Endocrine responses to environmental variation

Hormones regulate most physiological functions and life history from embryonic development to reproduction. In addition to their roles in growth and development, hormones also mediate responses to the abiotic, social and nutritional environments. Hormone signalling is responsive to environmental changes to adjust phenotypes to prevailing conditions. Both hormone levels and receptor densities can change to provide a flexible system of regulation. Endocrine flexibility connects the environment to organismal function, and it is central to understanding environmental impacts and their effect on individuals and populations. Hormones may also act as a 'sensor' to link environmental signals to epigenetic processes and thereby effect phenotypic plasticity within and across generations. Many environmental parameters are now changing in unprecedented ways as a result of human activity. The knowledge base of organism-environmental interactions was established in environments that differ in many ways from current conditions as a result of ongoing human impacts. It is an urgent contemporary challenge to understand how evolved endocrine responses will modulate phenotypes in response to anthropogenic environmental impacts including climate change, light-at-night and chemical pollution. Endocrine responses play a central role in ecology, and their integration into conservation can lead to more effective outcomes. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Thyroid hormone links environmental signals to DNA methylation

Environmental conditions experienced within and across generations can impact individual phenotypes via so-called 'epigenetic' processes. Here we suggest that endocrine signalling acts as a 'sensor' linking environmental inputs to epigenetic modifications. We focus on thyroid hormone signalling and DNA methylation, but other mechanisms are likely to act in a similar manner. DNA methylation is one of the most important epigenetic mechanisms, which alters gene expression patterns by methylating cytosine bases via DNA methyltransferase enzymes. Thyroid hormone is mechanistically linked to DNA methylation, at least partly by regulating the activity of DNA methyltransferase 3a, which is the principal enzyme that mediates epigenetic responses to environmental change. Thyroid signalling is sensitive to natural and anthropogenic environmental impacts (e.g. light, temperature, endocrine-disrupting pollution), and here we propose that thyroid hormone acts as an environmental sensor to mediate epigenetic modifications. The nexus between thyroid hormone signalling and DNA methylation can integrate multiple environmental signals to modify phenotypes, and coordinate phenotypic plasticity at different time scales, such as within and across generations. These dynamics can have wide-ranging effects on health and fitness of animals, because they influence the time course of phenotypic adjustments and potentially the range of environmental stimuli that can elicit epigenetic responses. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

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'.

Warming and pollution interact to alter energy transfer efficiency, performance and fitness across generations in zebrafish (Danio rerio)

Energy transfer efficiency across different trophic levels, from food to new biomass, can determine population dynamics and food-web function. Here we show that the energy needed to produce a unit of new biomass increases with warming and exposure to bisphenol A (BPA), an endocrine disrupting compound. These environmental effects are at least partially transmitted across generations via DNA methylation. We raised parental (F0) and their offspring (F1) zebrafish (Danio rerio) of two genotypes (DNA methyltransferase 3a knock-out [DNMT3a-/-] and wild type [DNMT3a+/+]) at different temperatures (24 and 30 °C), with and without BPA (0 and 10 μg l-1) to test whether the effects of BPA are i) temperature specific, ii) mediated by DNA methylation, and iii) transmitted across generations even if offspring are not exposed. All experimental factors interacted to influence growth in length and mass, and metabolic rates with the result that wild-type F0 and F1 fish experienced the greatest energetic cost of growth under warm conditions in the presence of BPA. However, this response was not observed in DNMT3a-/- fish, indicating that DNA methylation is at least partly responsible for mediating these effects. Under the same conditions (warm + BPA) wild-type parents had reduced swimming performance, and reduced fecundity, and offspring embryonic survival was reduced significantly; genotype affected these responses significantly. Our results indicate that the conditions that are becoming increasingly common globally - warming and endocrine disrupting compounds from plastic pollution and production - can have detrimental effects on energy transfer efficiency and thereby potentially on food-web structure. These effects can be transmitted across generations even if offspring are not exposed to the pollutant, and are likely to have ramifications for conservation and fisheries.

New horizons for comparative studies and meta-analyses

Comparative analyses and meta-analyses are key tools to elucidate broad biological principles, yet the two approaches often appear different in purpose. We propose an integrated approach that can generate deeper insights into ecoevolutionary processes. Marrying comparative and meta-analytic approaches will allow for (i) a more accurate investigation of drivers of biological variation, (ii) a greater ability to account for sources of non-independence in experimental data, (iii) more effective control of publication bias, and (iv) improved transparency and reproducibility. Stronger integration of meta-analytic and comparative studies can also broaden the scope from species-centric investigations to community-level responses and function-valued traits (e.g., reaction norms). We illuminate commonalities, differences, and the transformative potential of combining these methodologies for advancing ecology and evolutionary biology.

Feeding frequency does not interact with BPA exposure to influence metabolism or behaviour in zebrafish (Danio rerio)

Resource limitation can constrain energy (ATP) production, and thereby affect locomotion and behaviour such as exploration of novel environments and boldness. Consequently, ecological processes such as dispersal and interactions within and between species may be influenced by food availability. Energy metabolism, and behaviour are regulated by endocrine signalling, and may therefore be impacted by endocrine disrupting compounds (EDCs) including bisphenol A (BPA) derived from plastic manufacture and pollution. It is important to determine the impacts of these novel environmental contexts to understand how human activity alters individual physiology and behaviour and thereby populations. Our aim was to determine whether BPA exposure interacts with feeding frequency to alter metabolism and behaviour. In a fully factorial experiment, we show that low feeding frequency reduced zebrafish (Danio rerio) mass, condition, resting metabolic rates, total distance moved and speed in a novel arena, as well as anxiety indicated by the number of times fish returned to a dark shelter. However, feeding frequency did not significantly affect maximal metabolic rates, aerobic scope, swimming performance, latency to leave a shelter, or metabolic enzyme activities (citrate synthase and lactate dehydrogenase). Natural or anthropogenic fluctuation in food resources can therefore impact energetics and movement of animals with repercussions for ecological processes such as dispersal. BPA exposure reduced LDH activity and body mass, but did not interact with feeding frequency. Hence, behaviour of adult fish is relatively insensitive to disruption by BPA. However, alteration of LDH activity by BPA could disrupt lactate metabolism and signalling and together with reduction in body mass could affect size-dependent reproductive output. BPA released by plastic manufacture and pollution can thereby impact conservation and management of natural resources.

Sex-specific transgenerational plasticity: developmental temperatures of mothers and fathers have different effects on sons and daughters

Each parent can influence offspring phenotype via provisioning of the zygote or sex-specific DNA methylation. Transgenerational plasticity may therefore depend on the environmental conditions experienced by each parent. We tested this hypothesis by conducting a fully factorial experiment across three generations of guppies (Poecilia reticulata), determining the effects of warm (28°C) and cold (21°C) thermal backgrounds of mothers and fathers on mass and length, and thermal performance (sustained and sprint swimming speeds, citrate synthase and lactate dehydrogenase activities; 18, 24, 28, 32 and 36°C test temperatures) of sons and daughters. Offspring sex was significant for all traits except for sprint speed. Warmer mothers produced sons and daughters with reduced mass and length, and warmer fathers produced shorter sons. Sustained swimming speed (Ucrit) of male offspring was greatest when both parents were raised at 28°C, and warmer fathers produced daughters with greater Ucrit. Similarly, warmer fathers produced sons and daughters with greater metabolic capacity. We show that the thermal variation experienced by parents can modify offspring phenotype, and that predicting the impacts of environmental change on populations would require knowledge of the thermal background of each mother and father, particularly where sexes are spatially segregated.

How can physiology best contribute to wildlife conservation in a warming world?

Global warming is now predicted to exceed 1.5°C by 2033 and 2°C by the end of the 21st century. This level of warming and the associated environmental variability are already increasing pressure on natural and human systems. Here we emphasize the role of physiology in the light of the latest assessment of climate warming by the Intergovernmental Panel on Climate Change. We describe how physiology can contribute to contemporary conservation programmes. We focus on thermal responses of animals, but we acknowledge that the impacts of climate change are much broader phylogenetically and environmentally. A physiological contribution would encompass environmental monitoring, coupled with measuring individual sensitivities to temperature change and upscaling these to ecosystem level. The latest version of the widely accepted Conservation Standards designed by the Conservation Measures Partnership includes several explicit climate change considerations. We argue that physiology has a unique role to play in addressing these considerations. Moreover, physiology can be incorporated by institutions and organizations that range from international bodies to national governments and to local communities, and in doing so, it brings a mechanistic approach to conservation and the management of biological resources.

Can animals tune tissue mechanics in response to changing environments caused by anthropogenic impacts?

Anthropogenic climate change and pollution are impacting environments across the globe. This Review summarises the potential impact of such anthropogenic effects on animal tissue mechanics, given the consequences for animal locomotor performance and behaviour. More specifically, in light of current literature, this Review focuses on evaluating the acute and chronic effects of temperature on the mechanical function of muscle tissues. For ectotherms, maximal muscle performance typically occurs at temperatures approximating the natural environment of the species. However, species vary in their ability to acclimate to chronic changes in temperature, which is likely to have longer-term effects on species range. Some species undergo periods of dormancy to avoid extreme temperature or drought. Whilst the skeletal muscle of such species generally appears to be adapted to minimise muscle atrophy and maintain performance for emergence from dormancy, the increased occurrence of extreme climatic conditions may reduce the survival of individuals in such environments. This Review also considers the likely impact of anthropogenic pollutants, such as hormones and heavy metals, on animal tissue mechanics, noting the relative paucity of literature directly investigating this key area. Future work needs to determine the direct effects of anthropogenic environmental changes on animal tissues and related changes in locomotor performance and behaviour, including accounting for currently unknown interactions between environmental factors, e.g. temperature and pollutants.

Bisphenol S reduces locomotor performance and modifies muscle protein levels but not mitochondrial bioenergetics in adult zebrafish

Human activity has now introduced novel chemicals into most aquatic ecosystems. Endocrine-disrupting compounds originating from plastic pollution and manufacture can have pronounced biological effects by disrupting hormone-mediated processes. Bisphenol A (BPA) is one of the most commonly produced endocrine-disrupting compounds, which interferes with signalling by a broad range of hormones. In recognition of its potentially harmful effects, BPA is being replaced by substitutes such as bisphenol S (BPS). However, toxicological studies revealed that BPS too can bind to hormone receptors and disrupt signalling, particularly of thyroid hormone. The aim of this study was to test whether BPS exposure impacts locomotor performance and muscle function in zebrafish (Danio rerio). Locomotor performance depends on thyroid hormone signalling, and it is closely related to fitness so that its disruption can have negative ecological and evolutionary consequences. BPS exposure of 15 μg l^-1 [∼60 nM] and 30 μg l^-1 (but not 60 μg l^-1) decreased sustained swimming performance (U_crit), but not sprint speed. In a fully factorial design, we show that living in flowing water increased U_crit compared to a still water control, and that BPS reduced U_crit under both conditions but did not eliminate the training effect. In a second factorial experiment, we show that BPS did not affect mitochondrial bioenergetics in skeletal muscle (state 3 and 4 rates, respiratory control ratios, ROS production), but that induced hypothyroidism decreased state 3 and 4 rates of respiration. However, both hypothyroidism and BPS exposure decreased activity of AMP-activated protein kinase (pAMPK:total AMPK) but increased protein levels of myocyte enhancer factor 2, and slow and fast myosin heavy chains. Our data indicate that BPS is not a safe alternative for BPA and that exposure to BPS can have ecological consequences, which are likely to be at least partly mediated via thyroid hormone disruption.

Biological mechanisms matter in contemporary wildlife conservation

Given limited resources for wildlife conservation paired with an urgency to halt declines and rebuild populations, it is imperative that management actions are tactical and effective. Mechanisms are about how a system works and can inform threat identification and mitigation such that conservation actions that work can be identified. Here, we call for a more mechanistic approach to wildlife conservation and management where behavioral and physiological tools and knowledge are used to characterize drivers of decline, identify environmental thresholds, reveal strategies that would restore populations, and prioritize conservation actions. With a growing toolbox for doing mechanistic conservation research as well as a suite of decision-support tools (e.g., mechanistic models), the time is now to fully embrace the concept that mechanisms matter in conservation ensuring that management actions are tactical and focus on actions that have the potential to directly benefit and restore wildlife populations.

Interactive effects of anthropogenic environmental drivers on endocrine responses in wildlife

Anthropogenic activity has created unique environmental drivers, which may interact to produce unexpected effects. My aim was to conduct a systematic review of the interactive effects of anthropogenic drivers on endocrine responses in non-human animals. The interaction between temperature and light can disrupt reproduction and growth by impacting gonadotropins, thyroid hormones, melatonin, and growth hormone. Temperature and endocrine disrupting compounds (EDCs) interact to modify reproduction with differential effects across generations. The combined effects of light and EDCs can be anxiogenic, so that light-at-night could increase anxiety in wildlife. Light and noise increase glucocorticoid release by themselves, and together can modify interactions between individuals and their environment. The literature detailing interactions between drivers is relatively sparse and there is a need to extend research to a broader range of taxa and interactions. I suggest that incorporating endocrine responses into Adverse Outcome Pathways would be beneficial to improve predictions of environmental effects.

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

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

Bisphenols impact hormone levels in animals: A meta-analysis

Bisphenols are used in the manufacture of plastics and are endocrine disrupting compounds detectable in free living organisms and environments globally. The original bisphenol, bisphenol A (BPA), is best known as a xenoestrogen, but it also disrupts other steroid hormones and other classes of hormones including thyroid and pituitary hormones. When its toxicity became better known, BPA was replaced by presumably less toxic alternatives, including bisphenols S, F, and AF. However, recent data suggest that all bisphenols can have endocrine disrupting effects, although their impacts remain unresolved particularly in non-human animals. Our aim was to establish the current state-of-knowledge of the effects of different bisphenols on circulating hormone levels in non-human animals. Our meta-analysis showed that a diverse range of hormones (including thyroid hormones, corticosterone, follicle stimulating hormone, luteinizing hormone, and estradiol) are strongly impacted by exposure to any bisphenol type, and that in laboratory rats (Rattus norvegicus) the effect was modified by life-stage. Although there were qualitative differences, BPA alternatives had as great or greater effects on hormone levels as BPA. However, data coverage across hormones was uneven, and most studies measured the effects of BPA on vertebrate reproductive hormones. Similarly, taxonomic coverage was poor. Over 80% of data originated from laboratory rats and zebrafish (Danio rerio) and there are no data for whole classes of invertebrates and vertebrates (e.g., amphibians). Our results show that all bisphenols alter circulating levels of a broad range of hormones. However, the current state-of-knowledge is incomplete so that the ecological impacts of bisphenols are difficult to gauge, although based on the available data bisphenols are likely to be detrimental to a broad range of taxa and ecosystems.

Evolution of plasticity: metabolic compensation for fluctuating energy demands at the origin of life

Phenotypic plasticity of physiological functions enables rapid responses to changing environments and may thereby increase the resilience of organisms to environmental change. Here, we argue that the principal hallmarks of life itself, self-replication and maintenance, are contingent on the plasticity of metabolic processes ('metabolic plasticity'). It is likely that the Last Universal Common Ancestor (LUCA), 4 billion years ago, already possessed energy-sensing molecules that could adjust energy (ATP) production to meet demand. The earliest manifestation of metabolic plasticity, switching cells from growth and storage (anabolism) to breakdown and ATP production (catabolism), coincides with the advent of Darwinian evolution. Darwinian evolution depends on reliable translation of information from information-carrying molecules, and on cell genealogy where information is accurately passed between cell generations. Both of these processes create fluctuating energy demands that necessitate metabolic plasticity to facilitate replication of genetic material and (proto)cell division. We propose that LUCA possessed rudimentary forms of these capabilities. Since LUCA, metabolic networks have increased in complexity. Generalist founder enzymes formed the basis of many derived networks, and complexity arose partly by recruiting novel pathways from the untapped pool of reactions that are present in cells but do not have current physiological functions (the so-called 'underground metabolism'). Complexity may thereby be specific to environmental contexts and phylogenetic lineages. We suggest that a Boolean network analysis could be useful to model the transition of metabolic networks over evolutionary time. Network analyses can be effective in modelling phenotypic plasticity in metabolic functions for different phylogenetic groups because they incorporate actual biochemical regulators that can be updated as new empirical insights are gained.

Rates of warming impact oxidative stress in zebrafish (Danio rerio)

Potentially negative effects of thermal variation on physiological functions may be modulated by compensatory responses, but their efficacy depends on the timescale of phenotypic adjustment relative to the rate of temperature change. Increasing temperatures in particular can affect mitochondrial bioenergetics and rates of reactive oxygen species (ROS) production. Our aim was to test whether different rates of temperature increase impact mitochondrial bioenergetics and modulate oxidative stress. We exposed zebrafish (Danio rerio) to warming from 20 to 28°C over 3, 6, 24, or 48 h, and compared these to a control group that was kept at constant 20°C. Fish exposed to the fastest (3 h) and slowest (48 h) rates of warming had significantly higher rates of H2O2 production relative to the control treatment, and the proportion of O2 converted to H2O2 (H2O2/O2 ratio) was significantly greater in these groups. However, ROS production was not paralleled by differences in mitochondrial substrate oxidation rates, leak respiration rates, or coupling (respiratory control ratios). Increased rates of ROS production did not lead to damage of proteins or membranes, which may be explained by a moderate increase in catalase activity at the fastest, but not the slowest rate of warming. The increase in ROS production at the slowest rate of heating indicates that even seemingly benign environments may be stressful. Understanding how animals respond to different rates of temperature change is important, because the rate determines the time period for phenotypic adjustments and it also alters the environmental thermal signal that triggers compensatory pathways.

Endocrine disruption from plastic pollution and warming interact to increase the energetic cost of growth in a fish

Energetic cost of growth determines how much food-derived energy is needed to produce a given amount of new biomass and thereby influences energy transduction between trophic levels. Growth and development are regulated by hormones and are therefore sensitive to changes in temperature and environmental endocrine disruption. Here, we show that the endocrine disruptor bisphenol A (BPA) at an environmentally relevant concentration (10 µgl^-1) decreased fish (Danio rerio) size at 30°C water temperature. Under the same conditions, it significantly increased metabolic rates and the energetic cost of growth across development. By contrast, BPA decreased the cost of growth at cooler temperatures (24°C). BPA-mediated changes in cost of growth were not associated with mitochondrial efficiency (P/O ratios (i.e. adenosine diphosphate (ADP) used/oxygen consumed) and respiratory control ratios) although BPA did increase mitochondrial proton leak. In females, BPA decreased age at maturity at 24°C but increased it at 30°C, and it decreased the gonadosomatic index suggesting reduced investment into reproduction. Our data reveal a potentially serious emerging problem: increasing water temperatures resulting from climate warming together with endocrine disruption from plastic pollution can impact animal growth efficiency, and hence the dynamics and resilience of animal populations and the services these provide.

Physiological Performance Curves: When Are They Useful?

This review serves as an introduction to a special issue of Frontiers in Physiology, focused on the importance of physiological performance curves across phylogenetic and functional boundaries. Biologists have used performance curves to describe the effects of changing environmental conditions on animal physiology since the late 1800s (at least). Animal physiologists have studied performance curves extensively over the past decades, and there is a good foundation to understanding how the environment affects physiological functions of individuals. Our goal here was to build upon this research and address outstanding questions regarding the mutability and applicability of performance curves across taxonomic groups and levels of biological organization. Performance curves are not fixed at a taxonomic, population, or individual level – rather they are dynamic and can shift in response to evolutionary pressures (e.g., selection) and epigenetic programming (e.g., plasticity). The mechanisms underlying these shifts are being increasingly used to predict the efficacy with which plasticity and heritability of performance curves can render individuals and populations less vulnerable to climate change. Individual differences in physiological performance curves (and plasticity of performance curves) can also have cascading effects at higher levels of biological organization. For instance, individual physiology likely influences group behaviors in non-additive ways. There is a need therefore to extend the concept of performance curves to social interactions and sociality. Collectively, this special issue emphasizes the power of how within- and between-individual shifts in performance curves might scale up to the population-, species-, and community-level dynamics that inform conservation management strategies.

Two Locomotor Traits Show Different Patterns of Developmental Plasticity Between Closely Related Clonal and Sexual Fish

The capacity to compensate for environmental change determines population persistence and biogeography. In ectothermic organisms, performance at different temperatures can be strongly affected by temperatures experienced during early development. Such developmental plasticity is mediated through epigenetic mechanisms that induce phenotypic changes within the animal's lifetime. However, epigenetic modifiers themselves are encoded by DNA so that developmental plasticity could itself be contingent on genetic diversity. In this study, we test the hypothesis that the capacity for developmental plasticity depends on a species' among-individual genetic diversity. To test this, we exploited a unique species complex that contains both the clonal, genetically identical Amazon molly (Poecilia formosa), and the sexual, genetically diverse Atlantic molly (Poecilia mexicana). We predicted that the greater among-individual genetic diversity in the Atlantic molly may increase their capacity for developmental plasticity. We raised both clonal and sexual mollies at either warm (28°C) or cool (22°C) temperatures and then measured locomotor capacity (critical sustained swimming performance) and unforced movement in an open field across a temperature gradient that simulated environmental conditions often experienced by these species in the wild. In the clonal Amazon molly, differences in the developmental environment led to a shift in the thermal performance curve of unforced movement patterns, but much less so in maximal locomotor capacity. In contrast, the sexual Atlantic mollies exhibited the opposite pattern: developmental plasticity was present in maximal locomotor capacity, but not in unforced movement. Thus our data show that developmental plasticity in clones and their sexual, genetically more diverse sister species is trait dependent. This points toward mechanistic differences in how genetic diversity mediates plastic responses exhibited in different traits.

Plasticity of Performance Curves in Ectotherms: Individual Variation Modulates Population Responses to Environmental Change

Many ectothermic animals can respond to changes in their environment by altering the sensitivities of physiological rates, given sufficient time to do so. In other words, thermal acclimation and developmental plasticity can shift thermal performance curves so that performance may be completely or partially buffered against the effects of environmental temperature changes. Plastic responses can thereby increase the resilience to temperature change. However, there may be pronounced differences between individuals in their capacity for plasticity, and these differences are not necessarily reflected in population means. In a bet-hedging strategy, only a subsection of the population may persist under environmental conditions that favour either plasticity or fixed phenotypes. Thus, experimental approaches that measure means across individuals can not necessarily predict population responses to temperature change. Here, we collated published data of 608 mosquitofish (Gambusia holbrooki) each acclimated twice, to a cool and a warm temperature in random order, to model how diversity in individual capacity for plasticity can affect populations under different temperature regimes. The persistence of both plastic and fixed phenotypes indicates that on average, neither phenotype is selectively more advantageous. Fish with low acclimation capacity had greater maximal swimming performance in warm conditions, but their performance decreased to a greater extent with decreasing temperature in variable environments. In contrast, the performance of fish with high acclimation capacity decreased to a lesser extent with a decrease in temperature. Hence, even though fish with low acclimation capacity had greater maximal performance, high acclimation capacity may be advantageous when ecologically relevant behaviour requires submaximal locomotor performance. Trade-offs, developmental effects and the advantages of plastic phenotypes together are likely to explain the observed population variation.

Diet and temperature modify the relationship between energy use and ATP production to influence behavior in zebrafish (Danio rerio)

Food availability and temperature influence energetics of animals and can alter behavioral responses such as foraging and spontaneous activity. Food availability, however, is not necessarily a good indicator of energy (ATP) available for cellular processes. The efficiency of energy transduction from food-derived substrate to ATP in mitochondria can change with environmental context. Our aim was to determine whether the interaction between food availability and temperature affects mitochondrial efficiency and behavior in zebrafish (Danio rerio). We conducted a fully factorial experiment to test the effects of feeding frequency, acclimation temperature (three weeks to 18 or 28°C), and acute test temperature (18 and 28°C) on whole-animal oxygen consumption, mitochondrial bioenergetics and efficiency (ADP consumed per oxygen atom; P:O ratio), and behavior (boldness and exploration). We show that infrequently fed (once per day on four days per week) zebrafish have greater mitochondrial efficiency than frequently fed (three times per day on five days per week) animals, particularly when warm-acclimated. The interaction between temperature and feeding frequency influenced exploration of a novel environment, but not boldness. Both resting rate of producing ATP and scope for increasing it were positively correlated with time spent exploring and distance moved in standardized trials. In contrast, behavior was not associated with whole-animal aerobic (oxygen consumption) scope, but exploration was positively correlated with resting oxygen consumption rates. We highlight the importance of variation in both metabolic (oxygen consumption) rate and efficiency of producing ATP in determining animal performance and behavior. Oxygen consumption represents energy use, and P:O ratio is a variable that determines how much of that energy is allocated to ATP production. Our results emphasize the need to integrate whole-animal responses with subcellular traits to evaluate the impact of environmental conditions on behavior and movement.

Social rank and not physiological capacity determines competitive success in zebrafish (Danio rerio)

Competition for resources shapes ecological and evolutionary relationships. Physiological capacities such as in locomotor performance can influence the fitness of individuals by increasing competitive success. Social hierarchy too can affect outcomes of competition by altering locomotor behaviour or because higher ranking individuals monopolize resources. Here, we tested the hypotheses that competitive success is determined by sprint performance or by social status. We show that sprint performance of individuals measured during escape responses (fast start) or in an accelerated sprint test did not correlate with realized sprint speed while competing for food within a social group of five fish; fast start and accelerated sprint speed were higher than realized speed. Social status within the group was the best predictor of competitive success, followed by realized speed. Social hierarchies in zebrafish are established within 7 days of their first encounter, and interestingly, there was a positive correlation between social status and realized speed 1 and 4 days after fish were placed in a group, but not after 7 days. These data indicate that physiological performance decreases in importance as social relationships are established. Also, maximal physiological capacities were not important for competitive success, but swimming speed changed with social context.

DNA methyltransferase 3a mediates developmental thermal plasticity

BACKGROUND

Thermal plasticity is pivotal for evolution in changing climates and in mediating resilience to its potentially negative effects. The efficacy to respond to environmental change depends on underlying mechanisms. DNA methylation induced by DNA methyltransferase 3 enzymes in the germline or during early embryonic development may be correlated with responses to environmental change. This developmental plasticity can interact with reversible acclimation within adult organisms, which would increase the speed of response and could alleviate potential mismatches between parental or early embryonic environments and those experienced at later life stages. Our aim was to determine whether there is a causative relationship between DNMT3 enzyme and developmental thermal plasticity and whether either or both interact with short-term acclimation to alter fitness and thermal responses in zebrafish (Danio rerio).

RESULTS

We developed a novel DNMT3a knock-out model to show that sequential knock-out of DNA methyltransferase 3a isoforms (DNMT3aa-/- and DNMT3aa-/-ab-/-) additively decreased survival and increased deformities when cold developmental temperatures in zebrafish offspring mismatched warm temperatures experienced by parents. Interestingly, short-term cold acclimation of parents before breeding rescued DNMT3a knock-out offspring by restoring survival at cold temperatures. DNMT3a knock-out genotype interacted with developmental temperatures to modify thermal performance curves in offspring, where at least one DNMT3a isoform was necessary to buffer locomotion from increasing temperatures. The thermal sensitivity of citrate synthase activity, an indicator of mitochondrial density, was less severely affected by DNMT3a knock-out, but there was nonetheless a significant interaction between genotype and developmental temperatures.

CONCLUSIONS

Our results show that DNMT3a regulates developmental thermal plasticity and that the phenotypic effects of different DNMT3a isoforms are additive. However, DNMT3a interacts with other mechanisms, such as histone (de)acetylation, induced during short-term acclimation to buffer phenotypes from environmental change. Interactions between these mechanisms make phenotypic compensation for climate change more efficient and make it less likely that thermal plasticity incurs a cost resulting from environmental mismatches.

Bisphenols alter thermal responses and performance in zebrafish (Danio rerio)

Plastic pollutants are novel environmental stressors that are now persistent components of natural ecosystems. Endocrine disrupting chemicals such as bisphenols that leach out of plastics can modify physiological responses of animals by interfering with hormone signalling. Here, we tested whether three commonly produced bisphenols, bisphenol A (BPA), bisphenol F (BPF) and bisphenol S (BPS), impair thermal acclimation of swimming performance and metabolic enzyme [citrate synthase (CS) and lactate dehydrogenase (LDH)] activities in adult zebrafish (Danio rerio). We found that exposure to 30-μg l-1 BPF and BPS, but not BPA, reduced swimming performance, and no interactions between bisphenol exposure and acclimation (3 weeks to 18°C and 28°C) or acute test (18°C and 28°C) temperatures were found. BPA interacted with acclimation and acute test temperatures to determine CS activity, an indicator of mitochondrial density and aerobic metabolic capacity. BPS reduced CS activity and an interaction (at a one-tailed significance) between acclimation temperature and BPF exposure determined CS activity. LDH activity reflects anaerobic ATP production capacity, and BPA and BPF altered the effects of thermal acclimation and acute test temperatures on LDH activity. Our data show that all bisphenols we tested at ecologically relevant concentrations can disrupt the thermal responses of fish. BPS and BPF are used as environmentally safer alternatives to BPA, but our data show that these bisphenols are also of concern, particularly in thermally variable environments.

Integrating Mitochondrial Aerobic Metabolism into Ecology and Evolution

Biologists have long appreciated the critical role that energy turnover plays in understanding variation in performance and fitness among individuals. Whole-organism metabolic studies have provided key insights into fundamental ecological and evolutionary processes. However, constraints operating at subcellular levels, such as those operating within the mitochondria, can also play important roles in optimizing metabolism over different energetic demands and time scales. Herein, we explore how mitochondrial aerobic metabolism influences different aspects of organismal performance, such as through changing adenosine triphosphate (ATP) and reactive oxygen species (ROS) production. We consider how such insights have advanced our understanding of the mechanisms underpinning key ecological and evolutionary processes, from variation in life-history traits to adaptation to changing thermal conditions, and we highlight key areas for future research.

One hundred research questions in conservation physiology for generating actionable evidence to inform conservation policy and practice

Environmental change and biodiversity loss are but two of the complex challenges facing conservation practitioners and policy makers. Relevant and robust scientific knowledge is critical for providing decision-makers with the actionable evidence needed to inform conservation decisions. In the Anthropocene, science that leads to meaningful improvements in biodiversity conservation, restoration and management is desperately needed. Conservation Physiology has emerged as a discipline that is well-positioned to identify the mechanisms underpinning population declines, predict responses to environmental change and test different in situ and ex situ conservation interventions for diverse taxa and ecosystems. Here we present a consensus list of 10 priority research themes. Within each theme we identify specific research questions (100 in total), answers to which will address conservation problems and should improve the management of biological resources. The themes frame a set of research questions related to the following: (i) adaptation and phenotypic plasticity; (ii) human-induced environmental change; (iii) human-wildlife interactions; (iv) invasive species; (v) methods, biomarkers and monitoring; (vi) policy, engagement and communication; (vii) pollution; (viii) restoration actions; (ix) threatened species; and (x) urban systems. The themes and questions will hopefully guide and inspire researchers while also helping to demonstrate to practitioners and policy makers the many ways in which physiology can help to support their decisions.

Thermal adaptation in the honeybee (Apis mellifera) via changes to the structure of malate dehydrogenase

In honeybees there are three alleles of cytosolic malate dehydrogenase gene: F, M and S. Allele frequencies are correlated with environmental temperature, suggesting that the alleles have temperature-dependent fitness benefits. We determined the enzyme activity of each allele across a range of temperatures in vitro The F and S alleles have higher activity and are less sensitive to high temperatures than the M allele, which loses activity after incubation at temperatures found in the thorax of foraging bees in hot climates. Next, we predicted the protein structure of each allele and used molecular dynamics simulations to investigate their molecular flexibility. The M allozyme is more flexible than the S and F allozymes at 50°C, suggesting a plausible explanation for its loss of activity at high temperatures, and has the greatest structural flexibility at 15°C, suggesting that it can retain some enzyme activity at cooler temperatures. MM bees recovered from 2 h of cold narcosis significantly better than all other genotypes. Combined, these results explain clinal variation in malate dehydrogenase allele frequencies in the honeybee at the molecular level.

Age-related changes in isolated mouse skeletal muscle function are dependent on sex, muscle, and contractility mode

The present study aimed to simultaneously examine the age-related, muscle-specific, sex-specific, and contractile mode-specific changes in isolated mouse skeletal muscle function and morphology across multiple ages. Measurements of mammalian muscle morphology, isometric force and stress (force/cross-sectional area), absolute and normalized (power/muscle mass) work-loop power across a range of contractile velocities, fatigue resistance, and myosin heavy chain (MHC) isoform concentration were measured in 232 isolated mouse (CD-1) soleus, extensor digitorum longus (EDL), and diaphragm from male and female animals aged 3, 10, 30, 52, and 78 wk. Aging resulted in increased body mass and increased soleus and EDL muscle mass, with atrophy only present for female EDL by 78 wk despite no change in MHC isoform concentration. Absolute force and power output increased up to 52 wk and to a higher level for males. A 23–36% loss of isometric stress exceeded the 14–27% loss of power normalized to muscle mass between 10 wk and 52 wk, although the loss of normalized power between 52 and 78 wk continued without further changes in stress ( P > 0.23). Males had lower power normalized to muscle mass than females by 78 wk, with the greatest decline observed for male soleus. Aging did not cause a shift toward slower contractile characteristics, with reduced fatigue resistance observed in male EDL and female diaphragm. Our findings show that the loss of muscle quality precedes the loss of absolute performance as CD-1 mice age, with the greatest effect seen in male soleus, and in most instances without muscle atrophy or an alteration in MHC isoforms.

What do warming waters mean for fish physiology and fisheries?

Environmental signals act primarily on physiological systems, which then influence higher-level functions such as movement patterns and population dynamics. Increases in average temperature and temperature variability associated with global climate change are likely to have strong effects on fish physiology and thereby on populations and fisheries. Here we review the principal mechanisms that transduce temperature signals and the physiological responses to those signals in fish. Temperature has a direct, thermodynamic effect on biochemical reaction rates. Nonetheless, plastic responses to longer-term thermal signals mean that fishes can modulate their acute thermal responses to compensate at least partially for thermodynamic effects. Energetics are particularly relevant for growth and movement, and therefore for fisheries, and temperature can have pronounced effects on energy metabolism. All energy (ATP) production is ultimately linked to mitochondria, and temperature has pronounced effects on mitochondrial efficiency and maximal capacities. Mitochondria are dependent on oxygen as the ultimate electron acceptor so that cardiovascular function and oxygen delivery link environmental inputs with energy metabolism. Growth efficiency, that is the conversion of food into tissue, changes with temperature, and there are indications that warmer water leads to decreased conversion efficiencies. Moreover, movement and migration of fish relies on muscle function, which is partially dependent on ATP production but also on intracellular calcium cycling within the myocyte. Neuroendocrine processes link environmental signals to regulated responses at the level of different tissues, including muscle. These physiological processes within individuals can scale up to population responses to climate change. A mechanistic understanding of thermal responses is essential to predict the vulnerability of species and populations to climate change.

Effect of the plastic pollutant bisphenol A on the biology of aquatic organisms: A meta-analysis

Plastic pollution is a global environmental concern. In particular, the endocrine-disrupting chemical bisphenol A (BPA) is nearly ubiquitous in aquatic environments globally, and it continues to be produced and released into the environment in large quantities. BPA disrupts hormone signalling and can thereby have far-reaching physiological and ecological consequences. However, it is not clear whether BPA has consistent effects across biological traits and phylogenetic groups. Hence, the aim of this study was to establish the current state of knowledge of the effect of BPA in aquatic organisms. We show that overall BPA exposure affected aquatic organisms negatively. It increased abnormalities, altered behaviour and had negative effects on the cardiovascular system, development, growth and survival. Early life stages were the most sensitive to BPA exposure in invertebrates and vertebrates, and invertebrates and amphibians seem to be particularly affected. These data provide a context for management efforts in the face of increasing plastic pollution. However, data availability is highly biased with respect to taxonomic groups and traits studies, and in the geographical distribution of sample collection. The latter is the case for both measurements of the biological responses and assessing pollution levels in water ways. Future research effort should be directed towards biological systems, such as studying endocrine disruption directly, and geographical areas (particularly in Africa and Asia) which we identify to be currently undersampled.

Mismatched light and temperature cues disrupt locomotion and energetics via thyroid-dependent mechanisms

Animals integrate information from different environmental cues to maintain performance across environmental gradients. Increasing average temperature and variability induced by climate change can lead to mismatches between seasonal cues. We used mosquitofish (Gambusia holbrooki) to test the hypotheses that mismatches between seasonal temperature and light regimes (short days and warm temperature and vice versa) decrease swimming performance, metabolic rates and mitochondrial efficiency and that the responses to light and temperature are mediated by thyroid hormone. We show that day length influenced thermal acclimation of swimming performance through thyroid-dependent mechanisms. Oxygen consumption rates were influenced by acclimation temperature and thyroid hormone. Mitochondrial substrate oxidation rates (state three rates) were modified by the interaction between temperature and day length, and mitochondrial efficiency (P/O ratios) increased with warm acclimation. Using P/O ratios to calibrate metabolic (oxygen consumption) scope showed that oxygen consumption did not predict adenosine triphosphate (ATP) production. Unlike oxygen consumption, ATP production was influenced by day length in a thyroid-dependent manner. Our data indicate that oxygen consumption alone should not be used as a predictor of ATP production. Overall, the effects of thyroid hormone on locomotion and energetics were reversed by mismatches such as warm temperatures on short days. We predict that mid to high latitudes in North America and Asia will be particularly affected by mismatches as a result of high seasonality and predicted warming over the next 50 years.

Is Endothermy an Evolutionary By-Product?

Endothermy alters the energetic relationships between organisms and their environment and thereby influences fundamental niches. Endothermy is closely tied to energy metabolism. Regulation of energy balance is indispensable for all life and regulatory pathways increase in complexity from bacteria to vertebrates. Increasing complexity of metabolic networks also increase the probability for endothermic phenotypes to appear. Adaptive arguments are problematic epistemologically because the regulatory mechanisms enabling endothermy have not evolved for the 'purpose' of endothermy and the utility of current traits is likely to have changed over evolutionary time. It is most parsimonious to view endothermy as the evolutionary by-product of energy balance regulation rather than as an adaptation and interpret its evolution in the context of metabolic networks.

Importance of adipocyte browning in the evolution of endothermy

Endothermy changes the relationship between organisms and their environment fundamentally, and it is therefore of major ecological and evolutionary significance. Endothermy is characterized by non-shivering thermogenesis, that is metabolic heat production in the absence of muscular activity. In many eutherian mammals, brown adipose tissue (BAT) is an evolutionary innovation that facilitates non-shivering heat production in mitochondria by uncoupling food-derived substrate oxidation from chemical energy (ATP) production. Consequently, energy turnover is accelerated resulting in increased heat release. The defining characteristics of BAT are high contents of mitochondria and vascularization, and the presence of uncoupling protein 1. Recent insights, however, reveal that a range of stimuli such as exercise, diet and the immune system can cause the browning of white adipocytes, thereby increasing energy expenditure and heat production even in the absence of BAT. Here, we review the molecular mechanisms that cause browning of white adipose tissue, and their potential contribution to thermoregulation. The significance for palaeophysiology lies in the presence of adipose tissue and the mechanisms that cause its browning and uncoupling in all amniotes. Hence, adipocytes may have played a role in the evolution of endothermy beyond the more specific evolution of BAT in eutherians. This article is part of the theme issue 'Vertebrate palaeophysiology'.

The impacts of climate change on the biomechanics of animals: Themed Issue Article: Biomechanics and Climate Change

Anthropogenic climate change induces unprecedented variability in a broad range of environmental parameters. These changes will impact material properties and animal biomechanics, thereby affecting animal performance and persistence of populations. Climate change implies warming at the global level, and it may be accompanied by altered wind speeds, wave action, ocean circulation, acidification as well as increased frequency of hypoxic events. Together, these environmental drivers affect muscle function and neural control and thereby movement of animals such as bird migration and schooling behaviour of fish. Altered environmental conditions will also modify material properties of animals. For example, ocean acidification, particularly when coupled with increased temperatures, compromises calcified shells and skeletons of marine invertebrates and byssal threads of mussels. These biomechanical consequences can lead to population declines and disintegration of habitats. Integrating biomechanical research with ecology is instrumental in predicting the future responses of natural systems to climate change and the consequences for ecosystem services such as fisheries and ecotourism.

Reframing conservation physiology to be more inclusive, integrative, relevant and forward-looking: reflections and a horizon scan

Applying physiological tools, knowledge and concepts to understand conservation problems (i.e. conservation physiology) has become commonplace and confers an ability to understand mechanistic processes, develop predictive models and identify cause-and-effect relationships. Conservation physiology is making contributions to conservation solutions; the number of 'success stories' is growing, but there remain unexplored opportunities for which conservation physiology shows immense promise and has the potential to contribute to major advances in protecting and restoring biodiversity. Here, we consider how conservation physiology has evolved with a focus on reframing the discipline to be more inclusive and integrative. Using a 'horizon scan', we further explore ways in which conservation physiology can be more relevant to pressing conservation issues of today (e.g. addressing the Sustainable Development Goals; delivering science to support the UN Decade on Ecosystem Restoration), as well as more forward-looking to inform emerging issues and policies for tomorrow. Our horizon scan provides evidence that, as the discipline of conservation physiology continues to mature, it provides a wealth of opportunities to promote integration, inclusivity and forward-thinking goals that contribute to achieving conservation gains. To advance environmental management and ecosystem restoration, we need to ensure that the underlying science (such as that generated by conservation physiology) is relevant with accompanying messaging that is straightforward and accessible to end users.

Increased physical activity does not improve obesity-induced decreases in muscle quality in zebrafish (Danio rerio)

Obesity has a negative effect on muscle contractile function, and the effects of obesity are not reversed by weight loss. It is therefore important to determine how muscle function can be restored, and exercise is the most promising approach. We tested the hypothesis (in zebrafish, Danio rerio) that moderate aerobic exercise (forced swimming for 30 min/day, raising metabolic rates to at least twice resting levels) will alleviate the negative effects of obesity on muscle function. We allocated zebrafish randomly to experimental treatments in a fully factorial design with diet treatment [three levels: lean control, diet-induced obese, obese followed by weight loss (obese-lean)], and exercise (exercise and sedentary control) as independent factors. Treatments were conducted for 10 wk, and we measured locomotor performance, isolated muscle mechanics, and myosin heavy chain composition. Obesity led to decreased muscle force production per unit area ( P = 0.01), and slowed muscle contraction ( P = 0.004) and relaxation rates ( P = 0.02). These effects were not reversible by weight loss or exercise. However, at the level implemented in our experimental animals, neither diet nor exercise affected swimming performance or myosin heavy chain concentrations. The moderate levels of exercise we implemented therefore are not sufficient to reverse the effects of obesity on muscle function, and higher intensity or a combination of modes of exercise may be necessary to improve muscle quality during obesity and following weight loss.

NEW & NOTEWORTHY Obesity can have a negative effect on muscle function and thereby compromise mobility. Even though aerobic exercise has many physiological benefits in obese and normal-weight individuals, we show that in zebrafish aerobic exercise does not improve obesity-induced reductions in muscle contractile function. A combination between different modes of exercise may be more effective than aerobic exercise alone.

Cost of transport is a repeatable trait but is not determined by mitochondrial efficiency in zebrafish (Danio rerio)

The energy used to move a given distance (cost of transport; CoT) varies significantly between individuals of the same species. A lower CoT allows animals to allocate more of their energy budget to growth and reproduction. A higher CoT may cause animals to adjust their movement across different environmental gradients to reduce energy allocated to movement. The aim of this project was to determine whether CoT is a repeatable trait within individuals, and to determine its physiological causes and ecological consequences. We found that CoT is a repeatable trait in zebrafish (Danio rerio). We rejected the hypothesis that mitochondrial efficiency (P/O ratios) predicted CoT. We also rejected the hypothesis that CoT is modulated by temperature acclimation, exercise training or their interaction, although CoT increased with increasing acute test temperature. There was a weak but significant negative correlation between CoT and dispersal, measured as the number of exploration decisions made by fish, and the distance travelled against the current in an artificial stream. However, CoT was not correlated with the voluntary speed of fish moving against the current. The implication of these results is that CoT reflects a fixed physiological phenotype of an individual, which is not plastic in response to persistent environmental changes. Consequently, individuals may have fundamentally different energy budgets as they move across environments, and may adjust movement patterns as a result of allocation trade-offs. It was surprising that mitochondrial efficiency did not explain differences in CoT, and our working hypothesis is that the energetics of muscle contraction and relaxation may determine CoT. The increase in CoT with increasing acute environmental temperature means that warming environments will increase the proportion of the energy budget allocated to locomotion unless individuals adjust their movement patterns.

Increased wave action promotes muscle performance but increasing temperatures cause a tenacity-endurance trade-off in intertidal snails (Nerita atramentosa)

Concurrent increases in wave action and sea surface temperatures increase the physical impact on intertidal organisms and affect their physiological capacity to respond to that impact. Our aim was to determine whether wave exposure altered muscle function in intertidal snails (Nerita atramentosa) and whether responses to wave action and temperature are plastic, leading to compensation for altered environmental conditions. We show that field snails from exposed shores had greater endurance and vertical tenacity than snails from matched protected shores (n = 5 pairs of shores). There were no differences in muscle metabolic capacities (strombine/lactate dehydrogenase, citrate synthase and cytochrome c oxidase activities) between shore types. Maximum stress (force/foot area) produced by isolated foot muscle did not differ between shore types, but foot muscle from snails on exposed shores had greater endurance. A laboratory experiment showed that vertical tenacity was greater in animals acclimated for 3 weeks to cool winter temperatures (15 C) compared to summer temperatures (25 C), but endurance was greater in snails acclimated to 25°C. Acclimation to water flow that mimicked wave action in the field increased vertical tenacity but decreased endurance. Our data show that increased wave action elicits a training effect on muscle, but that increasing sea surface temperature can cause a trade-off between tenacity and endurance. Ocean warming would negate the beneficial increase in tenacity that could render snails more resistant to acute impacts of wave action, while promoting longer term resistance to dislodgment by waves.

Zebrafish (Danio rerio) as a Model for Sprint Exercise Training

Sprint performance is important ecologically and physiologically, and it can influence fitness by determining outcomes of predator-prey relationships, for example, and it can confer substantial human health benefits. In this article we test whether zebrafish (Danio rerio) are a suitable model to test hypotheses about the effects and consequences of sprint exercise training, and the physiological underpinnings of sprint performance. We show that stage 3 c-starts that capture the initial escape response of fish lasting <1 s were repeatable within individuals. In addition, somewhat longer constant acceleration protocols lasting 10 s (U_10s) or 30 s (U_30s) were highly repeatable within individuals over 3, 6, and 23 days. C-starts within individuals were not correlated with either U_10s or U_30s, indicating that these measures reflect different physiological aspects of sprinting. Stage 3 c-starts and U_10s responded positively to sprint exercise training. Our exercise training protocol (5 × 10 s sprints with 5-min rest periods on 4 days per week for 3 weeks) was based on the human sporting literature, and together, our results indicate that zebrafish are a good model to assess the physiological and behavioral consequences of sprint exercise training.

The effects of obesity on skeletal muscle contractile function

Obesity can cause a decline in contractile function of skeletal muscle, thereby reducing mobility and promoting obesity-associated health risks. We reviewed the literature to establish the current state-of-knowledge of how obesity affects skeletal muscle contraction and relaxation. At a cellular level, the dominant effects of obesity are disrupted calcium signalling and 5'-adenosine monophosphate-activated protein kinase (AMPK) activity. As a result, there is a shift from slow to fast muscle fibre types. Decreased AMPK activity promotes the class II histone deacetylase (HDAC)-mediated inhibition of the myocyte enhancer factor 2 (MEF2). MEF2 promotes slow fibre type expression, and its activity is stimulated by the calcium-dependent phosphatase calcineurin. Obesity-induced attenuation of calcium signalling via its effects on calcineurin, as well as on adiponectin and actinin affects excitation-contraction coupling and excitation-transcription coupling in the myocyte. These molecular changes affect muscle contractile function and phenotype, and thereby in vivo and in vitro muscle performance. In vivo, obesity can increase the absolute force and power produced by increasing the demand on weight-supporting muscle. However, when normalised to body mass, muscle performance of obese individuals is reduced. Isolated muscle preparations show that obesity often leads to a decrease in force produced per muscle cross-sectional area, and power produced per muscle mass. Obesity and ageing have similar physiological consequences. The synergistic effects of obesity and ageing on muscle function may exacerbate morbidity and mortality. Important future research directions include determining: the relationship between time course of weight gain and changes in muscle function; the relative effects of weight gain and high-fat diet feeding per se; the effects of obesity on muscle function during ageing; and if the effects of obesity on muscle function are reversible.

Collective Behaviour: Physiology Determines Position

An animal's position within a group affects feeding - front positions generally offer richer pickings. However, a new study shows that position can be influenced by feeding because big meals reduce scope for locomotion.

Physiological mechanisms underlying animal social behaviour

Many species of animal live in groups, and the group represents the organizational level within which ecological and evolutionary processes occur. Understanding these processes, therefore, relies on knowledge of the mechanisms that permit or constrain group formation. We suggest that physiological capacities and differences in physiology between individuals modify fission-fusion dynamics. Differences between individuals in locomotor capacity and metabolism may lead to fission of groups and sorting of individuals into groups with similar physiological phenotypes. Environmental impacts such as hypoxia can influence maximum group sizes and structure in fish schools by altering access to oxygenated water. The nutritional environment determines group cohesion, and the increase in information collected by the group means that individuals should rely more on social information and form more cohesive groups in uncertain environments. Changing environmental contexts require rapid responses by individuals to maintain group coordination, which are mediated by neuroendocrine signalling systems such as nonapeptides and steroid hormones. Brain processing capacity may constrain social complexity by limiting information processing. Failure to evaluate socially relevant information correctly limits social interactions, which is seen, for example, in autism. Hence, functioning of a group relies to a large extent on the perception and appropriate processing of signals from conspecifics. Many if not all physiological systems are mechanistically linked, and therefore have synergistic effects on social behaviour. A challenge for the future lies in understanding these interactive effects, which will improve understanding of group dynamics, particularly in changing environments.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.