Body size shapes song in honeyeaters

Birdsongs are among the most distinctive animal signals. Their evolution is thought to be shaped simultaneously by habitat structure and by the constraints of morphology. Habitat structure affects song transmission and detectability, thus influencing song (the acoustic adaptation hypothesis), while body size and beak size and shape necessarily constrain song characteristics (the morphological constraint hypothesis). Yet, support for the acoustic adaptation and morphological constraint hypotheses remains equivocal, and their simultaneous examination is infrequent. Using a phenotypically diverse Australasian bird clade, the honeyeaters (Aves: Meliphagidae), we compile a dataset consisting of song, environmental, and morphological variables for 163 species and jointly examine predictions of these two hypotheses. Overall, we find that body size constrains song frequency and pace in honeyeaters. Although habitat type and environmental temperature influence aspects of song, that influence is indirect, likely via effects of environmental variation on body size, with some evidence that elevation constrains the evolution of song peak frequency. Our results demonstrate that morphology has an overwhelming influence on birdsong, in support of the morphological constraint hypothesis, with the environment playing a secondary role generally via body size rather than habitat structure. These results suggest that changing body size (a consequence of both global effects such as climate change and local effects such as habitat transformation) will substantially influence the nature of birdsong.

Per capita sperm metabolism is density dependent

From bacteria to metazoans, higher density populations have lower per capita metabolic rates than lower density populations. The negative covariance between population density and metabolic rate is thought to represent a form of adaptive metabolic plasticity. A relationship between density and metabolism was actually first noted 100 years ago, and was focused on spermatozoa; even then, it was postulated that adaptive plasticity drove this pattern. Since then, contemporary studies of sperm metabolism specifically assume that sperm concentration has no effect on metabolism and that sperm metabolic rates show no adaptive plasticity. We did a systematic review to estimate the relationship between sperm aerobic metabolism and sperm concentration, for 198 estimates spanning 49 species, from protostomes to humans from 88 studies. We found strong evidence that per capita metabolic rates are concentration dependent: both within and among species, sperm have lower metabolisms in dense ejaculates, but increase their metabolism when diluted. On average, a 10-fold decrease in sperm concentration increased per capita metabolic rate by 35%. Metabolic plasticity in sperm appears to be an adaptive response, whereby sperm maximize their chances of encountering eggs.

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

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

Cold adaptation does not handicap warm tolerance in the most abundant Arctic seabird

Arctic birds and mammals are physiologically adapted to survive in cold environments but live in the fastest warming region on the planet. They should therefore be most threatened by climate change. We fitted a phylogenetic model of upper critical temperature (TUC) in 255 bird species and determined that TUC for dovekies (Alle alle; 22.4°C)-the most abundant seabird in the Arctic-is 8.8°C lower than predicted for a bird of its body mass (150 g) and habitat latitude. We combined our comparative analysis with in situ physiological measurements on 36 dovekies from East Greenland and forward-projections of dovekie energy and water expenditure under different climate scenarios. Based on our analyses, we demonstrate that cold adaptation in this small Arctic seabird does not handicap acute tolerance to air temperatures up to at least 15°C above their current maximum. We predict that climate warming will reduce the energetic costs of thermoregulation for dovekies, but their capacity to cope with rising temperatures will be constrained by water intake and salt balance. Dovekies evolved 15 million years ago, and their thermoregulatory physiology might also reflect adaptation to a wide range of palaeoclimates, both substantially warmer and colder than the present day.

How and Why Does Metabolism Scale with Body Mass?

Most explanations for the relationship between body size and metabolism invoke physical constraints; such explanations are evolutionarily inert, limiting their predictive capacity. Contemporary approaches to metabolic rate and life history lack the pluralism of foundational work. Here, we call for reforging of the lost links between optimization approaches and physiology.

Thermoregulatory strategies of songbird nestlings reveal limited capacity for cooling and high risk of dehydration

How the accelerating pace of global warming will affect animal populations depends on the effects of increasing temperature across the life cycle. Developing young are sensitive to environmental challenges, often with life-long consequences, but the risks of climate warming during this period are insufficiently understood. This may be due to limited insight into physiological sensitivity and the temperatures that represent a thermal challenge for young. Here we examined the physiological and behavioural effects of increasing temperatures by measuring metabolic rate, water loss, and heat dissipation behaviours between 25-45 °C in nestlings of a small free-living songbird of temperate SE-Australia, the superb fairy-wren. We found a high and relatively narrow thermoneutral zone from 33.1 to 42.3 °C, with metabolic rate increasing and all nestlings panting above this range. Evaporative water loss sharply increased above 33.5 °C; at the same temperature, nestlings changed their posture (extended their wings) to facilitate passive heat loss. However, at all temperatures measured, water loss was insufficient to dissipate metabolically produced heat, indicating poor cooling capabilities, which persisted even when individuals were panting. While nestlings are relatively tolerant to higher temperatures, with no evidence for hyperthermia at temperatures below 42 °C, they are at a high risk of dehydration even at lower temperatures, with limited ability to mitigate this. Thus, climate warming is likely to elevate the risk dehydration, which is concerning, since it is accompanied by drier conditions.

Linking physiology and climate to infer species distributions in Australian skinks

Climate has a key impact on animal physiology, which in turn can have a profound influence on geographic distributions. Yet, the mechanisms linking climate, physiology and distribution are not fully resolved. Using an integrative framework, we tested the predictions of the climatic variability hypothesis (CVH), which states that species with broader distributions have broader physiological tolerance than range-restricted species, in a group of Lampropholis skinks (8 species, 196 individuals) along a latitudinal gradient in eastern Australia. We investigated several physiological aspects including metabolism, water balance, thermal physiology, thermoregulatory behaviour and ecological performance. Additionally, to test whether organismal information (e.g. behaviour and physiology) can enhance distribution models, hence providing evidence that physiology and climate interact to shape range sizes, we tested whether species distribution models incorporating physiology better predict the range sizes than models using solely climatic layers. In agreement with the CVH, our results confirm that widespread species can tolerate and perform better at broader temperature ranges than range-restricted species. We also found differences in field body temperatures, but not thermal preference, between widespread and range-restricted species. However, metabolism and water balance did not correlate with range size. Biophysical modelling revealed that the incorporation of physiological and behavioural data improves predictions of Lampropholis distributions compared with models based solely on macroclimatic inputs, but mainly for range-restricted species. By integrating several aspects of the physiology and niche modelling of a group of ectothermic animals, our study provides evidence that physiology correlates with species distributions. Physiological responses to climate are central in establishing geographic ranges of skinks, and the incorporation of processes occurring at local scales (e.g. behaviour) can improve species distribution models.

Long-term effects of incubation temperature on growth and thermal physiology in a small ectotherm

Thermal conditions in the developmental environment can substantially affect an individual's phenotype, particularly in egg-laying ectotherms. However, whether these effects persist into adulthood is rarely examined. To investigate this, we incubated delicate skink, Lampropholis delicata, eggs at either cool (22°C), mild (26°C) or hot (30°C) temperatures. After hatching, we measured growth, thermal performance curves of locomotor activity, and thermal sensitivity of resting metabolic rate of offspring as juveniles (4-6 weeks of age), sub-adults (approx. 200 days of age), and adults (approx. 2 years of age), and then measured developmental temperature impacts on male fertility. Incubation temperature had a lasting effect on growth and locomotor performance, with cool and hot incubation temperatures resulting in faster growth and larger maximum size, and hot incubation temperatures reducing locomotor performance at all timepoints. Effects on resting metabolic rate were only present in sub-adults, with a higher metabolic rate at high and average body mass and negative metabolic scaling exponent in cool-incubated lizards. Additionally, cool and hot incubation treatments resulted in shorter sperm midpieces and heads. Incubation temperature did not affect testis mass or sperm count. Overall, our results demonstrate that incubation temperature can have lasting effects on later life stages, highlighting the importance of maternal nest-site selection, but that some effects are age dependent. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

Optimisation and constraint: explaining metabolic patterns in biology

Constraint-based explanations have dominated theories of size-related patterns in nature for centuries. Explanations for metabolic scaling - the way in which metabolism changes with body mass - have been based on the geometry of circulatory networks through which resources are distributed, the need to dissipate heat produced as a by-product of metabolic processes, and surface-area-to-volume constraints on the flux of nutrients or waste. As an alternative to these constraint-based approaches, we recently developed a new theory that predicts that metabolic allometry arises as a consequence of the optimisation of growth and reproduction to maximise fitness within a finite life. Our theory is free of physical geometric constraints that limit the possibilities available to evolution, and we therefore argue that metabolic allometry can be explained without the need to invoke any of the assumed constraints traditionally imposed by metabolic theories. Our findings also suggest that metabolism, growth and reproduction have co-evolved to maximise fitness (i.e. lifetime reproduction) and that the observed patterns in these fundamental characteristics of life can similarly be explained by optimisation rather than constraint. In this Centenary Commentary, we present an overview of our approach and a critique of its limitations. We propose a suite of empirical tests that we hope will move the field forward, discuss the dangers of model overparameterisation and highlight the need to remain open to non-adaptive hypotheses for the origin of biological patterns.

Does thermal biology differ between two colour pattern morphs of a widespread Australian lizard?

Alternative phenotypes allow individuals to pursue different adaptive pathways in response to the same selective challenge. Colour polymorphic species with geographically varying morph frequencies may reflect multiple adaptations to spatial variables such as temperature and climate. We examined whether thermal biology differed between colour morphs of an Australian lizard, the delicate skink, Lampropholis delicata. The delicate skink has two colour pattern morphs, with frequencies varying across latitude and sex: plain (darker, more common at temperate latitudes, more common in males) or striped (lighter, more common at lower latitudes, more common in females). We tested heating and cooling rate, sprint speed, thermal preference, field body temperature and metabolic rate in both morphs and sexes to determine any link between colour and morph frequency distribution. Plain individuals heated more quickly, but other thermal traits showed little variation among morphs. Lampropholis delicata colour influences rates of heat exchange, but the relationship does not appear to be adaptive, suggesting that behavioural thermoregulation homogenises body temperature in the field. While we find no substantial evidence of thermal differences between the two colour morphs, morph-specific behaviour may buffer against differences in heat exchange. Latitudinal variation in species colour may be driven by selection pressures other than temperature.

Life history optimisation drives latitudinal gradients and responses to global change in marine fishes

Within many species, and particularly fish, fecundity does not scale with mass linearly; instead, it scales disproportionately. Disproportionate intraspecific size-reproduction relationships contradict most theories of biological growth and present challenges for the management of biological systems. Yet the drivers of reproductive scaling remain obscure and systematic predictors of how and why reproduction scaling varies are lacking. Here, we parameterise life history optimisation model to predict global patterns in the life histories of marine fishes. Our model predict latitudinal trends in life histories: Polar fish should reproduce at a later age and show steeper reproductive scaling than tropical fish. We tested and confirmed these predictions using a new, global dataset of marine fish life histories, demonstrating that the risks of mortality shape maturation and reproductive scaling. Our model also predicts that global warming will profoundly reshape fish life histories, favouring earlier reproduction, smaller body sizes, and lower mass-specific reproductive outputs, with worrying consequences for population persistence.

Response to Comments on "Metabolic scaling is the product of life-history optimization"

Froese and Pauly argue that our model is contradicted by the observation that fish reproduce before their growth rate decreases. Kearney and Jusup show that our model incompletely describes growth and reproduction for some species. Here we discuss the costs of reproduction, the relationship between reproduction and growth, and propose tests of models based on optimality and constraint.

Macroevolutionary patterns in marine hermaphroditism

Most plants and many animals are hermaphroditic; whether the same forces are responsible for hermaphroditism in both groups is unclear. The well-established drivers of hermaphroditism in plants (e.g., seed dispersal potential, pollination mode) have analogues in animals (e.g., larval dispersal potential, fertilization mode), allowing us to test the generality of the proposed drivers of hermaphroditism across both groups. Here, we test these theories for 1153 species of marine invertebrates, from three phyla. Species with either internal fertilization, restricted offspring dispersal, or small body sizes are more likely to be hermaphroditic than species that are external fertilizers, planktonic developers, or larger. Plants and animals show different biogeographical patterns, however: animals are less likely to be hermaphroditic at higher latitudes-the opposite to the trend in plants. Overall, our results suggest that similar forces, namely, competition among offspring or gametes, shape the evolution of hermaphroditism across plants and three invertebrate phyla.

Univariate and multivariate plasticity in response to incubation temperature in an Australian lizard

Environments, particularly developmental environments, can generate a considerable amount of phenotypic variation through phenotypic plasticity. Plasticity in response to incubation temperature is well characterised in egg-laying reptiles. However, traits do not always vary independently of one another, and studies encompassing a broad range of traits spanning multiple categories are relatively rare but crucial to better understand whole-organism responses to environmental change, particularly if covariation among traits may constrain plasticity. In this study, we investigated multivariate plasticity in response to incubation across three temperatures in the delicate skink, Lampropholis delicata, and whether this was affected by covariation among traits. At approximately 1 month of age, a suite of growth, locomotor performance, thermal physiology and behavioural traits were measured. Plasticity in the multivariate phenotype of delicate skinks was distinct for different incubation temperatures. Cool temperatures drove shifts in growth, locomotor performance and thermal physiology, while hot temperatures primarily caused changes in locomotor performance and behaviour. These differences are likely due to variation in thermal reaction norms, as there was little evidence that covariation among traits or phenotypic integration influenced plasticity, and there was no effect of incubation temperature on the direction or strength of covariation. While there were broad themes in terms of which trait categories were affected by different incubation treatments, traits appeared to be affected independently by developmental temperature. Comparing reaction norms of a greater range of traits and temperatures will enable better insight into these patterns among trait categories, as well as the impacts of environmental change.

Metabolic scaling is the product of life-history optimization

Organisms use energy to grow and reproduce, so the processes of energy metabolism and biological production should be tightly bound. On the basis of this tenet, we developed and tested a new theory that predicts the relationships among three fundamental aspects of life: metabolic rate, growth, and reproduction. We show that the optimization of these processes yields the observed allometries of metazoan life, particularly metabolic scaling. We conclude that metabolism, growth, and reproduction are inextricably linked; that together they determine fitness; and, in contrast to longstanding dogma, that no single component drives another. Our model predicts that anthropogenic change will cause animals to evolve decreased scaling exponents of metabolism, increased growth rates, and reduced lifetime reproductive outputs, with worrying consequences for the replenishment of future populations.

Relationship between capillaries, mitochondria and maximum power of the heart: a meta-study from shrew to elephant

This meta-study uses phylogenetic scaling models across more than 30 species, spanning five orders of magnitude in body mass, to show that cardiac capillary numerical density and mitochondrial volume density decrease with body mass raised to the -0.07 ± 0.03 and -0.04 ± 0.01 exponents, respectively. Thus, while an average 10 g mammal has a cardiac capillary density of approximately 4150 mm^-2 and a mitochondrial density of 33%, a 1 t mammal has considerably lower corresponding values of 1850 mm^-2 and 21%. These similar scaling trajectories suggest quantitative matching for the primary oxygen supply and oxygen consuming structures of the heart, supporting economic design at the cellular level of the oxygen cascade in this aerobic organ. These scaling trajectories are nonetheless somewhat shallower than the exponent of -0.11 calculated for the maximum external mechanical power of the cardiac tissue, under conditions of heavy exercise, when oxygen flow between capillaries and mitochondria is probably fully exploited. This mismatch, if substantiated, implies a declining external mechanical efficiency of the heart with increasing body mass, whereby larger individuals put more energy in but get less energy out, a scenario with implications for cardiovascular design, aerobic capacity and limits of body size.

Predicting the response of disease vectors to global change: The importance of allometric scaling

The distribution of disease vectors such as mosquitoes is changing. Climate change, invasions and vector control strategies all alter the distribution and abundance of mosquitoes. When disease vectors undergo a range shift, so do disease burdens. Predicting such shifts is a priority to adequately prepare for disease control. Accurate predictions of distributional changes depend on how factors such as temperature and competition affect mosquito life-history traits, particularly body size and reproduction. Direct estimates of both body size and reproduction in mosquitoes are logistically challenging and time-consuming, so the field has long relied upon linear (isometric) conversions between wing length (a convenient proxy of size) and reproductive output. These linear transformations underlie most models projecting species' distributions and competitive interactions between native and invasive disease vectors. Using a series of meta-analyses, we show that the relationship between wing length and fecundity are nonlinear (hyperallometric) for most mosquito species. We show that whilst most models ignore reproductive hyperallometry (with respect to wing length), doing so introduces systematic biases into estimates of population growth. In particular, failing to account for reproductive hyperallometry overestimates the effects of temperature and underestimates the effects of competition. Assuming isometry also increases the potential to misestimate the efficacy of vector control strategies by underestimating the contribution of larger females in population replenishment. Finally, failing to account for reproductive hyperallometry and variation in body size can lead to qualitative errors via the counter-intuitive effects of Jensen's inequality. For example, if mean sizes decrease, but variance increases, then reproductive outputs may actually increase. We suggest that future disease vector models incorporate hyperallometric relationships to more accurately predict changes in mosquito distribution in response to global change.

Metabolic phenotype mediates the outcome of competitive interactions in a response-surface field experiment

Competition and metabolism should be linked. Intraspecific variation in metabolic rates and, hence, resource demands covary with competitive ability. The effects of metabolism on conspecific interactions, however, have mostly been studied under laboratory conditions. We used a trait-specific response-surface design to test for the effects of metabolism on pairwise interactions of the marine colonial invertebrate, Bugula neritina in the field. Specifically, we compared the performance (survival, growth, and reproduction) of focal individuals, both in the presence and absence of a neighbor colony, both of which had their metabolic phenotype characterized. Survival of focal colonies depended on the metabolic phenotype of the neighboring individual, and on the combination of both the focal and neighbor colony metabolic phenotypes that were present. Surprisingly, we found pervasive effects of neighbor metabolic phenotypes on focal colony growth and reproduction, although the sign and strength of these effects showed strong microenvironmental variability. Overall, we find that the metabolic phenotype changes the strength of competitive interactions, but these effects are highly contingent on local conditions. We suggest future studies explore how variation in metabolic rate affects organisms beyond the focal organism alone, particularly under field conditions.

Metabolism drives demography in an experimental field test

Metabolism should drive demography by determining the rates of both biological work and resource demand. Long-standing "rules" for how metabolism should covary with demography permeate biology, from predicting the impacts of climate change to managing fisheries. Evidence for these rules is almost exclusively indirect and in the form of among-species comparisons, while direct evidence is exceptionally rare. In a manipulative field experiment on a sessile marine invertebrate, we created experimental populations that varied in population size (density) and metabolic rate, but not body size. We then tested key theoretical predictions regarding relationships between metabolism and demography by parameterizing population models with lifetime performance data from our field experiment. We found that populations with higher metabolisms had greater intrinsic rates of increase and lower carrying capacities, in qualitative accordance with classic theory. We also found important departures from theory-in particular, carrying capacity declined less steeply than predicted, such that energy use at equilibrium increased with metabolic rate, violating the long-standing axiom of energy equivalence. Theory holds that energy equivalence emerges because resource supply is assumed to be independent of metabolic rate. We find this assumption to be violated under real-world conditions, with potentially far-reaching consequences for the management of biological systems.

The roles of diffusion and convection in ventilation of animal burrows

The relationship between body mass and the respiratory microenvironment of burrowing animals is examined using artificial burrows containing surrogate animals that simulate O₂ consumption by removal of air and simultaneous replacement with N₂. Allometric relationships between body mass and burrow radius, nest chamber radius, and O₂ consumption rate show that published mathematical predictions of diffusion-mediated gas exchange are adequate to describe the respiratory environments of animals in small blind-ending burrows through porous substrata. Diffusion is sufficient to ventilate burrows containing small mammals weighing less than 340 g, or subterranean nest chambers connected to the surface by one or more tunnels containing mammals weighing less than 30 kg. Outside of these limits, convection prevails and prevents the development of hypoxic conditions, particularly in burrows of mammals weighing more than 1300 g.

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.

Artificial mass loading disrupts stable social order in pigeon dominance hierarchies

Dominance hierarchies confer benefits to group members by decreasing the incidences of physical conflict, but may result in certain lower ranked individuals consistently missing out on access to resources. Here, we report a linear dominance hierarchy remaining stable over time in a closed population of birds. We show that this stability can be disrupted, however, by the artificial mass loading of birds that typically comprise the bottom 50% of the hierarchy. Mass loading causes these low-ranked birds to immediately become more aggressive and rise-up the dominance hierarchy; however, this effect was only evident in males and was absent in females. Removal of the artificial mass causes the hierarchy to return to its previous structure. This interruption of a stable hierarchy implies a strong direct link between body mass and social behaviour and suggests that an individual's personality can be altered by the artificial manipulation of body mass.

Chronic exposure to a pervasive pharmaceutical pollutant erodes among-individual phenotypic variation in a fish

Pharmaceutical pollution is now recognised as a major emerging agent of global change. Increasingly, pharmaceutical pollutants are documented to disrupt ecologically important physiological and behavioural traits in exposed wildlife. However, little is known about potential impacts of pharmaceutical exposure on among-individual variation in these traits, despite phenotypic diversity being critical for population resilience to environmental change. Furthermore, although wildlife commonly experience multiple stressors contemporaneously, potential interactive effects between pharmaceuticals and biological stressors-such as predation threat-remain poorly understood. To redress this, we investigated the impacts of long-term exposure to the pervasive pharmaceutical pollutant fluoxetine (Prozac®) on among-individual variation in metabolic and behavioural traits, and the combined impacts of fluoxetine exposure and predation threat on mean metabolic and behavioural traits in a freshwater fish, the guppy (Poecilia reticulata). Using a mesocosm system, guppy populations were exposed for 15 months to one of two field-realistic levels of fluoxetine (nominal concentrations: 30 and 300 ng/L) or a solvent control. Fish from these populations were then tested for metabolic rate (oxygen uptake) and behaviour (activity), both before and after experiencing one of three levels of a predation treatment: an empty tank, a non-predatory fish (Melanotaenia splendida) or a predatory fish (Leiopotherapon unicolor). Guppies from both fluoxetine treatments had ∼70% lower among-individual variation in their activity levels, compared to unexposed fish. Similarly, fluoxetine exposure at the higher dosage was associated with a significant (26%) reduction in individual-level variation in oxygen uptake relative to unexposed fish. In addition, mean baseline metabolic rate was disrupted in low-fluoxetine exposed fish, although mean metabolic and behavioural responses to predation threat were not affected. Overall, our study demonstrates that long-term exposure to a pervasive pharmaceutical pollutant alters ecologically relevant traits in fish and erodes among-individual variability, which may be detrimental to the stability of contaminated populations globally.

Flight activity and glycogen depletion on a low-carbohydrate diet

Glycogen is a critical store for locomotion. Depleted glycogen stores are associated with increased fatigue during exercise. The reduced effectiveness of low-carbohydrate diets for weight loss over longer time periods may arise because such diets reduce glycogen stores and thereby energy expenditure via physical activity. To explore the effect of a low-carbohydrate diet on activity and glycogen utilisation, we fed adult Drosophila melanogaster a standard or low-carbohydrate diet for 9 days and measured patterns of flight activity and rates of glycogen depletion. We hypothesised that flight activity and rates of glycogen depletion would be reduced on a low-carbohydrate diet. Flight activity was elevated in the low-carbohydrate group but glycogen depletion rates were unchanged. We conclude that increased activity is probably a foraging response to carbohydrate deficiency and speculate that the previously demonstrated metabolic depression that occurs on a low-carbohydrate diet in this species may allow for increased flight activity without increased glycogen depletion.

Metabolic rate, context-dependent selection, and the competition-colonization trade-off

Metabolism is linked with the pace‐of‐life, co‐varying with survival, growth, and reproduction. Metabolic rates should therefore be under strong selection and, if heritable, become less variable over time. Yet intraspecific variation in metabolic rates is ubiquitous, even after accounting for body mass and temperature. Theory predicts variable selection maintains trait variation, but field estimates of how selection on metabolism varies are rare. We use a model marine invertebrate to estimate selection on metabolic rates in the wild under different competitive environments. Fitness landscapes varied among environments separated by a few centimeters: interspecific competition selected for higher metabolism, and a faster pace‐of‐life, relative to competition‐free environments. Populations experience a mosaic of competitive regimes; we find metabolism mediates a competition‐colonization trade‐off across these regimes. Although high metabolic phenotypes possess greater competitive ability, in the absence of competitors, low metabolic phenotypes are better colonizers. Spatial heterogeneity and the variable selection on metabolic rates that it generates is likely to maintain variation in metabolic rate, despite strong selection in any single environment.

The effect of ambient oxygen on the thermal performance of a cockroach, Nauphoeta cinerea

The oxygen and capacity-limited thermal tolerance (OCLTT) hypothesis proposes that the thermal tolerance of an animal is shaped by its capacity to deliver oxygen in relation to oxygen demand. Studies testing this hypothesis have largely focused on measuring short-term performance responses in animals under acute exposure to critical thermal maximums. The OCLTT hypothesis, however, emphasises the importance of sustained animal performance over acute tolerance. The present study tested the effect of chronic hypoxia and hyperoxia during development on moderate to long-term performance indicators at temperatures spanning the optimal temperature for growth in the speckled cockroach, Nauphoeta cinerea In contrast to the predictions of the OCLTT hypothesis, development under hypoxia did not significantly reduce growth rate or running performance, and development under hyperoxia did not significantly increase growth rate or running performance. The effects of developmental temperature and oxygen on tracheal morphology and metabolic rate were also not consistent with OCLTT predictions, suggesting that oxygen delivery capacity is not the primary driver shaping thermal tolerance in this species. Collectively, these findings suggest that the OCLTT hypothesis does not explain moderate to long-term thermal performance in N.cinerea, which raises further questions about the generality of the hypothesis.

Developmental cost theory predicts thermal environment and vulnerability to global warming

Metazoans must develop from zygotes to feeding organisms. In doing so, developing offspring consume up to 60% of the energy provided by their parent. The cost of development depends on two rates: metabolic rate, which determines the rate that energy is used; and developmental rate, which determines the length of the developmental period. Both development and metabolism are highly temperature-dependent such that developmental costs should be sensitive to the local thermal environment. Here, we develop, parameterize and test developmental cost theory, a physiologically explicit theory that reveals that ectotherms have narrow thermal windows in which developmental costs are minimized (T_opt). Our developmental cost theory-derived estimates of T_opt predict the natural thermal environment of 71 species across seven phyla remarkably well (R² ~0.83). Developmental cost theory predicts that costs of development are much more sensitive to small changes in temperature than classic measures such as survival. Warming-driven changes to developmental costs are predicted to strongly affect population replenishment and developmental cost theory provides a mechanistic foundation for determining which species are most at risk. Developmental cost theory predicts that tropical aquatic species and most non-nesting terrestrial species are likely to incur the greatest increase in developmental costs from future warming.

Phylogenetic investigation of skin sloughing rates in frogs: relationships with skin characteristics and disease-driven declines

Amphibian skin is highly variable in structure and function across anurans, and plays an important role in physiological homeostasis and immune defence. For example, skin sloughing has been shown to reduce pathogen loads on the skin, such as the lethal fungus Batrachochytrium dendrobatidis ( Bd), but interspecific variation in sloughing frequency is largely unknown. Using phylogenetic linear mixed models, we assessed the relationship between skin turnover rate, skin morphology, ecological traits and overall evidence of Bd-driven declines. We examined skin sloughing rates in 21 frog species from three continents, as well as structural skin characteristics measured from preserved specimens. We found that sloughing rate varies significantly with phylogenetic group, but was not associated with evidence of Bd-driven declines, or other skin characteristics examined. This is the first comparison of sloughing rate across a wide range of amphibian species, and creates the first database of amphibian sloughing behaviour. Given the strong phylogenetic signal observed in sloughing rate, approximate sloughing rates of related species may be predicted based on phylogenetic position. While not related to available evidence of declines, understanding variation in sloughing rate may help explain differences in the severity of infection in genera with relatively slow skin turnover rates (e.g. Atelopus).

Optimization of a Liquid Chromatographic Method for Determination of Oxytetracycline, Tetracycline, and Chlortetracycline in Milk

A liquid chromatographic (LC) method was developed for the simultaneous identification and quantitation of oxytetracycline, tetracycline, and chlortetracycline in milk. Milk samples (5 mL) were deproteinized by adding 1 mL 1N HCI and 24 mL acetonitrile, and filtering. Dichloromethane and hexane were added to 15 mL filtrate to separate the water layer. The organic layer was washed with 1 mL deionized water, and the combined water layers were diluted to 4 mL. Sample aliquots of 1000 μL were then injected directly and analyzed on an LC system. The sensitivity limit of the method is 5 ppb for each antibiotic; no interferences are present at their retention times. Mean recoveries from milk spiked at 0.01-1 ppm ranged from 87 to 99%, and precision was good.

A model to estimate seabird field metabolic rates

For free-ranging animals, field metabolic rate (FMR) is the sum of their energy expenditure over a specified period. This quantity is a key component of ecological processes at every biological level. We applied a phylogenetically informed meta-analytical approach to identify the large-scale determinants of FMR in seabirds during the breeding season. Using data from 64 studies of energetics in 47 species, we created a model to estimate FMR for any seabird population. We found that FMR was positively influenced by body mass and colony latitude and that it increased throughout the breeding season from incubation to brood to crèche. FMR was not impacted by colony-relative predation pressure or species average brood size. Based on this model, we present an app through which users can generate estimates of FMR for any population of breeding seabird. We encourage the use of this app to complement behavioural studies and increase understanding of how energetic demands influence the role of seabirds as driving components of marine systems.

Impacts of "supermoon" events on the physiology of a wild bird

The position of the Moon in relation to the Earth and the Sun gives rise to several predictable cycles, and natural changes in nighttime light intensity are known to cause alterations to physiological processes and behaviors in many animals. The limited research undertaken to date on the physiological responses of animals to the lunar illumination has exclusively focused on the synodic lunar cycle (full moon to full moon, or moon phase) but the moon's orbit-its distance from the Earth-may also be relevant. Every month, the moon moves from apogee, its most distant point from Earth-and then to perigee, its closest point to Earth. Here, we studied wild barnacle geese (Branta leucopsis) to investigate the influence of multiple interacting lunar cycles on the physiology of diurnally active animals. Our study, which uses biologging technology to continually monitor body temperature and heart rate for an entire annual cycle, asks whether there is evidence for a physiological response to natural cycles in lunar brightness in wild birds, particularly "supermoon" phenomena, where perigee coincides with a full moon. There was a three-way interaction between lunar phase, lunar distance, and cloud cover as predictors of nighttime mean body temperature, such that body temperature was highest on clear nights when the full moon coincided with perigee moon. Our study is the first to report the physiological responses of wild birds to "supermoon" events; the wild geese responded to the combination of two independent lunar cycles, by significantly increasing their body temperature at night. That wild birds respond to natural fluctuations in nighttime ambient light levels support the documented responses of many species to anthropogenic sources of artificial light, that birds seem unable to override. As most biological systems are arguably organized foremost by light, this suggests that any interactions between lunar cycles and local weather conditions could have significant impacts on the energy budgets of birds.

Interspecific scaling of blood flow rates and arterial sizes in mammals

This meta-study investigated the relationships between blood flow rate (Q̇; cm³ s^-1), wall shear stress (τ_w; dyn cm^-2) and lumen radius (r _i; cm) in 20 named systemic arteries of nine species of mammals, ranging in mass from 23 g mice to 652 kg cows, at rest. In the dataset, derived from 50 studies, lumen radius varied between 3.7 µm in a cremaster artery of a rat and 11.2 mm in the aorta of a human. The 92 logged data points of [Formula: see text] and r _i are described by a single second-order polynomial curve with the equation: [Formula: see text] The slope of the curve increased from approximately 2 in the largest arteries to approximately 3 in the smallest ones. Thus, da Vinci's rule ([Formula: see text]) applies to the main arteries and Murray's law ([Formula: see text]) applies to the microcirculation. A subset of the data, comprising only cephalic arteries in which [Formula: see text] is fairly constant, yielded the allometric power equation: [Formula: see text] These empirical equations allow calculation of resting perfusion rates from arterial lumen size alone, without reliance on theoretical models or assumptions on the scaling of wall shear stress in relation to body mass. As expected, [Formula: see text] of individual named arteries is strongly affected by body mass; however, [Formula: see text] of the common carotid artery from six species (mouse to horse) is also sensitive to differences in whole-body basal metabolic rate, independent of the effect of body mass.

Linking life-history theory and metabolic theory explains the offspring size-temperature relationship

Temperature often affects maternal investment in offspring. Across and within species, mothers in colder environments generally produce larger offspring than mothers in warmer environments, but the underlying drivers of this relationship remain unresolved. We formally evaluated the ubiquity of the temperature-offspring size relationship and found strong support for a negative relationship across a wide variety of ectotherms. We then tested an explanation for this relationship that formally links life-history and metabolic theories. We estimated the costs of development across temperatures using a series of laboratory experiments on model organisms, and a meta-analysis across 72 species of ectotherms spanning five phyla. We found that both metabolic and developmental rates increase with temperature, but developmental rate is more temperature sensitive than metabolic rate, such that the overall costs of development decrease with temperature. Hence, within a species' natural temperature range, development at relatively cooler temperatures requires mothers to produce larger, better provisioned offspring.

Flight feather moult drives minimum daily heart rate in wild geese

Waterfowl undergo an annual simultaneous flight-feather moult that renders them flightless for the duration of the regrowth of the flight feathers. In the wild, this period of flightlessness could restrict the capacity of moulting birds to forage and escape predation. Selection might therefore favour a short moult, but feather growth is constrained and presumably energetically demanding. We therefore tested the hypothesis that for birds that undergo a simultaneous flight-feather moult, this would be the period in the annual cycle with the highest minimum daily heart rates, reflecting these increased energetic demands. Implantable heart rate data loggers were used to record year-round heart rate in six wild barnacle geese (Branta leucopsis), a species that undergoes a simultaneous flight-feather moult. The mean minimum daily heart rate was calculated for each individual bird over an 11-month period, and the annual cycle was divided into seasons based on the life-history of the birds. Mean minimum daily heart rate varied significantly between seasons and was significantly elevated during wing moult, to 200 ± 32 beats min^-1, compared to all other seasons of the annual cycle, including both the spring and autumn migrations. The increase in minimum daily heart rate during moult is likely due to feather synthesis, thermoregulation and the reallocation of minerals and protein.

A widespread thermodynamic effect, but maintenance of biological rates through space across life's major domains

For over a century, the hypothesis of temperature compensation, the maintenance of similar biological rates in species from different thermal environments, has remained controversial. An alternative idea, that fitness is greater at higher temperatures (the thermodynamic effect), has gained increasing traction. This alternative hypothesis is also being used to understand large-scale biodiversity responses to environmental change. Yet evidence in favour of each of these contrasting hypotheses continues to emerge. In consequence, the fundamental nature of organismal thermal responses and its implications remain unresolved. Here, we investigate these ideas explicitly using a global dataset of 619 observations of four categories of organismal performance, spanning 14 phyla and 403 species. In agreement with both hypotheses, we show a positive relationship between the temperature of maximal performance rate (Topt) and environmental temperature (Tenv) for developmental rate and locomotion speed, but not growth or photosynthesis rate. Next, we demonstrate that relationships between Tenv and the maximal performance rate (Umax) are rarely significant and positive, as expected if a thermodynamic effect predominates. By contrast, a positive relationship between Topt and Umax is always present, but markedly weaker than theoretically predicted. These outcomes demonstrate that while some form of thermodynamic effect exists, ample scope is present for biochemical and physiological adaptation to thermal environments in the form of temperature compensation.

Legs of male fiddler crabs evolved to compensate for claw exaggeration and enhance claw functionality during waving displays

Many exaggerated morphological traits evolve under sexual selection. However, the optimal level of exaggeration is dictated by a trade-off between natural and sexual selection, representing a balance between its benefits and associated costs. Male fiddler crabs wave an enlarged major claw during behavioural displays that eliminates the need for direct combat, and determines courtship outcomes. The outcomes of these displays often depend on claw size, exposing males to selection for larger claws to improve mating and combat success. Applying phylogenetic comparative methods to 27 fiddler crab species, we examined the evolution of major claw morphologies, leg morphologies, and waving displays to determine whether these traits coevolved to optimise functioning of the exaggerated claw, or to mitigate potential metabolic or locomotor costs. We found legs to be sexually dimorphic, with males having longer legs than females. Legs were also longer in species that waved laterally rather than vertically, in species with larger major claws, and in species whose major claws were relatively elongate. These results suggest that leg morphology has coevolved with claw enlargement to enhance functionality of the major claw during waving displays, in addition to compensating for any negative effects of claw size.

Short-duration respirometry underestimates metabolic rate for discontinuous breathers

Metabolic rate is commonly estimated from rates of gas exchange. An underappreciated factor that can influence estimates is patterns of pulmonary respiration. Amphibians display discontinuous respiratory patterns, often including long apnoeas, in addition to cutaneous gas exchange. The contribution of cutaneous exchange increases at low temperatures when metabolic rate is low. Because of the relatively low permeability of skin, measurements that disproportionately capture cutaneous exchange can produce underestimates of metabolic rate. The permeability of amphibian skin to CO₂ is greater than that to O₂; therefore, calculating the ratio of whole-animal CO₂ emission to O₂ uptake (the respiratory exchange ratio, RER) can be used to avoid underestimates of metabolic rate by ensuring that observed values of RER fall within the normal physiological range (∼0.7 to 1). Using data for cane toads, Rhinella marina, we show that short-duration measurements lead to underestimates of metabolic rate and overestimates of RER. At low temperatures, this problem is exacerbated, requiring over 12 h for RER to fall within the normal physiological range. Many published values of metabolic rate in animals that utilise cutaneous exchange may be underestimates.

Loss of maternal EED results in postnatal overgrowth

BACKGROUND

Investigating how epigenetic information is transmitted through the mammalian germline is the key to understanding how this information impacts on health and disease susceptibility in offspring. EED is essential for regulating the repressive histone modification, histone 3 lysine 27 tri-methylation (H3K27me3) at many developmental genes.

RESULTS

In this study, we used oocyte-specific Zp3-Cre recombinase (Zp3Cre) to delete Eed specifically in mouse growing oocytes, permitting the study of EED function in oocytes and the impact of depleting EED in oocytes on outcomes in offspring. As EED deletion occurred only in growing oocytes and females were mated to normal wild type males, this model allowed the study of oocyte programming without confounding factors such as altered in utero environment. Loss of EED from growing oocytes resulted in a significant overgrowth phenotype that persisted into adult life. Significantly, this involved increased adiposity (total fat) and bone mineral density in offspring. Similar overgrowth occurs in humans with Cohen-Gibson (OMIM 617561) and Weaver (OMIM 277590) syndromes, that result from de novo germline mutations in EED or its co-factor EZH2, respectively. Consistent with a role for EZH2 in human oocytes, we demonstrate that de novo germline mutations in EZH2 occurred in the maternal germline in some cases of Weaver syndrome. However, deletion of Ezh2 in mouse oocytes resulted in a distinct phenotype compared to that resulting from oocyte-specific deletion of Eed.

CONCLUSIONS

This study provides novel evidence that altering EED-dependent oocyte programming leads to compromised offspring growth and development in the next generation.