Oxidative stress in ecology and evolution: lessons from avian studies

Organohalogenated contaminants in white-tailed eagle (Haliaeetus albicilla) nestlings: An assessment of relationships to immunoglobulin levels, telomeres and oxidative stress

Biomagnifying organohalogenated compounds (OHCs) may have adverse effects on the health of birds, especially marine avian top predators that accumulate high OHC loads. Contaminants may impair the humoral immunity and also influence the antioxidant enzyme activity (i.e. oxidative stress). Moreover, physical conditions and oxidative stress during development may reduce telomere lengths, one of the main mechanisms explaining cell senescence. To examine the potential effects of environmental contaminants on physiological biomarkers of health, OHCs with different 'physicochemical' properties were related to immunoglobulin Y levels (IgY; humoral immunity), superoxide dismutase enzyme (SOD) activity in blood plasma, and telomere length (measured in red blood cells) in individual 7-8weeks old nestlings (n=35) of white-tailed eagles (Haliaeetus albicilla) in the Norwegian Sub-Arctic. Different organochlorines (OCs) and perfluoroalkylated substances (PFASs) were measured in blood plasma of nestlings, demonstrating higher concentrations of the emerging contaminants (PFASs), notably perfluorooctane sulfonate (PFOS), compared to legacy OCs. There were no relationships between the contaminant loads and plasma IgY levels. Moreover, differences between years were found for telomere lengths, but this was not related to contaminants and more likely a result of different developmental conditions. However, there were significant and negative relationships between the OC loadings and the SOD activity. This suggests that some legacy OCs challenge the antioxidant capacity in nestlings of white-tailed eagles.

The oxidative debt of fasting: evidence for short to medium-term costs of advanced fasting in adult king penguins

In response to prolonged periods of fasting, animals have evolved metabolic adaptations helping to mobilize body reserves and/or reducing metabolic rate, to ensure a longer usage of reserves. Those metabolic changes can however be associated with higher exposure to oxidative stress, raising the question how species that naturally fast during their life cycle avoid an accumulation of oxidative damage over time. King penguins repeatedly cope with fasting periods up to several weeks. Here we investigated how adult male penguins deal with oxidative stress after an experimentally induced moderate fasting period (PII) or an advanced fasting period (PIII). After fasting in captivity, birds were released to forage at sea. We measured plasmatic oxidative stress on the same individuals at the start and end of the fasting period and when they returned from foraging at sea. We found an increase in activity of the antioxidant enzyme superoxide dismutase along with fasting. However, PIII individuals showed higher oxidative damage at the end of the fast compared to PII individuals. When they returned from re-feeding at sea, all birds had recovered their initial body mass and exhibited low levels of oxidative damage. Notably, levels of oxidative damage after the foraging trip were correlated to the rate of mass gain at sea in PIII individuals but not in PII individuals. Altogether, our results suggest that fasting induces a transitory exposure to oxidative stress and that effort to recover in body mass after an advanced fasting period may be a neglected carry-over cost of fasting.

Oxidative stress during courtship affects male and female reproductive effort differentially in a wild bird with biparental care

Oxidative stress has been suggested as one of the physiological mechanisms modulating reproductive effort, including investment in mate choice. Here, we evaluated whether oxidative stress influences breeding decisions by acting as a cost of or constraint on reproduction in the brown booby (Sula leucogaster), a long-lived seabird with prolonged biparental care. We found that during courtship, levels of lipid peroxidation (LP) of males and females were positively associated with gular skin color, a trait presumably used in mate choice, while levels of reactive oxygen species (ROS) were higher as laying approached and in early breeding pairs. Evidence of a constraining effect of oxidative stress for females was suggested by the fact that females with higher ROS during courtship laid smaller first eggs and had chicks with lower rates of body mass gain, and higher female LP was associated with lower offspring attendance time. No evidence of an oxidative cost of parental effort was found; from courtship to parental care male and female' ROS decreased, and changes in LP levels were non-significant. Finally, using a cross-fostering experiment we found that offspring ROS was unrelated to rearing and genetic parents' ROS. Interestingly, offspring LP was positively associated with the LP during courtship of both the rearing parents and the genetic father, suggesting that offspring LP might have both a genetic and an environmental component. Hence, in the brown booby oxidative stress may be a cost of investment in reproductive traits before egg laying and constrain females' investment in eggs and parental care.

Oxidative stress and life histories: unresolved issues and current needs

Life‐history theory concerns the trade‐offs that mold the patterns of investment by animals between reproduction, growth, and survival. It is widely recognized that physiology plays a role in the mediation of life‐history trade‐offs, but the details remain obscure. As life‐history theory concerns aspects of investment in the soma that influence survival, understanding the physiological basis of life histories is related, but not identical, to understanding the process of aging. One idea from the field of aging that has gained considerable traction in the area of life histories is that life‐history trade‐offs may be mediated by free radical production and oxidative stress. We outline here developments in this field and summarize a number of important unresolved issues that may guide future research efforts. The issues are as follows. First, different tissues and macromolecular targets of oxidative stress respond differently during reproduction. The functional significance of these changes, however, remains uncertain. Consequently there is a need for studies that link oxidative stress measurements to functional outcomes, such as survival. Second, measurements of oxidative stress are often highly invasive or terminal. Terminal studies of oxidative stress in wild animals, where detailed life‐history information is available, cannot generally be performed without compromising the aims of the studies that generated the life‐history data. There is a need therefore for novel non‐invasive measurements of multi‐tissue oxidative stress. Third, laboratory studies provide unrivaled opportunities for experimental manipulation but may fail to expose the physiology underpinning life‐history effects, because of the benign laboratory environment. Fourth, the idea that oxidative stress might underlie life‐history trade‐offs does not make specific enough predictions that are amenable to testing. Moreover, there is a paucity of good alternative theoretical models on which contrasting predictions might be based. Fifth, there is an enormous diversity of life‐history variation to test the idea that oxidative stress may be a key mediator. So far we have only scratched the surface. Broadening the scope may reveal new strategies linked to the processes of oxidative damage and repair. Finally, understanding the trade‐offs in life histories and understanding the process of aging are related but not identical questions. Scientists inhabiting these two spheres of activity seldom collide, yet they have much to learn from each other.

Sex- and melanism-specific variations in the oxidative status of adult tawny owls in response to manipulated reproductive effort

Oxidative stress, determined by the balance between the production of damaging reactive oxygen species (ROS) and antioxidant defences, is hypothesized to play an important role in shaping the cost of reproduction and life history trade-offs. To test this hypothesis, we manipulated reproductive effort in 94 breeding pairs of tawny owls (Strix aluco) to investigate the sex- and melanism-specific effects on markers of oxidative stress in red blood cells (RBCs). This colour polymorphic bird species shows sex-specific division of labour and melanism-specific history strategies. Brood sizes at hatching were experimentally enlarged or reduced to increase or decrease reproductive effort, respectively. We obtained an integrative measure of the oxidative balance by measuring ROS production by RBCs, intracellular antioxidant glutathione levels and membrane resistance to ROS. We found that light melanic males (the sex undertaking offspring food provisioning) produced more ROS than darker conspecifics, but only when rearing an enlarged brood. In both sexes, light melanic individuals had also a larger pool of intracellular antioxidant glutathione than darker owls under relaxed reproductive conditions (i.e. reduced brood), but not when investing substantial effort in current reproduction (enlarged brood). Finally, resistance to oxidative stress was differently affected by the brood size manipulation experiment in males and females independently of their plumage coloration. Altogether, our results support the hypothesis that reproductive effort can alter the oxidative balance in a sex- and colour-specific way. This further emphasizes the close link between melanin-based coloration and life history strategies.

Decadal re-evaluation of contaminant exposure and productivity of ospreys (Pandion haliaetus) nesting in Chesapeake Bay Regions of Concern

The last large-scale ecotoxicological study of ospreys (Pandion haliaetus) in Chesapeake Bay was conducted in 2000-2001 and focused on U.S. EPA-designated Regions of Concern (ROCs; Baltimore Harbor/Patapsco, Anacostia/middle Potomac, and Elizabeth Rivers). In 2011-2012, ROCs were re-evaluated to determine spatial and temporal trends in productivity and contaminants. Concentrations of p,p'-DDE were low in eggs and below the threshold associated with eggshell thinning. Eggs from the Anacostia/middle Potomac Rivers had lower total PCB concentrations in 2011 than in 2000; however, concentrations remained unchanged in Baltimore Harbor. Polybrominated diphenyl ether flame retardants declined by 40%, and five alternative brominated flame retardants were detected at low levels. Osprey productivity was adequate to sustain local populations, and there was no relation between productivity and halogenated contaminants. Our findings document continued recovery of the osprey population, declining levels of many persistent halogenated compounds, and modest evidence of genetic damage in nestlings from industrialized regions.

Energy Expenditure and Metabolic Changes of Free-Flying Migrating Northern Bald Ibis

Many migrating birds undertake extraordinary long flights. How birds are able to perform such endurance flights of over 100-hour durations is still poorly understood. We examined energy expenditure and physiological changes in Northern Bald Ibis Geronticus eremite during natural flights using birds trained to follow an ultra-light aircraft. Because these birds were tame, with foster parents, we were able to bleed them immediately prior to and after each flight. Flight duration was experimentally designed ranging between one and almost four hours continuous flights. Energy expenditure during flight was estimated using doubly-labelled-water while physiological properties were assessed through blood chemistry including plasma metabolites, enzymes, electrolytes, blood gases, and reactive oxygen compounds. Instantaneous energy expenditure decreased with flight duration, and the birds appeared to balance aerobic and anaerobic metabolism, using fat, carbohydrate and protein as fuel. This made flight both economic and tolerable. The observed effects resemble classical exercise adaptations that can limit duration of exercise while reducing energetic output. There were also in-flight benefits that enable power output variation from cruising to manoeuvring. These adaptations share characteristics with physiological processes that have facilitated other athletic feats in nature and might enable the extraordinary long flights of migratory birds as well.

Interactions between Biliverdin, Oxidative Damage, and Spleen Morphology after Simulated Aggressive Encounters in Veiled Chameleons

Stressors frequently increase oxidative damage–unless organisms simultaneously mount effective antioxidant responses. One putative mitigative mechanism is the use of biliverdin, an antioxidant produced in the spleen during erythrocyte degradation. We hypothesized that both wild and captive-bred male veiled chameleons (Chamaeleo calyptratus), which are highly aggressive to conspecifics, would respond to agonistic displays with increased levels of oxidative damage, but that increased levels of biliverdin would limit this increase. We found that even just visual exposure to a potential combatant resulted in decreased body mass during the subsequent 48-hour period, but that hematocrit, biliverdin concentration in the bile, relative spleen size, and oxidative damage in plasma, liver, and spleen were unaffected. Contrary to our predictions, we found that individuals with smaller spleens exhibited greater decreases in hematocrit and higher bile biliverdin concentrations, suggesting a revision to the idea of spleen-dependent erythrocyte processing. Interestingly, individuals with larger spleens had reduced oxidative damage in both the liver and spleen, demonstrating the spleen’s importance in modulating oxidative damage. We also uncovered differences in spleen size and oxidative damage between wild and captive-bred chameleons, highlighting environmentally dependent differences in oxidative physiology. Lastly, we found no relationship between oxidative damage and biliverdin concentration, calling into question biliverdin’s antioxidant role in this species.

Migrating songbirds on stopover prepare for, and recover from, oxidative challenges posed by long-distance flight

Managing oxidative stress is an important physiological function for all aerobic organisms, particularly during periods of prolonged high metabolic activity, such as long-distance migration across ecological barriers. However, no previous study has investigated the oxidative status of birds at different stages of migration and whether that oxidative status depends on the condition of the birds. In this study, we compared (1) energy stores and circulating oxidative status measures in (a) two species of Neotropical migrants with differing migration strategies that were sampled at an autumn stopover site before an ecological barrier; and (b) a species of trans-Saharan migrant sampled at a spring stopover site after crossing an ecological barrier; and (2) circulating oxidative measures and indicators of fat metabolism in a trans-Saharan migrant after stopovers of varying duration (0–8 nights), based on recapture records. We found fat stores to be positively correlated with circulating antioxidant capacity in Blackpoll Warblers and Red-eyed Vireos preparing for fall migration on Block Island, USA, but uncorrelated in Garden Warblers on the island of Ponza, Italy, after a spring crossing of the Sahara Desert and Mediterranean Sea. In all circumstances, fat stores were positively correlated with circulating lipid oxidation levels. Among Garden Warblers on the island of Ponza, fat anabolism increased with stopover duration while oxidative damage levels decreased. Our study provides evidence that birds build antioxidant capacity as they build fat stores at stopover sites before long flights, but does not support the idea that antioxidant stores remain elevated in birds with high fuel levels after an ecological barrier. Our results further suggest that lipid oxidation may be an inescapable hazard of using fats as the primary fuel for flight. Yet, we also show that birds on stopover are capable of recovering from the oxidative damage they have accrued during migration, as lipid oxidation levels decrease with time on stopover. Thus, the physiological strategy of migrating songbirds may be to build prophylactic antioxidant capacity in concert with fuel stores at stopover sites before a long-distance flight, and then repair oxidative damage while refueling at stopover sites after long-distance flight.

Reproduction alters oxidative status when it is traded-off against longevity

Oxidative stress has been proposed to mediate one of the most important aspects of life-history evolution: the trade-off between reproduction and self-maintenance. However, empirical studies have cast doubt on the generality of this intriguing notion. Here, we hypothesize that reproduction alters oxidative status only when a trade-off between reproduction and self-maintenance occurs. Accordingly, in female Bicyclus anynana butterflies, we found that reproduction affected oxidative markers only under challenging thermal conditions that made the trade-off between reproduction and longevity emerge. Interestingly, under such conditions, butterflies favored longevity over reproduction, suggesting that self-maintenance mechanisms were activated. Accordingly, butterflies reproducing under challenging thermal conditions exhibited enhanced antioxidant defenses and stable oxidative damage. Altogether, our results indicate that a trade-off between reproduction and self-maintenance is indeed a necessary condition for reproduction to alter oxidative status, and that the effects of such a trade-off on oxidative status depend on whether priority is given to self-maintenance or reproduction. Assessing the existence of the trade-off between self-maintenance and reproduction, and whether self-maintenance is prioritized relative to reproduction is therefore crucial for understanding variation in oxidative status in reproducing animals, which may clarify the general implication of oxidative stress in the resolution of life-history trade-offs.

Oxidative stress as an indicator of the costs of reproduction among free-ranging rhesus macaques

Sex differences in longevity may reflect sex-specific costs of intra-sexual competition and reproductive effort. As male rhesus macaques experience greater intrasexual competition and die younger, we predicted that males would experience greater oxidative stress than females and that oxidative stress would reflect sex-specific measures of reproductive effort. Males, relative to females, had higher concentrations of 8-OHdG and malondialdehyde, which are markers of DNA oxidative damage and lipid peroxidation, respectively. Older macaques had lower 8-OHdG levels than younger ones, suggesting that oxidative stress decreases in parallel with known age-related declines in reproductive investment. Among males, a recent period of social instability affected oxidative status: males who attacked others at higher rates had higher 8-OHdG levels. Multiparous lactating females with daughters had higher 8-OHdG levels than those with sons. No differences in antioxidant capacity were found. These results lend initial support for the use of oxidative stress markers to assess trade-offs between reproductive effort and somatic maintenance in primates.

Oxidative stress predicts long-term resight probability and reproductive success in Scopoli's shearwater (Calonectris diomedea)

A major challenge in conservation physiology is to find out biomarkers that reliably reflect individual variation in wear and tear. Recent work has suggested that biomarkers of oxidative stress may provide an additional tool to assess the health state of individuals and to predict fitness perspectives. In this study, we assessed whether three biomarkers of plasma oxidative status predicted the following factors: (i) the resight probability as breeder in the next seasons; and (ii) the cumulative reproductive output over multiple years in Scopoli's shearwaters (Calonectris diomedea) using a 7 year individual-based data set. Our results show that shearwaters having higher levels of a marker of oxidative damage (reactive oxygen metabolites) in 2008 had a lower resight probability in the next years and a lower number of chicks raised from 2008 to 2014. In contrast, two biomarkers of antioxidant defences (non-enzymatic antioxidant capacity of plasma and thiols) did not have any predictive value. Increased concentrations of plasma reactive oxygen metabolites, together with the significant individual repeatability over time in this metric of oxidative stress found in numerous studies, suggest that this metric might serve as a blood-derived biomarker for health and fitness perspectives in birds and, possibly, also in other taxa.

Pollutant resilience in embryos of the Antarctic sea urchin Sterechinus neumayeri reflects maternal antioxidant status

Legacy pollutants, including polycyclic aromatic hydrocarbons (PAHs) and metals, can occur in high concentrations in some Antarctic marine environments, particularly near scientific research stations. Oxidative stress is an important unifying feature underlying the toxicity of many chemical contaminants to aquatic organisms. However, the potential impacts of pollutants on the oxidative physiology of Antarctic marine invertebrates are not well documented. Sterechinus neumayeri is a common animal in the shallow subtidal benthos surrounding Antarctica, and is considered an important keystone species. The aim of the present study was to collect baseline oxidative biomarker data for S. neumayeri and to investigate the impacts of field exposure to chemical contaminants on gamete health and parent-to-offspring transfer of oxidative stress resilience. We analysed antioxidant enzyme activities, levels of the molecular antioxidant glutathione, protein carbonylation, lipid peroxidation and levels of 8-OHdG as oxidative stress biomarkers in S. neumayeri from a contaminant-impacted site near McMurdo Station and a relatively pristine site at Cape Evans. Biomarkers were analysed in adult gamete tissue and in early stage embryos exposed to AN8 fuel oil. PAHs were quantified as a proxy for contamination and were found to be elevated in urchins from the contaminated site (up to 231.67ng/g DW). These contaminant-experienced adult urchins produced eggs with greater levels of a broad suite of antioxidants, particularly superoxide dismutase, catalase and glyoxalase-I, than those from Cape Evans. In addition, embryos that were derived from contaminant-experienced mothers were endowed with higher baseline levels of antioxidants, which conferred an enhanced capacity to minimize oxidative damage to lipids, proteins and DNA when exposed to AN8 fuel. This pattern was strongest following exposure to 900ppm AN8, where lipid and protein damage was 5-7 times greater than baseline levels in contaminant-naïve female embryos in comparison to 3-4 times greater in contaminant-experienced female embryos. Despite this inherited resilience against oxidative stress, abnormal development was as high in these embryos when exposed to AN8 as in those derived from contaminant-naïve mothers (up to 80% abnormality), implying the conferred advantage may not translate to a fitness or survival gain, at least up to the blastulae stage. Our findings document the first evidence for parent-to-offspring transfer of oxidative stress resilience in an Antarctic marine invertebrate.

Oxidative shielding and the cost of reproduction

Life-history theory assumes that reproduction and lifespan are constrained by trade-offs which prevent their simultaneous increase. Recently, there has been considerable interest in the possibility that this cost of reproduction is mediated by oxidative stress. However, empirical tests of this theory have yielded equivocal support. We carried out a meta-analysis to examine associations between reproduction and oxidative damage across markers and tissues. We show that oxidative damage is positively associated with reproductive effort across females of various species. Yet paradoxically, categorical comparisons of breeders versus non-breeders reveal that transition to the reproductive state is associated with a step-change reduction in oxidative damage in certain tissues and markers. Developing offspring may be particularly sensitive to harm caused by oxidative damage in mothers. Therefore, such reductions could potentially function to shield reproducing mothers, gametes and developing offspring from oxidative insults that inevitably increase as a consequence of reproductive effort. According to this perspective, we hypothesise that the cost of reproduction is mediated by dual impacts of maternally-derived oxidative damage on mothers and offspring, and that mothers may be selected to diminish such damage. Such oxidative shielding may explain why many existing studies have concluded that reproduction has little or no oxidative cost. Future advance in life-history theory therefore needs to take account of potential transgenerational impacts of the mechanisms underlying life-history trade-offs.

Oxidative stress is a potential cost of breeding in male and female northern elephant seals

The trade-off between current reproductive effort and survival is a key concept of life history theory. A variety of studies support the existence of this trade-off but the underlying physiological mechanisms are not well-understood. Oxidative stress has been proposed as a potential mechanism underlying the observed inverse relationship between reproductive investment and lifespan. Prolonged fasting is associated with oxidative stress including increases in the production of reactive oxygen species, oxidative damage and inflammation.Northern elephant seals (NES) undergo prolonged fasts while maintaining high metabolic rates during breeding. We investigated NES of both sexes to assess oxidative stress associated with extended breeding fasts. We measured changes in the plasma activity or concentrations of markers for oxidative stress in 30 adult male and 33 adult female northern elephant seals across their 1-3 month breeding fasts. Markers assessed included a pro-oxidant enzyme, several antioxidant enzymes, markers for oxidative damage to lipids, proteins and DNA, and markers for systemic inflammation.Plasma xanthine oxidase (XO), a pro-oxidant enzyme that increases production of oxidative radicals, and several protective antioxidant enzymes increased over breeding in both sexes. Males showed increased oxidative damage to lipids and DNA and increased systemic inflammation, while oxidative damage to proteins declined across breeding. In contrast, females showed no oxidative damage to lipids or DNA or changes in inflammation, but showed increases in oxidative damage to proteins. XO activity, antioxidant enzymes, oxidative damage markers, and inflammatory markers were strongly correlated in males but these relationships were weaker or non-existent in females.NES provide evidence for oxidative stress as a physiological cost of reproduction in a capital breeding mammal. Both sexes strongly up-regulated antioxidant defenses during breeding. Despite this response, and in contrast to similar duration non-breeding fasts in previous studies on conspecifics, there was evidence of oxidative damage to tissues. These data demonstrate the utility of using plasma markers to examine oxidative stress but also suggest the necessity of measuring a broad suite of plasma markers to assess systemic oxidative stress.

Repeated stressors in adulthood increase the rate of biological ageing

BACKGROUND

Individuals of the same age can differ substantially in the degree to which they have accumulated tissue damage, akin to bodily wear and tear, from past experiences. This accumulated tissue damage reflects the individual's biological age and may better predict physiological and behavioural performance than the individual's chronological age. However, at present it remains unclear how to reliably assess biological age in individual wild vertebrates.

METHODS

We exposed hand-raised adult Eurasian blackbirds (Turdus merula) to a combination of repeated immune and disturbance stressors for over one year to determine the effects of chronic stress on potential biomarkers of biological ageing including telomere shortening, oxidative stress load, and glucocorticoid hormones. We also assessed general measures of individual condition including body mass and locomotor activity.

RESULTS

By the end of the experiment, stress-exposed birds showed greater decreases in telomere lengths. Stress-exposed birds also maintained higher circulating levels of oxidative damage compared with control birds. Other potential biomarkers such as concentrations of antioxidants and glucocorticoid hormone traits showed greater resilience and did not differ significantly between treatment groups.

CONCLUSIONS

The current data demonstrate that repeated exposure to experimental stressors affects the rate of biological ageing in adult Eurasian blackbirds. Both telomeres and oxidative damage were affected by repeated stress exposure and thus can serve as blood-derived biomarkers of biological ageing.

Are there intergenerational and population-specific effects of oxidative stress in sockeye salmon (Oncorhynchus nerka)?

Intergenerational effects of stress have been reported in a wide range of taxa; however, few researchers have examined the intergenerational consequences of oxidative stress. Oxidative stress occurs in living organisms when reactive oxygen species remain unquenched by antioxidant defense systems and become detrimental to cells. In fish, it is unknown how maternal oxidative stress and antioxidant capacity influence offspring quality. The semelparous, migratory life history of Pacific salmon (Oncorhynchus spp.) provides a unique opportunity to explore intergenerational effects of oxidative stress. This study examined the effects of population origin on maternal and developing offspring oxidative stress and antioxidant capacity, and elucidated intergenerational relationships among populations of sockeye salmon (Oncorhynchus nerka) with varying migration effort. For three geographically distinct populations of Fraser River sockeye salmon (British Columbia, Canada), antioxidant capacity and oxidative stress were measured in adult female plasma, heart, brain, and liver, as well as in developing offspring until time of emergence. Maternal and offspring oxidative stress and antioxidant capacity varied among populations but patterns were not consistent across tissue/developmental stage. Furthermore, maternal oxidative stress and antioxidant capacity did not affect offspring oxidative stress and antioxidant capacity across any of the developmental stages or populations sampled. Our results revealed that offspring develop their endogenous antioxidant systems at varying rates across populations; however, this variability is overcome by the time of emergence. While offspring may be relying on maternally derived antioxidants in the initial stages of development, they rapidly develop their own antioxidant systems (mainly glutathione) during later stages of development.

The janus face of stress on reproduction: from health to disease

Parenthood is a fundamental feature of all known life. However, infertility has been recognized as a public health issue worldwide. But even when the offspring are conceived, in utero problems can lead to immediate (abortion), early (birth), and late (adulthood) consequences. One of the most studied factors is stress. However, stress response is, per se, of adaptive nature allowing the organism to cope with challenges. Stressors lead to deterioration if one is faced with too long lasting, too many, and seemingly unsolvable situations. In stress adaptation the hypothalamus-pituitary-adrenocortical axis and the resulting glucocorticoid elevation are one of the most important mechanisms. At cellular level stress can be defined as an unbalance between production of free radicals and antioxidant defenses. Oxidative stress is widely accepted as an important pathogenic mechanism in different diseases including infertility. On the other hand, the goal of free radical production is to protect the cells from infectious entities. This review aims to summarize the negative and positive influence of stress on reproduction as a process leading to healthy progeny. Special emphasis was given to the balance at the level of the organism and cells.

Carotenoids, birdsong and oxidative status: administration of dietary lutein is associated with an increase in song rate and circulating antioxidants (albumin and cholesterol) and a decrease in oxidative damage

Despite the appealing hypothesis that carotenoid-based colouration signals oxidative status, evidence supporting the antioxidant function of these pigments is scarce. Recent studies have shown that lutein, the most common carotenoid used by birds, can enhance the expression of non-visual traits, such as birdsong. Nevertheless, the underlying physiological mechanisms remain unclear. In this study we hypothesized that male European starlings (Sturnus vulgaris) fed extra lutein increase their song rate as a consequence of an improved oxidative status. Although birdsong may be especially sensitive to the redox status, this has, to the best of our knowledge, never been tested. Together with the determination of circulating oxidative damage (ROMs, reactive oxygen metabolites), we quantified uric acid, albumin, total proteins, cholesterol, and testosterone, which are physiological parameters potentially sensitive to oxidation and/or related to both carotenoid functions and birdsong expression. We found that the birds fed extra lutein sang more frequently than control birds and showed an increase of albumin and cholesterol together with a decrease of oxidative damage. Moreover, we could show that song rate was associated with high levels of albumin and cholesterol and low levels of oxidative damage, independently from testosterone levels. Our study shows for the first time that song rate honestly signals the oxidative status of males and that dietary lutein is associated with the circulation of albumin and cholesterol in birds, providing a novel insight to the theoretical framework related to the honest signalling of carotenoid-based traits.

Experimental food restriction reveals individual differences in corticosterone reaction norms with no oxidative costs

Highly plastic endocrine traits are thought to play a central role in allowing organisms to respond rapidly to environmental change. Yet, not all individuals display the same degree of plasticity in these traits, and the costs of this individual variation in plasticity are unknown. We studied individual differences in corticosterone levels under varying conditions to test whether there are consistent individual differences in (1) baseline corticosterone levels; (2) plasticity in the hormonal response to an ecologically relevant stressor (food restriction); and (3) whether individual differences in plasticity are related to fitness costs, as estimated by oxidative stress levels. We took 25 wild-caught house sparrows into captivity and assigned them to repeated food restricted and control treatments (60% and 110% of their daily food intake), such that each individual experienced both food restricted and control diets twice. We found significant individual variation in baseline corticosterone levels and stress responsiveness, even after controlling for changes in body mass. However, these individual differences in hormonal responsiveness were not related to measures of oxidative stress. These results have implications for how corticosterone levels may evolve in natural populations and raise questions about what we can conclude from phenotypic correlations between hormone levels and fitness measures.

Sex-specific effects of prenatal and postnatal nutritional conditions on the oxidative status of great tit nestlings

The early life period is characterized by fast growth and development, which can lead to high reactive oxygen species (ROS) production. Young animals thus have to balance their investment in growth versus ROS defence, and this balance is likely mediated by resource availability. Consequently resources transferred prenatally by the mother and nutritional conditions experienced shortly after birth may crucially determine the oxidative status of young animals. Here, we experimentally investigated the relative importance of pre- and early postnatal nutritional conditions on the oxidative status of great tit nestlings (Parus major). We show that resources transferred by the mother through the egg and nutritional conditions encountered after hatching affect the oxidative status of nestling in a sex-specific way. Daughters of non-supplemented mothers and daughters which did not receive extra food during the early postnatal period had higher oxidative damage than sons, while no differences between sons and daughters were found when extra food was provided pre- or postnatally. No effect of the food supplementations on growth, fledging mass or tarsus length was observed, indicating that female nestlings maintained their investment in growth at the expense of ROS defence mechanisms when resources were limited. The lower priority of the antioxidant defence system for female nestlings was also evidenced by lower levels of specific antioxidant components. These results highlight the important role of early parental effects in shaping oxidative stress in the offspring, and show that the sensitivity to these parental effects is sex-specific.

Reproduction is associated with a tissue-dependent reduction of oxidative stress in eusocial female Damaraland mole-rats (Fukomys damarensis)

Oxidative stress has been implicated as both a physiological cost of reproduction and a driving force on an animal's lifespan. Since increased reproductive effort is generally linked with a reduction in survival, it has been proposed that oxidative stress may influence this relationship. Support for this hypothesis is inconsistent, but this may, in part, be due to the type of tissues that have been analyzed. In Damaraland mole-rats the sole reproducing female in the colony is also the longest lived. Therefore, if oxidative stress does impact the trade-off between reproduction and survival in general, this species may possess some form of enhanced defense. We assessed this relationship by comparing markers of oxidative damage (malondialdehyde, MDA; protein carbonyls, PC) and antioxidants (total antioxidant capacity, TAC; superoxide dismutase, SOD) in various tissues including plasma, erythrocytes, heart, liver, kidney and skeletal muscle between wild-caught reproductive and non-reproductive female Damaraland mole-rats. Reproductive females exhibited significantly lower levels of PC across all tissues, and lower levels of MDA in heart, kidney and liver relative to non-reproductive females. Levels of TAC and SOD did not differ significantly according to reproductive state. The reduction in oxidative damage in breeding females may be attributable to the unusual social structure of this species, as similar relationships have been observed between reproductive and non-reproductive eusocial insects.

Regulation of plasma testosterone, corticosterone, and metabolites in response to stress, reproductive stage, and social challenges in a desert male songbird

In many male vertebrates, the secretion of reproductive (gonadal androgens) and adrenocortical (glucocorticoids) hormones varies seasonally and in response to environmental stimuli, and these hormones exert numerous behavioral and metabolic effects. We performed two field studies on adult male Rufous-winged Sparrows, Peucaea carpalis, a Sonoran Desert rain-dependent sedentary species, to (a) determine seasonal changes in initial (baseline) and acute stress-induced plasma testosterone (T), corticosterone (CORT), and two metabolites (uric acid and glucose) and (b) compare the effects of two types of social challenge (song playback or simulated territorial intrusion consisting of song playback plus exposure to a live decoy bird) on plasma T, CORT, these metabolites, and territorial behavior. Initial plasma T was higher during the summer breeding period than during post-breeding molt. Acute stress resulting from capture and restraint for 30 min decreased plasma T in breeding condition birds but not in the fall, revealing that this decrease is seasonally regulated. Initial plasma CORT did not change seasonally, but plasma CORT increased in response to acute stress. This increase was likewise seasonally regulated, being relatively smaller during autumnal molt than in the summer. We found no evidence that acute stress levels of CORT are functionally related to stress-depressed plasma T and, therefore, that plasma T decreases during stress as a result of elevated plasma CORT. Thirty minutes of exposure to simulated territorial intrusion resulted in different behavior than 30 min of exposure to song playback, with increased time spent near the decoy and decreased number of overhead flights. Neither type of social challenge influenced plasma T, thus offering no support for the hypothesis that plasma T either responds to or mediates the behavioral effects of social challenge. Exposure to both social challenges elevated plasma CORT, but simulated territorial intrusion was more effective in this respect than song playback. Plasma uric acid and glucose decreased during acute stress, but only plasma uric acid decreased during social challenge. Thus, an elevation in plasma CORT was consistently associated with a decrease in plasma uric acid, but not with a change in glycemia. These results enhance our understanding of the short-term relationships between T, CORT, and avian territorial behavior. They provide novel information on the endocrine effects of acute stress, in particular on plasma T, in free-ranging birds, and are among the first in these birds to link these effects to metabolic changes.

Coenzyme Q10 treatments influence the lifespan and key biochemical resistance systems in the honeybee, Apis mellifera

Natural bioactive preparations that will boost apian resistance, aid body detoxification, or fight crucial bee diseases are in demand. Therefore, we examined the influence of coenzyme Q10 (CoQ10, 2,3-dimethoxy, 5-methyl, 6-decaprenyl benzoquinone) treatment on honeybee lifespan, Nosema resistance, the activity/concentration of antioxidants, proteases and protease inhibitors, and biomarkers. CoQ10 slows age-related metabolic processes. Workers that consumed CoQ10 lived longer than untreated controls and were less infested with Nosema spp. Relative to controls, the CoQ10-treated workers had higher protein concentrations that increased with age but then they decreased in older bees. CoQ10 treatments increased the activities of antioxidant enzymes (superoxide dismutase, GPx, catalase, glutathione S-transferase), protease inhibitors, biomarkers (aspartate aminotransferase, alkaline phosphatase, alanine aminotransferase), the total antioxidant potential level, and concentrations of uric acid and creatinine. The activities of acidic, neutral, and alkaline proteases, and concentrations of albumin and urea were lower in the bees that were administered CoQ10. CoQ10 could be taken into consideration as a natural diet supplement in early spring before pollen sources become available in the temperate Central European climate. A response to CoQ10 administration that is similar to mammals supports our view that Apis mellifera is a model organism for biochemical gerontology.

Experimentally induced stress, oxidative load and changes in immunity in a tropical wild bird, Perdicula asiatica: involvement of melatonin and glucocorticoid receptors

Throughout the year, birds encounter various environmental challenges such as extreme temperatures, rainfall and shortage of food. Here we report on the effect of stress on the general immunity of wild birds as measured by several assays including melatonin, an anti-stress hormone. We selected Perdicula asiatica, a wild tropical bird, and exposed them to experimental stressors such as water deprivation, food deprivation and immobilization, i.e., stressors that they would encounter in a natural environment. We measured the oxidative load in the spleen in terms of superoxide dismutase (SOD), catalase activity and thiobarbituric acid-reactive substance (TBARS) levels. The immune status was judged by total leukocyte count (TLC), heterophil/lymphocyte ratios (H/L) and percent stimulation ratio of splenocytes (%SR). The peripheral levels of melatonin and corticosterone were also determined and correlated with the expression of melatonin (Mel(1a)/Mel(1b)) and glucocorticoid receptors. Our results showed a significant (p < 0.05) decrease in splenic SOD and catalase activity, while a significant (p < 0.05) increase in TBARS and a corticosterone level was observed. Stressful conditions also decreased the immune status as reflected by the low values of H/L ratios, TLC and %SR. In contrast, melatonin pretreatment significantly (p < 0.05) reduced the oxidative stress and improved the immune parameters when compared to untreated control birds. This suggests that melatonin prevents/alleviates oxidative damage and suppresses the immune status induced by stressful conditions via its membrane receptor expression (Mel(1a) and Mel(1b)) in P. asiatica.

Oxidative stress in endurance flight: an unconsidered factor in bird migration

Migrating birds perform extraordinary endurance flights, up to 200 h non-stop, at a very high metabolic rate and while fasting. Such an intense and prolonged physical activity is normally associated with an increased production of reactive oxygen and nitrogen species (RONS) and thus increased risk of oxidative stress. However, up to now it was unknown whether endurance flight evokes oxidative stress. We measured a marker of oxidative damage (protein carbonyls, PCs) and a marker of enzymatic antioxidant capacity (glutathione peroxidase, GPx) in the European robin (Erithacus rubecula), a nocturnal migrant, on its way to the non-breeding grounds. Both markers were significantly higher in European robins caught out of their nocturnal flight than in conspecifics caught during the day while resting. Independently of time of day, both markers showed higher concentrations in individuals with reduced flight muscles. Adults had higher GPx concentrations than first-year birds on their first migration. These results show for the first time that free-flying migrants experience oxidative stress during endurance flight and up-regulate one component of antioxidant capacity. We discuss that avoiding oxidative stress may be an overlooked factor shaping bird migration strategies, e.g. by disfavouring long non-stop flights and an extensive catabolism of the flight muscles.

Cellular respiration: the nexus of stress, condition, and ornamentation

A fundamental hypothesis for the evolution and maintenance of ornamental traits is that ornaments convey information to choosing females about the quality of prospective mates. A diverse array of ornaments (e.g., colors, morphological features, and behaviors) has been associated with a wide range of measures of individual quality, but decades of study of such indicator traits have failed to produce general mechanisms of honest signaling. Here, I propose that efficiency of cellular respiration, as a product of mitochondrial function, underlies the associations between ornamentation and performance for a broad range of traits across taxa. A large biomedical literature documents the fundamental biochemical links between oxidative phosphorylation (OXPHOS) and the production of reactive oxygen species (ROS), the process of metabolism, the function of the immune system, the synthesis of proteins, and the development and function of the nervous system. The production of virtually all ornaments whose expressions have been demonstrated to be condition-dependent is directly affected by the efficiency of cellular respiration, suggesting that the signaling of respiratory efficiency may be the primary function of such traits. Furthermore, the production of ornaments links to stress-response systems, including particularly the neuroendocrine system, through mitochondrial function, thereby makes ornamental traits effective signals of the capacity to withstand environmental perturbations. The identification of a unifying mechanism of honest signaling holds the potential to connect many heretofore-disparate fields of study related to stress and ornamentation, including neuroendocrinology, respiratory physiology, metabolic physiology, and immunology.