Plastic but repeatable: rapid adjustments of mitochondrial function and density during reproduction in a wild bird species

The relationship between mitochondrial respiration, resting metabolic rate and blood cell count in great tits

Although mitochondrial respiration is believed to explain a substantial part of the variation in resting metabolic rate (RMR), few studies have empirically studied the relationship between organismal and cellular metabolism. We therefore investigated the relationship between RMR and mitochondrial respiration of permeabilized blood cells in wild great tits (Parus major L.). We also studied the correlation between mitochondrial respiration traits and blood cell count, as normalizing mitochondrial respiration by the cell count is a method commonly used to study blood metabolism. In contrast to previous studies, our results show that there was no relationship between RMR and mitochondrial respiration in intact blood cells (i.e. with the ROUTINE respiration). However, when cells were permeabilized and interrelation re-assessed under saturating substrate availability, we found that RMR was positively related to phosphorylating respiration rates through complexes I and II (i.e. OXPHOS respiration) and to the mitochondrial efficiency to produce energy (i.e. net phosphorylation efficiency), though variation explained by the models was low (i.e. linear model: R2=0.14 to 0.21). However, unlike studies in mammals, LEAK respiration without [i.e. L(n)] and with [i.e. L(Omy)] adenylates was not significantly related to RMR. These results suggest that phosphorylating respiration in blood cells can potentially be used to predict RMR in wild birds, but that this relationship may have to be addressed in standardized conditions (permeabilized cells) and that the prediction risks being imprecise. We also showed that, in our conditions, there was no relationship between any mitochondrial respiration trait and blood cell count. Hence, we caution against normalising respiration rates using this parameter as is sometimes done. Future work should address the functional explanations for the observed relationships, and determine why these appear labile across space, time, taxon, and physiological state.

Non-lethal sampling for assessment of mitochondrial function does not affect metabolic rate and swimming performance

A fundamental issue in the metabolic field is whether it is possible to understand underlying mechanisms that characterize individual variation. Whole-animal performance relies on mitochondrial function as it produces energy for cellular processes. However, our lack of longitudinal measures to evaluate how mitochondrial function can change within and among individuals and with environmental context makes it difficult to assess individual variation in mitochondrial traits. The aims of this study were to test the repeatability of muscle mitochondrial metabolism by performing two biopsies of red muscle, and to evaluate the effects of biopsies on whole-animal performance in goldfish Carassius auratus. Our results show that basal mitochondrial respiration and net phosphorylation efficiency are repeatable at 14-day intervals. We also show that swimming performance (optimal cost of transport and critical swimming speed) was repeatable in biopsied fish, whereas the repeatability of individual oxygen consumption (standard and maximal metabolic rates) seemed unstable over time. However, we noted that the means of individual and mitochondrial traits did not change over time in biopsied fish. This study shows that muscle biopsies allow the measurement of mitochondrial metabolism without sacrificing animals and that two muscle biopsies 14 days apart affect the intraspecific variation in fish performance without affecting average performance of individuals. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Repeat Sampling of Female Passerines During Reproduction Reveals Surprising Higher Plasma Oxidative Damage During Resting Compared to Active State

Traditional models of oxidative stress predict accumulation of damage caused by reactive oxygen species (ROS) production as highly correlated with aerobic metabolism, a prediction under increasing scrutiny. Here, we repeat sampled female great tits (Parus major) at two opposite levels of energy use during the period of maximum food provisioning to nestlings, once at rest and once during activity. Our results were in contrast to the above prediction, namely significantly higher levels of oxidative damage during rest opposed to active phase. This discrepancy could not be explained neither using levels of "first line" antioxidant enzymes activity measured from erythrocytes, nor from total nonenzymatic antioxidant capacity measured from plasma, as no differences were found between states. Significantly higher levels of uric acid, a potent antioxidant, were seen in the plasma during the active phase than in rest phase, which may explain the lower levels of oxidative damage despite high levels of physical activity. Our results challenge the hypothesis that oxidative stress is elevated during times with high energy use and call for more profound understanding of potential drivers of the modulation of oxidative stress such as metabolic state of the animal, and thus also the time of sampling in general.

Mitochondrial metabolism in blood more reliably predicts whole-animal energy needs compared to other tissues

Understanding energy metabolism in free-ranging animals is crucial for ecological studies. In birds, red blood cells (RBCs) offer a minimally invasive method to estimate metabolic rate (MR). In this study with European starlings Sturnus vulgaris, we examined how RBC oxygen consumption relates to oxygen use in key tissues (brain, liver, heart, and pectoral muscle) and versus the whole organism measured at basal levels. The pectoral muscle accounted for 34%-42% of organismal MR, while the heart and liver, despite their high mass-specific metabolic rate, each contributed 2.5%-3.0% to organismal MR. Despite its low contribution to organismal MR (0.03%-0.04%), RBC MR best predicted organismal MR (r = 0.70). Oxygen consumption of the brain and pectoralis was also associated with whole-organism MR, unlike that of heart and liver. Overall, our findings demonstrate that the metabolism of a systemic tissue like blood is a superior proxy for organismal energy metabolism than that of other tissues.

Early-life environmental effects on mitochondrial aerobic metabolism: a brood size manipulation in wild great tits

In avian species, the number of chicks in the nest and subsequent sibling competition for food are major components of the offspring's early-life environment. A large brood size is known to affect chick growth, leading in some cases to long-lasting effects for the offspring, such as a decrease in size at fledgling and in survival after fledging. An important pathway underlying different growth patterns could be the variation in offspring mitochondrial metabolism through its central role in converting energy. Here, we performed a brood size manipulation in great tits (Parus major) to unravel its impact on offspring mitochondrial metabolism and reactive oxygen species (ROS) production in red blood cells. We investigated the effects of brood size on chick growth and survival, and tested for long-lasting effects on juvenile mitochondrial metabolism and phenotype. As expected, chicks raised in reduced broods had a higher body mass compared with enlarged and control groups. However, mitochondrial metabolism and ROS production were not significantly affected by the treatment at either chick or juvenile stages. Interestingly, chicks raised in very small broods were smaller in size and had higher mitochondrial metabolic rates. The nest of rearing had a significant effect on nestling mitochondrial metabolism. The contribution of the rearing environment in determining offspring mitochondrial metabolism emphasizes the plasticity of mitochondrial metabolism in relation to the nest environment. This study opens new avenues regarding the effect of postnatal environmental conditions in shaping offspring early-life mitochondrial metabolism.

Plasticity of mitochondrial function safeguards phosphorylating respiration during in vitro simulation of rest-phase hypothermia

Many animals downregulate body temperature to save energy when resting (rest-phase hypothermia). Small birds that winter at high latitudes have comparatively limited capacity for hypothermia and so pay large energy costs for thermoregulation during cold nights. Available evidence suggests this process is fueled by adenosine triphosphate (ATP)-dependent mechanisms. Most ATP is produced by oxidative phosphorylation in the mitochondria, but mitochondrial respiration may be lower during hypothermia because of the temperature dependence of biological processes. This can create conflict between increased organismal ATP demand and a lower mitochondrial capacity to provide it. We studied this in blood cell mitochondria of wild great tits (Parus major) by simulating rest-phase hypothermia via a 6°C reduction in assay temperature in vitro. The birds had spent the night preceding the experiment in thermoneutrality or in temperatures representing mild or very cold winter nights, but night temperatures never affected mitochondrial respiration. However, across temperature groups, endogenous respiration was 14% lower in hypothermia. This did not reflect general thermal suppression of mitochondrial function because phosphorylating respiration was unaffected by thermal state. Instead, hypothermia was associated with a threefold reduction of leak respiration, from 17% in normothermia to 4% in hypothermia. Thus, the coupling of total respiration to ATP production was 96% in hypothermia, compared to 83% in normothermia. Our study shows that the thermal insensitivity of phosphorylation combined with short-term plasticity of leak respiration may safeguard ATP production when endogenous respiration is suppressed. This casts new light on the process by which small birds endure harsh winter cold and warrants future tests across tissues in vivo.

Mitochondrial function declines with age within individuals but is not linked to the pattern of growth or mortality risk in zebra finch

Mitochondrial dysfunction is considered a highly conserved hallmark of ageing. However, most of the studies in both model and non-model organisms are cross-sectional in design; therefore, little is known, at the individual level, on how mitochondrial function changes with age, its link to early developmental conditions or its relationship with survival. Here we manipulated the postnatal growth in zebra finches (Taeniopygia guttata) via dietary modification that induced accelerated growth without changing adult body size. In the same individuals, we examined blood cells mitochondrial functioning (mainly erythrocytes) when they were young (ca. 36 weeks) and again in mid-aged (ca. 91 weeks) adulthood. Mitochondrial function was strongly influenced by age but not by postnatal growth conditions. Across all groups, within individual ROUTINE respiration, OXPHOS and OXPHOS coupling efficiency significantly declined with age, while LEAK respiration increased. However, we found no link between mitochondrial function and the probability of survival into relatively old age (ca. 4 years). Our results suggest that the association between accelerated growth and reduced longevity, evident in this as in other species, is not attributable to age-related changes in any of the measured mitochondrial function traits.

A whole blood approach improves speed and accuracy when measuring mitochondrial respiration in intact avian blood cells

Understanding mitochondrial biology and pathology is key to understanding the evolution of animal form and function. However, mitochondrial measurement often involves invasive, or even terminal, sampling, which can be difficult to reconcile in wild models or longitudinal studies. Non-mammal vertebrates contain mitochondria in their red blood cells, which can be exploited for minimally invasive mitochondrial measurement. Several recent bird studies have measured mitochondrial function using isolated blood cells. Isolation adds time in the laboratory and might be associated with physiological complications. We developed and validated a protocol to measure mitochondrial respiration in bird whole blood. Endogenous respiration was comparable between isolated blood cells and whole blood. However, respiration towards oxidative phosphorylation was higher in whole blood, and whole blood mitochondria were better coupled and had higher maximum working capacity. Whole blood measurement was also more reproducible than measurement on isolated cells for all traits considered. Measurements were feasible over a 10-fold range of sample volumes, although both small and large volumes were associated with changes to respiratory traits. The protocol was compatible with long-term storage: after 24 h at 5°C without agitation, all respiration traits but maximum working capacity remained unchanged, the latter decreasing by 14%. Our study suggests that whole blood measurement provides faster, more reproducible, and more biologically and physiologically relevant (mitochondrial integrity) assessment of mitochondrial respiration. We recommend future studies to take a whole blood approach unless specific circumstances require the use of isolated blood cells.

How telomere dynamics are influenced by the balance between mitochondrial efficiency, reactive oxygen species production and DNA damage

It is well known that oxidative stress is a major cause of DNA damage and telomere attrition. Most endogenous reactive oxygen species (ROS) are produced in the mitochondria, producing a link between mitochondrial function, DNA integrity and telomere dynamics. In this review we will describe how ROS production, rates of damage to telomeric DNA and DNA repair are dynamic processes. The rate of ROS production depends on mitochondrial features such as the level of inner membrane uncoupling and the proportion of time that ATP is actively being produced. However, the efficiency of ATP production (the ATP/O ratio) is positively related to the rate of ROS production, so leading to a trade-off between the body's energy requirements and its need to prevent oxidative stress. Telomeric DNA is especially vulnerable to oxidative damage due to features such as its high guanine content; while repair to damaged telomere regions is possible through a range of mechanisms, these can result in more rapid telomere shortening. There is increasing evidence that mitochondrial efficiency varies over time and with environmental context, as do rates of DNA repair. We argue that telomere dynamics can only be understood by appreciating that the optimal solution to the trade-off between energetic efficiency and telomere protection will differ between individuals and will change over time, depending on resource availability, energetic demands and life history strategy.

The relationships between growth rate and mitochondrial metabolism varies over time

Mitochondrial metabolism varies significantly between individuals of the same species and can influence animal performance, such as growth. However, growth rate is usually determined before the mitochondrial assay. The hypothesis that natural variation in mitochondrial metabolic traits is linked to differences in both previous and upcoming growth remains untested. Using biopsies to collect tissue in a non-lethal manner, we tested this hypothesis in a fish model (Dicentrarchus labrax) by monitoring individual growth rate, measuring mitochondrial metabolic traits in the red muscle, and monitoring the growth of the same individuals after the mitochondrial assay. Individual variation in growth rate was consistent before and after the mitochondrial assay; however, the mitochondrial traits that explained growth variation differed between the growth rates determined before and after the mitochondrial assay. While past growth was correlated with the activity of the cytochrome c oxidase, a measure of mitochondrial density, future growth was linked to mitochondrial proton leak respiration. This is the first report of temporal shift in the relationship between growth rate and mitochondrial metabolic traits, suggesting an among-individual variation in temporal changes in mitochondrial traits. Our results emphasize the need to evaluate whether mitochondrial metabolic traits of individuals can change over time.

Mice selected for a high basal metabolic rate evolved larger guts but not more efficient mitochondria

Intra-specific variation in both the basal metabolic rate (BMR) and mitochondrial efficiency (the amount of ATP produced per unit of oxygen consumed) has profound evolutionary and ecological consequences. However, the functional mechanisms responsible for this variation are not fully understood. Mitochondrial efficiency is negatively correlated with BMR at the interspecific level but it is positively correlated with performance capacity at the intra-specific level. This discrepancy is surprising, as theories explaining the evolution of endothermy assume a positive correlation between BMR and performance capacity. Here, we quantified mitochondrial oxidative phosphorylation activity and efficiency in two lines of laboratory mice divergently selected for either high (H-BMR) or low (L-BMR) levels of BMR. H-BMR mice had larger livers and kidneys (organs that are important predictors of BMR). H-BMR mice also showed higher oxidative phosphorylation activity in liver mitochondria but this difference can be hypothesized to be a direct effect of selection only if the heritability of this trait is low. However, mitochondrial efficiency in all studied organs did not differ between the two lines. We conclude that the rapid evolution of BMR can reflect changes in organ size rather than mitochondrial properties, and does not need to be accompanied obligatorily by changes in mitochondrial efficiency.

Mitochondria as the powerhouses of sexual selection: Testing mechanistic links between development, cellular respiration, and bird song

The developmental environment can affect the expression of sexually selected traits in adulthood. The physiological mechanisms that modulate such effects remain a matter of intense debate. Here, we test the role of the developmental environment in shaping adult mitochondrial function and link mitochondrial function to expression of a sexually selected trait in males (bird song). We exposed male zebra finches (Taeniopygia guttata) to corticosterone (CORT) treatment during development. After males reached adulthood, we quantified mitochondrial function from whole red blood cells and measured baseline CORT and testosterone levels, body condition/composition, and song structure. CORT-treated males had mitochondria that were less efficient (FCR_L/R) and used a lower proportion of maximum capacity (FCR_R/ETS) than control males. Additionally, CORT-treated males had higher baseline levels of CORT as adults compared to control males. Using structural equation modelling, we found that the effects of CORT treatment during development on adult mitochondrial function were indirect and modulated by baseline CORT levels, which are programmed by CORT treatment during development. Developmental treatment also had an indirect effect on song peak frequency. Males treated with CORT during development sang songs with higher peak frequency than control males, but this effect was modulated through increased CORT levels and by a decrease in FCR_R/ETS. CORT-treated males had smaller tarsi compared to control males; however, there were no associations between body size and measures of song frequency. Here, we provide the first evidence supporting links between the developmental environment, mitochondrial function, and the expression of a sexually selected trait (bird song).

Developmental plasticity of mitochondrial aerobic metabolism, growth and survival by prenatal glucocorticoids and thyroid hormones: an experimental test in wild great tits

Developmental plasticity is partly mediated by transgenerational effects, including those mediated by the maternal endocrine system. Glucocorticoid and thyroid hormones may play central roles in developmental programming through their action on metabolism and growth. However, the mechanisms by which they affect growth and development remain understudied. One hypothesis is that maternal hormones directly affect the production and availability of energy-carrying molecules (e.g. ATP) by their action on mitochondrial function. To test this hypothesis, we experimentally increased glucocorticoid and thyroid hormones in wild great tit eggs (Parus major) to investigate their impact on offspring mitochondrial aerobic metabolism (measured in blood cells), and subsequent growth and survival. We show that prenatal glucocorticoid supplementation affected offspring cellular aerobic metabolism by decreasing mitochondrial density, maximal mitochondrial respiration and oxidative phosphorylation, while increasing the proportion of the maximum capacity being used under endogenous conditions. Prenatal glucocorticoid supplementation only had mild effects on offspring body mass, size and condition during the rearing period, but led to a sex-specific (females only) decrease in body mass a few months after fledging. Contrary to our expectations, thyroid hormones supplementation did not affect offspring growth or mitochondrial metabolism. Recapture probabilities as juveniles or adults were not significantly affected by prenatal hormonal treatments. Our results demonstrate that prenatal glucocorticoids can affect post-natal mitochondrial density and aerobic metabolism. The weak effects on growth and apparent survival suggest that nestlings were mostly able to compensate for the transient decrease in mitochondrial aerobic metabolism induced by prenatal glucocorticoids.

Does IGF-1 Shape Life-History Trade-Offs? Opposite Associations of IGF-1 With Telomere Length and Body Size in a Free-Living Bird

Hormonal pathways have been proposed to be key at modulating how fast individuals grow and reproduce and how long they live (i.e., life history trajectory). Research in model species living under controlled environment is suggesting that insulin-like growth factor 1 (IGF-1), which is an evolutionarily conserved polypeptide hormone, has an important role in modulating animal life histories. Much remains, however, to be done to test the role played by IGF-1 in shaping the phenotype and life history of animals in the wild. Using a wild long-lived bird, the Alpine swift (Tachymarptis melba), we show that adults with higher levels of IGF-1 had longer wings and shorter telomeres. Hence, telomeres being a proxy of lifespan in this species, our results support a potential role of IGF-1 at shaping the life-history of wild birds and suggest that IGF-1 may influence the growth-lifespan trade-off.

Experimental demonstration of prenatal programming of mitochondrial aerobic metabolism lasting until adulthood

It is increasingly being postulated that among-individual variation in mitochondrial function underlies variation in individual performance (e.g. growth rate) and state of health. It has been suggested (but not adequately tested) that environmental conditions experienced before birth could programme postnatal mitochondrial function, with persistent effects potentially lasting into adulthood. We tested this hypothesis in an avian model by experimentally manipulating prenatal conditions (incubation temperature and stability) and then measuring mitochondrial aerobic metabolism in blood cells from the same individuals during the middle of the growth period and at adulthood. Mitochondrial aerobic metabolism changed markedly across life stages, and parts of these age-related changes were influenced by the prenatal temperature conditions. A high incubation temperature induced a consistent and long-lasting increase in mitochondrial aerobic metabolism. Postnatal mitochondrial aerobic metabolism was positively associated with oxidative damage on DNA but not telomere length. While we detected significant within-individual consistency in mitochondrial aerobic metabolism across life stages, the prenatal temperature regime only accounted for a relatively small proportion (less than 20%) of the consistent among-individual differences we observed. Our results demonstrate that prenatal conditions can programme consistent and long-lasting differences in mitochondrial function, which could potentially underlie among-individual variation in performance and health state.

Prenatal acoustic programming of mitochondrial function for high temperatures in an arid-adapted bird

Sound is an essential source of information in many taxa and can notably be used by embryos to programme their phenotypes for postnatal environments. While underlying mechanisms are mostly unknown, there is growing evidence for the involvement of mitochondria-main source of cellular energy (i.e. ATP)-in developmental programming processes. Here, we tested whether prenatal sound programmes mitochondrial metabolism. In the arid-adapted zebra finch, prenatal exposure to 'heat-calls'-produced by parents incubating at high temperatures-adaptively alters nestling growth in the heat. We measured red blood cell mitochondrial function, in nestlings exposed prenatally to heat- or control-calls, and reared in contrasting thermal environments. Exposure to high temperatures always reduced mitochondrial ATP production efficiency. However, as expected to reduce heat production, prenatal exposure to heat-calls improved mitochondrial efficiency under mild heat conditions. In addition, when exposed to an acute heat-challenge, LEAK respiration was higher in heat-call nestlings, and mitochondrial efficiency low across temperatures. Consistent with its role in reducing oxidative damage, LEAK under extreme heat was also higher in fast growing nestlings. Our study therefore provides the first demonstration of mitochondrial acoustic sensitivity, and brings us closer to understanding the underpinning of acoustic developmental programming and avian strategies for heat adaptation.

How Birds During Migration Maintain (Oxidative) Balance

Animals dynamically adjust their physiology and behavior to survive in changing environments, and seasonal migration is one life stage that demonstrates these dynamic adjustments. As birds migrate between breeding and wintering areas, they incur physiological demands that challenge their antioxidant system. Migrating birds presumably respond to these oxidative challenges by up-regulating protective endogenous systems or accumulating dietary antioxidants at stopover sites, although our understanding of the pre-migration preparations and mid-migration responses of birds to such oxidative challenges is as yet incomplete. Here we review evidence from field and captive-bird studies that address the following questions: (1) Do migratory birds build antioxidant capacity as they build fat stores in preparation for long flights? (2) Is oxidative damage an inevitable consequence of oxidative challenges such as flight, and, if so, how is the extent of damage affected by factors such as the response of the antioxidant system, the level of energetic challenge, and the availability of dietary antioxidants? (3) Do migratory birds ‘recover’ from the oxidative damage accrued during long-duration flights, and, if so, does the pace of this rebalancing of oxidative status depend on the quality of the stopover site? The answer to all these questions is a qualified ‘yes’ although ecological factors (e.g., diet and habitat quality, geographic barriers to migration, and weather) affect how the antioxidant system responds. Furthermore, the pace of this dynamic physiological response remains an open question, despite its potential importance for shaping outcomes on timescales ranging from single flights to migratory journeys. In sum, the antioxidant system of birds during migration is impressively dynamic and responsive to environmental conditions, and thus provides ample opportunities to study how the physiology of migratory birds responds to a changing and challenging world.

Mitochondria: Endosymbiont bacteria DNA sequence as a target against cancer

As the energy factory for the cell, the mitochondrion, through its role of adenosine triphosphate production by oxidative phosphorylation, can be regarded as the guardian of well regulated cellular metabolism; the integrity of mitochondrial functions, however, is particularly vulnerable in cancer due to the lack of superstructures such as histone and lamina folds to protect the mitochondrial genome from unintended exposure, which consequently elevates risks of mutation. In cancer, mechanisms responsible for enforcing quality control surveillance for identifying and eliminating defective mitochondria are often poorly regulated, and certain uneliminated mitochondrial DNA (mtDNA) mutations and polymorphisms can be advantageous for the proliferation, progression, and metastasis of tumor cells. Such pathogenic mtDNA aberrations are likely to increase and occasionally be homoplasmic in cancer cells and, intriguingly, in normal cells in the proximity of tumor microenvironments as well. Distinct characteristics of these abnormalities in mtDNA may provide a new path for cancer therapy. Here we discuss a promising novel therapeutic strategy, using the sequence‐specific properties of pyrrole‐imidazole polyamide‐triphenylphosphonium conjugates, against cancer for clearing abnormal mtDNA by reactivating mitochondrial quality control surveillance.

The Mystery of Energy Compensation

AbstractThe received wisdom on how activity affects energy expenditure is that the more activity is undertaken, the more calories will have been burned by the end of the day. Yet traditional hunter-gatherers, who lead physically hard lives, burn no more calories each day than Western populations living in labor-saving environments. Indeed, there is now a wealth of data, both for humans and other animals, demonstrating that long-term lifestyle changes involving increases in exercise or other physical activities do not result in commensurate increases in daily energy expenditure (DEE). This is because humans and other animals exhibit a degree of energy compensation at the organismal level, ameliorating some of the increases in DEE that would occur from the increased activity by decreasing the energy expended on other biological processes. And energy compensation can be sizable, reaching many hundreds of calories in humans. But the processes that are downregulated in the long-term to achieve energy compensation are far from clear, particularly in humans-we do not know how energy compensation is achieved. My review here of the literature on relevant exercise intervention studies, for both humans and other species, indicates conflict regarding the role, if any, of basal metabolic rate (BMR) or low-level activity such as fidgeting play, particularly once changes in body composition are factored out. In situations where BMR and low-level activity are not major components of energy compensation, what then drives it? I discuss how changes in mitochondrial efficiency and changes in circadian fluctuations in BMR may contribute to our understanding of energy management. Currently unexplored, these mechanisms and others may provide important insights into the mystery of how energy compensation is achieved.

Revisiting the question of nucleated versus enucleated erythrocytes in birds and mammals

Erythrocyte enucleation is thought to have evolved in mammals to support their energetic cost of high metabolic activities. However, birds face similar selection pressure yet possess nucleated erythrocytes. Current hypotheses on the mammalian erythrocyte enucleation claim that the absence of cell organelles allows erythrocytes to 1) pack more hemoglobin into the cells to increase oxygen carrying capacity and 2) decrease erythrocyte size for increased surface area-to-volume ratio, and improved ability to traverse small capillaries. In this article, we first empirically tested current hypotheses using both conventional and phylogenetically informed analysis comparing literature values of mean cell hemoglobin concentration (MCHC) and mean cell volume (MCV) between 181 avian and 194 mammalian species. We found no difference in MCHC levels between birds and mammals using both conventional and phylogenetically corrected analysis. MCV was higher in birds than mammals according to conventional analysis, but the difference was lost when we controlled for phylogeny. These results suggested that avian and mammalian erythrocytes may employ different strategies to solve a common problem. To further investigate existing hypotheses or develop new hypothesis, we need to understand the functions of various organelles in avian erythrocytes. Consequently, we covered potential physiological functions of various cell organelles in avian erythrocytes based on current knowledge, while making explicit comparisons to their mammalian counterparts. Finally, we proposed by taking an integrative and comparative approach, using tools from molecular biology to evolutionary biology, would allow us to better understand the fundamental physiological functions of various components of avian and mammalian erythrocytes.

Avian red blood cell mitochondria produce more heat in winter than in autumn

Endotherms in cold regions improve heat-producing capacity when preparing for winter. We know comparatively little about how this change is fueled by seasonal adaptation in cellular respiration. Thus, we studied the changes of mitochondrial function in red blood cells in sympatric Coal (Periparus ater), Blue (Cyanistes caeruleus), and Great (Parus major) tits between autumn and winter. These species differ more than twofold in body mass and in several aspects of their foraging ecology and social dominance, which could require differential seasonal adaptation of energy expenditure. Coal and Great tits in particular upregulated the mitochondrial respiration rate and mitochondrial volume in winter. This was not directed toward ATP synthesis, instead reflecting increased uncoupling of electron transport from ATP production. Because uncoupling is exothermic, this increased heat-producing capacity at the sub-cellular level in winter. This previously unexplored the route of thermogenesis in birds should be addressed in future work.

Inferring Whole-Organism Metabolic Rate From Red Blood Cells in Birds

Metabolic rate is a key ecological variable that quantifies the energy expenditure needed to fuel almost all biological processes in an organism. Metabolic rates are typically measured at the whole-organism level (woMR) with protocols that can elicit stress responses due to handling and confinement, potentially biasing resulting data. Improved, non-stressful methodology would be especially valuable for measures of field metabolic rate, which quantifies the energy expenditure of free-living individuals. Recently, techniques to measure cellular metabolic rate (cMR) in mitochondria of blood cells have become available, suggesting that blood-based cMR can be a proxy of organismal aerobic performance. Aerobic metabolism actually takes place in the mitochondria. Quantifying cMR from blood samples offers several advantages such as direct estimates of metabolism and minimized disturbance of individuals. To our knowledge, the hypothesis that blood-based cMR correlates with woMR has not yet been directly tested. We measured cMR in red blood cells of captive great tits (Parus major), first during their morning activity period and second after subjecting them to a 2.5 h day-time respirometry protocol to quantify woMR. We predicted cMR to decrease as individuals transitioned from an active to a resting state. In the two blood samples we also assessed circulating corticosterone concentrations to determine the perceived disturbance of individuals. From respirometry traces we extracted initial and final woMR measures to test for a predicted positive correlation with cMR measures, while accounting for corticosterone concentrations. Indeed, cMR declined from the first to the second measurement. Furthermore, woMR and cMR were positively related in individuals that had relatively low corticosterone concentrations and displayed little locomotor activity throughout respirometry. By contrast, woMR and cMR covaried negatively in birds that increased corticosterone concentrations and activity levels substantially. Our results show that red blood cell cMR represents a proxy for woMR when birds do not display signs of stress, i.e., either before increases in hormonal or behavioral parameters have occurred or after they have abated. This method represents a valuable tool for obtaining metabolic data repeatedly and in free-living individuals. Our findings also highlight the importance of accounting for individual stress responses when measuring metabolic rate at any level.

Effects of Heat Waves During Post-natal Development on Mitochondrial and Whole Body Physiology: An Experimental Study in Zebra Finches

Human-induced climate change is increasing the frequency, duration, and intensity of heat waves and exposure to these extreme temperatures impacts individual physiology and performance (e.g., metabolism, water balance, and growth). These traits may be susceptible to thermal conditions experienced during embryonic development, but experiments focusing on post-natal development are scant. Documented effects of heat waves on whole-body metabolism may reflect changes in mitochondrial function, but most studies do not measure physiological traits at both the cellular and whole organism levels. Here, we exposed nests of zebra finches to experimentally simulated heat waves for 18 days after hatching and measured body mass, growth rate, whole-body metabolic rate, body temperature, wet thermal conductance, evaporative water loss, and relative water economy of chicks at three ages corresponding to ectothermic (day 5), poikilothermic (day 12), and homoeothermic (day 50) stages. Additionally, we measured mitochondrial bioenergetics of blood cells 80 days post-hatch. While early-life exposure to heat wave conditions did not impact whole body metabolic and hygric physiology, body temperature was lower for birds from heated compared with control nests at both 12 and 50 days of age. There was also an effect of nest heating at the cellular level, with mitochondria from heated birds having higher endogenous and proton-leak related respiration, although oxidative phosphorylation, maximum respiratory capacity, and coupling efficiency were not impacted. Our results suggest that early-life exposure to high ambient temperature induces programming effects on cellular-level and thermal physiology that may not be apparent for whole-animal metabolism.

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.

Born to be young? Prenatal thyroid hormones increase early-life telomere length in wild collared flycatchers

The underlying mechanisms of the lifelong consequences of prenatal environmental condition on health and ageing remain little understood. Thyroid hormones (THs) are important regulators of embryogenesis, transferred from the mother to the embryo. Since prenatal THs can accelerate early-life development, we hypothesized that this might occur at the expense of resource allocation in somatic maintenance processes, leading to premature ageing. Therefore, we investigated the consequences of prenatal TH supplementation on potential hallmarks of ageing in a free-living avian model in which we previously demonstrated that experimentally elevated prenatal TH exposure accelerates early-life growth. Using cross-sectional sampling, we first report that mitochondrial DNA (mtDNA) copy number and telomere length significantly decrease from early-life to late adulthood, thus suggesting that these two molecular markers could be hallmarks of ageing in our wild bird model. Elevated prenatal THs had no effect on mtDNA copy number but counterintuitively increased telomere length both soon after birth and at the end of the growth period (equivalent to offsetting ca 4 years of post-growth telomere shortening). These findings suggest that prenatal THs might have a role in setting the 'biological' age at birth, but raise questions about the nature of the evolutionary costs of prenatal exposure to high TH levels.

Testing for context-dependent effects of prenatal thyroid hormones on offspring survival and physiology: an experimental temperature manipulation

Maternal effects via hormonal transfer from the mother to the offspring provide a tool to translate environmental cues to the offspring. Experimental manipulations of maternally transferred hormones have yielded increasingly contradictory results, which may be explained by differential effects of hormones under different environmental contexts. Yet context-dependent effects have rarely been experimentally tested. We therefore studied whether maternally transferred thyroid hormones (THs) exert context-dependent effects on offspring survival and physiology by manipulating both egg TH levels and post-hatching nest temperature in wild pied flycatchers (Ficedula hypoleuca) using a full factorial design. We found no clear evidence for context-dependent effects of prenatal THs related to postnatal temperature on growth, survival and potential underlying physiological responses (plasma TH levels, oxidative stress and mitochondrial density). We conclude that future studies should test for other key environmental conditions, such as food availability, to understand potential context-dependent effects of maternally transmitted hormones on offspring, and their role in adapting to changing environments.

Increased glucocorticoid concentrations in early life cause mitochondrial inefficiency and short telomeres

Telomeres are DNA structures that protect chromosome ends. However, telomeres shorten during cell replication and at critically low lengths can reduce cell replicative potential, induce cell senescence and decrease fitness. Stress exposure, which elevates glucocorticoid hormone concentrations, can exacerbate telomere attrition. This phenomenon has been attributed to increased oxidative stress generated by glucocorticoids ('oxidative stress hypothesis'). We recently suggested that glucocorticoids could increase telomere attrition during stressful periods by reducing the resources available for telomere maintenance through changes in the metabolic machinery ('metabolic telomere attrition hypothesis'). Here, we tested whether experimental increases in glucocorticoid levels affected telomere length and mitochondrial function in wild great tit (Parus major) nestlings during the energy-demanding early growth period. We monitored resulting corticosterone (Cort) concentrations in plasma and red blood cells, telomere lengths and mitochondrial metabolism (metabolic rate, proton leak, oxidative phosphorylation, maximal mitochondrial capacity and mitochondrial inefficiency). We assessed oxidative damage caused by reactive oxygen species (ROS) metabolites as well as the total non-enzymatic antioxidant protection in plasma. Compared with control nestlings, Cort-nestlings had higher baseline corticosterone, shorter telomeres and higher mitochondrial metabolic rate. Importantly, Cort-nestlings showed increased mitochondrial proton leak, leading to a decreased ATP production efficiency. Treatment groups did not differ in oxidative damage or antioxidants. Hence, glucocorticoid-induced telomere attrition is associated with changes in mitochondrial metabolism, but not with ROS production. These findings support the hypothesis that shortening of telomere length during stressful periods is mediated by glucocorticoids through metabolic rearrangements.