between fish. Changes in RMR were mirrored by changes in eye colour that indicate they were linked to stress levels. These contrasting and strong responses show that even the nearby presence of a conspecific can have profound and variable effects on an individual's energy budget; they also highlight the complex trade-offs involved in social interactions.

Effect of diet quality and soil hardness on metabolic rate in the subterranean rodent Ctenomys talarum

The present work is aimed to establish, in Ctenomys talarum, the physiological and behavioral adjustments undergone by individuals when they are allowed to dig burrows in soils with different hardness and fed with diets of different quality. For each soil-diet combination, we estimated: resting metabolic rate (RMR), body temperature (T(b)), body mass, digestibility, food consumption rate, transit time, reingestion rate, feces production and time devoted to feeding, resting, locomotor activity and coprophagy. Soil type and diet quality affected RMR, but response to soil hardness was verified later. Animals fed with high quality (HQ) diet showed similar body temperature irrespective of soil condition, while animals fed with low quality (LQ) diet showed lower T(b) under soft soil (SS). Individuals fed with LQ diet showed lower RMR and both, lower digestibility and high transit time of food than those fed with HQ diet. Moreover, increments in feeding and defecation rates were observed in the former group. Number of reingested feces did not differ between animals fed with diets of different quality. However, when incidence of reingestion was considered, animals fed with HQ diet showed higher values of feces ingestion. Either feeding, resting and activity patterns were arrhythmic. However, for animals fed with LQ diet a tendency to rhythmic coprophagy was observed and it could be considered as a way to optimize feeding. This study shows that RMR is limited by digestive efficiency which is influenced by diet quality, but also thermal stress may limit the conversion of assimilated energy into work and heat.

Genetic correlations in a wild rodent: grass-eaters and fast-growers evolve high basal metabolic rates

Basal metabolic rate (BMR), commonly used as a measure of the cost of living, is highly variable among species, and sources of the variation are subject to an enduring debate among comparative biologists. One of the hypotheses links the variation in BMR with diversity of food habits and life-history traits. We test this hypothesis by asking how BMR of a particular species, the bank vole Myodes (=Clethrionomys) glareolus, would change under selection for high growth rate (measured as a postweaning body mass change; MD(PW)) and the ability to cope with a low-quality herbivorous diet (measured as body mass change during a four-day test; MD(LQD)). We show that both of the traits are heritable in the narrow sense (MD(PW): h(2)= 0.30; MD(LQD): h(2)= 0.19), and are genetically correlated with mass-independent BMR (additive genetic correlation, r(A)= 0.28 for MD(PW) and 0.37 for MD(LQD)). Thus, both of the traits could change in response to a selection, and the selection would also result in a correlated evolution of the level of metabolism. The results are consistent with the hypothesis that a part of the interspecific variation in BMR evolved in response to selection for life-history and ecological traits such as food habits.

Basal metabolism is correlated with habitat productivity among populations of degus (Octodon degus)

Several competing hypotheses attempt to explain how environmental conditions affect mass-independent basal metabolic rate (BMR) in mammals. One of the most inclusive is the hypothesis that associates BMR with food habits, including habitat productivity. The effects of food habits have been widely investigated at the interspecific level, and variation between individuals and populations has been largely ignored. Intraspecific analysis of physiological traits has the potential to compensate for many pitfalls associated with interspecific analyses and serve as a useful approach for evaluating hypotheses regarding metabolic adaptation. Here we tested the effects of climatic variables (mean annual rainfall=PP, mean annual temperature=T(A)), net primary productivity (NPP) and the de Martonne index (DMi) of aridity on mass-independent BMR among four populations of the caviomorph rodent Octodon degus along a geographic gradient in Chile. BMR was measured on animals maintained in a common garden acclimation set-up, thus kept under the same environment and diet quality for at least 6 months. Mass-independent BMR was significantly different among degu populations showing a large intraspecific spread in metabolic rates. A very large fraction of interpopulational variability in mass-independent BMR was explained by NPP, PP and DMi. Our results were conclusive about the effects of habitat productivity on setting the level of mass-independent BMR at the intraspecific-interpopulational level.