Age and breeding effort as sources of individual variability in oxidative stress markers in a bird species

DNA damage (8-OHdG) and telomere length in captive Psittacidae birds with different longevity

Aging is a complex process influenced by internal and external factors. Oxidative stress damages DNA, leading to 8-hydroxy-2' deoxyguanosine formation (8-OHdG). Telomere shortening is considered a biomarker of aging and oxidative stress may enhance its attrition. The ability to manage and repair oxidative stress varies among species and life histories. Avian species, such as Psittacidae birds, exhibit exceptional lifespans despite their physiological characteristics that might suggest otherwise. This study investigates 8-OHdG levels in serum samples from long- and short-lived birds of the order Psittaciformes, examining their relationship with telomere length and antioxidant capacity based on lifespan strategies. Among 43 individuals analyzed 26 belonged to the "long-lived species" group and 17 belonged to the "short-lived species" one. Relative telomere length (rTL) was measured in DNA isolated from whole blood by qPCR, and oxidative stress markers, such as Total Antioxidant Capacity (TAC) and 8-OHdG, were determined by spectrophotometry in serum samples. Long-lived birds had longer rTL than short-lived ones [1.308 ± 0.11 vs. 0.565 ± 0.13, (p < 0.001)]. On the contrary, short-lived birds showed more DNA damage than their counterparts [3.847 ± 0.351 vs. 2.012 ± 0.308, respectively, (p < 0.001)]. Old birds had shorter rTL than young ones, for both longevity groups (p < 0.001). Although no correlation was found between 8-OHdG levels and age, nor 8-OHdG and telomere length, long-lived birds exhibited 75.42-unit increased TAC levels when increased 8-OHdG concentrations (p = 0.046). These findings highlight distinct patterns of telomere length and oxidative stress influenced by lifespan strategies among avian longevity groups.

Biomonitoring of heavy metals and their association with DNA damage in Indian peafowl (Pavo cristatus) under captivity

Environmental pollution and changing climatic conditions are likely to damage biodiversity not only on organismal level but on molecular level as well. The aim of the present study was to find the concentration of heavy metals in soil, water, feed, feathers, and blood and association of heavy metals with DNA damage of P. cristatus. The results showed that lead (Pb) was significantly (P < 0.01) higher in soil and cadmium (Cd) was significantly (P < 0.01) higher in soil and water. Chromium (Cr), zinc (Zn), nickel (Ni), and cobalt (Co) were significant (P < 0.01) in feed. Manganese (Mn) was significantly (P < 0.01) higher in feed and soil (surface). In addition, Pb and Cd concentrations were significant (P < 0.01) in feathers while Cr and Zn concentrations were significantly (P < 0.01) higher in feces. Nickel was significantly (P < 0.01) higher in feathers and eggshell while Mn and Co concentrations were significantly (P < 0.01) higher in blood and feces, respectively. Furthermore, significant positive correlation between Pb (r_s = 0.75; P < 0.05) and Cd (r_s = 0.67; P < 0.05) concentrations in blood with tail DNA was found. It was concluded that heavy metals exist in the soil, water, feathers, and blood and have association with DNA damage of P. cristatus.

An approach to the effects of longevity, sexual maturity, and reproduction on telomere length and oxidative stress in different Psittacidae species

Introduction: Aging is a multifactorial process that includes molecular changes such as telomere shortening. Telomeres shorten progressively with age in vertebrates, and their shortening rate has a significant role in determining the lifespan of a species. However, DNA loss can be enhanced by oxidative stress. The need for novel animal models has recently emerged as a tool to gather more information about the human aging process. Birds live longer than other mammals of the same size, and Psittacidae species are the most persevering of them, due to special key traits. Methods: We aimed to determine telomere length by qPCR, and oxidative stress status using colorimetric and fluorescence methods in different species of the order Psittaciformes with different lifespans. Results: We found that telomeres shorten with age for both long- and short-lived birds (p < 0.001 and p = 0.004, respectively), with long-lived birds presenting longer telomeres than short-lived ones (p = 0.001). In addition, short-lived birds accumulated more oxidative stress products than long-lived birds (p = 0.013), who showed a better antioxidant capacity (p < 0.001). Breeding was found related to telomere shortening in all species (p < 0.001 and p = 0.003 for long- and short-lived birds). Short-lived birds, especially breeding females, increased their oxidative stress products when breeding (p = 0.021), whereas long-lived birds showed greater resistance and even increased their antioxidant capacity (p = 0.002). Conclusion: In conclusion, the relationship between age and telomere length in Psittacidae was verified. The influence of breeding increased cumulative oxidative damage in short-lived species, while long-lived species may counteract this damage.

Understanding the ecological and evolutionary function of stopover in migrating birds

Global movement patterns of migratory birds illustrate their fascinating physical and physiological abilities to cross continents and oceans. During their voyages, most birds land multiple times to make so-called 'stopovers'. Our current knowledge on the functions of stopover is mainly based on the proximate study of departure decisions. However, such studies are insufficient to gauge fully the ecological and evolutionary functions of stopover. If we study how a focal trait, e.g. changes in energy stores, affects the decision to depart from a stopover without considering the trait(s) that actually caused the bird to land, e.g. unfavourable environmental conditions for flight, we misinterpret the function of the stopover. It is thus important to realise and acknowledge that stopovers have many different functions, and that not every migrant has the same (set of) reasons to stop-over. Additionally, we may obtain contradictory results because the significance of different traits to a migrant is context dependent. For instance, late spring migrants may be more prone to risk-taking and depart from a stopover with lower energy stores than early spring migrants. Thus, we neglect that departure decisions are subject to selection to minimise immediate (mortality risk) and/or delayed (low future reproductive output) fitness costs. To alleviate these issues, we first define stopover as an interruption of migratory endurance flight to minimise immediate and/or delayed fitness costs. Second, we review all probable functions of stopover, which include accumulating energy, various forms of physiological recovery and avoiding adverse environmental conditions for flight, and list potential other functions that are less well studied, such as minimising predation, recovery from physical exhaustion and spatiotemporal adjustments to migration. Third, derived from these aspects, we argue for a paradigm shift in stopover ecology research. This includes focusing on why an individual interrupts its migratory flight, which is more likely to identify the individual-specific function(s) of the stopover correctly than departure-decision studies. Moreover, we highlight that the selective forces acting on stopover decisions are context dependent and are expected to differ between, e.g. K-/r-selected species, the sexes and migration strategies. For example, all else being equal, r-selected species (low survival rate, high reproductive rate) should have a stronger urge to continue the migratory endurance flight or resume migration from a stopover because the potential increase in immediate fitness costs suffered from a flight is offset by the expected higher reproductive success in the subsequent breeding season. Finally, we propose to focus less on proximate mechanisms controlling landing and departure decisions, and more on ultimate mechanisms to identify the selective forces shaping stopover decisions. Our ideas are not limited to birds but can be applied to any migratory species. Our revised definition of stopover and the proposed paradigm shift has the potential to stimulate a fruitful discussion towards a better evolutionary ecological understanding of the functions of stopover. Furthermore, identifying the functions of stopover will support targeted measures to conserve and restore the functionality of stopover sites threatened by anthropogenic environmental changes. This is especially important for long-distance migrants, which currently are in alarming decline.

The Cost of Reproduction in a Cooperatively Breeding Mammal: Consequences of Seasonal Variation in Rainfall, Reproduction, and Reproductive Suppression

Biological investments, such as reproduction, are influenced by both biotic and abiotic factors and their interactions. The trade-off between reproduction and survival has been well established. Seasonally breeding species, therefore, may exhibit variations in these trade-offs, but there is a dearth of knowledge concerning this. This study investigated the physiological cost of reproduction (measured through oxidative stress) across seasons in the cooperatively breeding highveld mole-rat (Cryptomys hottentotus pretoriae), one of the few seasonal breeding mole-rats. Oxidative stress indicates elevated reactive oxygen species (ROS) levels, which can overwhelm antioxidant defences resulting in damaged proteins, lipids and DNA, which overall can reduce longevity and compromise reproduction. Oxidative markers such as total oxidant status (TOS-measure of total peroxides present), total antioxidant capacity (TAC), oxidative stress index (OSI), and malondialdehyde (MDA) are utilised to measure oxidative stress. In this study, breeding and non-breeding male (NBM) and female mole-rats were captured during the dry season (breeding period) and wet season (non-breeding period). There was an apparent cost of reproduction in the highveld mole-rat; however, the seasonality pattern to the cost of reproduction varied between the sexes. Breeding females (BFs) had significantly higher MDA during the breeding period/dry season in comparison to the non-breeding period/wet season; this is possibly a consequence of bearing and nursing offspring. Contrastingly, breeding males (BMs) showed increased oxidative damage in the non-breeding/wet season compared to the breeding/dry season, possibly due to increased activities of protecting their mating rights for the next breeding/dry season, but this was not significant. Interestingly, during the non-breeding period/wet season, non-breeding females (NBFs) are released from their reproductive suppression, which resulted in increases in TOS and OSI, which again indicated that just the mere ability to be able to breed results in a cost (oxidative stress). Therefore we can speculate that highveld mole-rats exhibited seasonal variation in redox balance brought about by variation in abiotic variables (e.g., rainfall), physiology and behaviour. We conclude that physiological changes associated with reproduction are sufficient to induce significant acute oxidative stress in the plasma of female highveld mole-rats, which become alleviated following transition to the non-breeding season/wet period suggesting a possible hormetic effect.

New Perspectives on Avian Models for Studies of Basic Aging Processes

Avian models have the potential to elucidate basic cellular and molecular mechanisms underlying the slow aging rates and exceptional longevity typical of this group of vertebrates. To date, most studies of avian aging have focused on relatively few of the phenomena now thought to be intrinsic to the aging process, but primarily on responses to oxidative stress and telomere dynamics. But a variety of whole-animal and cell-based approaches to avian aging and stress resistance have been developed-especially the use of primary cell lines and isolated erythrocytes-which permit other processes to be investigated. In this review, we highlight newer studies using these approaches. We also discuss recent research on age-related changes in neural function in birds in the context of sensory changes relevant to homing and navigation, as well as the maintenance of song. More recently, with the advent of "-omic" methodologies, including whole-genome studies, new approaches have gained momentum for investigating the mechanistic basis of aging in birds. Overall, current research suggests that birds exhibit an enhanced resistance to the detrimental effects of oxidative damage and maintain higher than expected levels of cellular function as they age. There is also evidence that genetic signatures associated with cellular defenses, as well as metabolic and immune function, are enhanced in birds but data are still lacking relative to that available from more conventional model organisms. We are optimistic that continued development of avian models in geroscience, especially under controlled laboratory conditions, will provide novel insights into the exceptional longevity of this animal taxon.

Fasting ameliorates oxidative stress: A review of physiological strategies across life history events in wild vertebrates

Fasting is a component of many species' life history due to environmental factors or behavioral patterns that limit access to food. Despite metabolic and physiological challenges associated with these life history stages, fasting-adapted wild vertebrates exhibit few if any signs of oxidative stress, suggesting that fasting promotes redox homeostasis. Here we review mammalian, avian, reptilian, amphibian, and piscine examples of animals undergoing fasting during prolonged metabolic suppression (e.g. hibernation and estivation) or energetically demanding processes (e.g. migration and breeding) to better understand the mechanisms underlying fasting tolerance in wild vertebrates. These studies largely show beneficial effects of fasting on redox balance via limited oxidative damage. Though some species exhibit signs of oxidative stress due to energetically or metabolically extreme processes, fasting wild vertebrates largely buffer themselves from the negative consequences of oxidative damage through specific strategies such as elevating antioxidants, selectively maintaining redox balance in critical tissues, or modifying behavioral patterns. We conclude with suggestions for future research to better elucidate the protective effects of fasting on oxidative stress as well as disentangle the impacts from other life history stages. Further research in these areas will facilitate our understanding of the mechanisms wild vertebrates use to mitigate the negative impacts associated with metabolically-extreme life history stages as well as potential translation into therapeutic interventions in non-fasting-adapted species including humans.

The diversity of maternal-age effects upon pre-adult survival across animal species

Maternal senescence is the detrimental effect of increased maternal age on offspring performance. Despite much recent interest given to describing this phenomenon, its distribution across animal species is poorly understood. A review of the published literature finds that maternal age affects pre-adult survival in 252 of 272 populations (93%) representing 97 animal species. Age effects tended to be deleterious in invertebrates and mammals, including humans, confirming the presence of senescence. However, bird species were a conspicuous exception, as pre-adult survival tended to increase with maternal age in surveyed populations. In all groups, maternal-age effects became more negative in older mothers. Invertebrates senesced faster than vertebrates, and humans aged faster than non-human mammals. Within invertebrates, lepidopterans demonstrated the most extreme rates of maternal-effect senescence. Among the surveyed studies, phylogeny, life history and environment (e.g. laboratory versus wild populations) were tightly associated; this made it difficult to make confident inferences regarding the causes of diversity for the phenomenon. However, we provide some testable suggestions, and we observe that some differences appear to be consistent with predictions from evolutionary theory. We discuss how future work may help clarify ultimate and proximate causes for this diversity.

Life-long testosterone and antiandrogen treatments affect the survival and reproduction of captive male red-legged partridges (Alectoris rufa)

Abstract

Sexual steroids can play an important role as life-history organizers. In males, high circulating testosterone levels induce physiological/behavioral costs and benefits, leading to trade-offs. However, studies simultaneously testing the impact of these levels in both fitness components (survival and fecundity) during lifetime are scarce and limited to wild birds. To determine the mortality causes or hormonal manipulation impacts on male fertility is, nonetheless, a difficult task in free-ranging animals that could be easier in captivity. We longitudinally monitored captive red-legged partridges (Alectoris rufa) and exposed males to high exogenous testosterone levels, anti-androgens, or a control treatment during each breeding period throughout their lives. Theory predicts that individuals maintaining high androgen levels should obtain higher fitness returns via reproduction, but suffer reduced longevity. Testosterone-treated male partridges, accordingly, lived shorter compared to controls, since they were more prone to die from a natural bacterial infection. However, the same birds seemed to have a lower capacity to fertilize eggs, probably due to endocrine feedback reducing testicular mass. These results show that exogenous testosterone can exert unpredicted effects on fitness parameters. Therefore, caution must be taken when drawing conclusions from non-fully controlled experiments in the wild. Males treated with the androgen-receptor blocker flutamide did not outlive controls as predicted by the life-history trade-off theory, but their mates laid eggs with higher hatching success. The latter could be due to mechanisms improving sperm quality/quantity or influencing maternal investment in egg quality. Testosterone receptor activity/amount could thus be as relevant to fitness as testosterone levels.

Significance statement

It has repeatedly been hypothesized that high testosterone levels induce a cost in terms of reduced lifetime reproductive success. This can be due to reduced fecundity or via shorter lifespan. This is, however, only supported by a handful of studies, mostly in wild birds. We tested this in captive male red-legged partridges, which allowed us to determine reproductive success and mortality causes. We increased testosterone levels or blocked its action with antiandrogens throughout life. High testosterone levels reduced the survival by making birds more prone to die by infection. The eggs produced by their mates also showed lower hatching success, a probable manipulation artifact that should be considered in avian studies in the wild. Interestingly, the androgen-receptor blocker flutamide increased lifetime hatching success compared to controls, suggesting that androgen receptor amounts/activity are even more relevant to fitness than testosterone levels.

Effect of the Age and Body Weight of the Broiler Breeders Male on the Presentation of Oxidative Stress and Its Correlation with the Quality of Testicular Parenchyma and Physiological Antioxidant Levels

Chicken meat is a food of high nutritional quality; its production requires birds called broilers breeders and looking after all aspects that influence their reproductive capacity. An ongoing controversy exists among researchers related to the weight of the rooster and its fertilization capacity. By histological and biochemical tests, the association between weight and age with oxidant damage, testicular parenchyma and antioxidant capacity was evaluated in Ross 308 roosters. Testicular integrity was assessed by histological analysis, oxidative stress was determined by malondialdehyde content, non-enzymatic antioxidant capacity was determined by oxygen radical absorbance capacity assay and enzymatic antioxidant capacity through glutathione peroxidase, glutathione reductase and glutathione-S-transferase activities. Histological analysis showed vacuolization of the epithelium from the seminiferous tubules. A significant negative association was observed between malondialdehyde and the deterioration of the integrity of the seminiferous epithelium, as well as between age and integrity of the seminiferous epithelium. It became evident that oxidative damage directly affects the quality of testicular parenchyma. Weight and age were not associated with the antioxidant enzymes activities, but with non-enzymatic capacity. The data obtained suggest that weight is not the most important factor that influences the fertility of the rooster.

Muscle myonuclear domain, but not oxidative stress, decreases with age in a long-lived seabird with high activity costs

In birds, many physiological parameters appear to remain constant with increasing age, showing no deterioration until 'catastrophic' mortality sets in. Given their high whole-organism metabolic rate and the importance of flight in foraging and predator avoidance, flight muscle deterioration and accumulated oxidative stress and tissue deterioration may be an important contributor to physiological senescence in wild birds. As a by-product of aerobic respiration, reactive oxygen species are produced and can cause structural damage within cells. The anti-oxidant system deters oxidative damage to macromolecules. We examined oxidative stress and muscle ultrastructure in thick-billed murres aged 8 to 37 years (N=50) in pectoralis muscle biopsies. When considered in general linear models with body mass, body size and sex, no oxidative stress parameter varied with age. In contrast, there was a decrease in myonuclear domain similar to that seen in human muscle aging. We conclude that for wild birds with very high flight activity levels, muscle ultrastructural changes may be an important contributor to demographic senescence. Such gradual, linear declines in muscle morphology may eventually contribute to 'catastrophic' failure in foraging or predator avoidance abilities, leading to demographic senescence.

Age-specific patterns of maternal investment in common gull egg yolk

While the general patterns of age-specific changes in reproductive success are quite well established in long-lived animals, we still do not know if allocation patterns of maternally transmitted compounds are related to maternal age. We measured the levels of yolk testosterone, carotenoids and vitamins A and E in a population of known-aged common gulls (Larus canus) and found an age-specific pattern in yolk lutein and vitamin A concentrations. Middle-aged mothers allocated more of these substances to yolk compared to young and old mothers. These results can be explained through differences in age-specific foraging, absorption or deposition patterns of carotenoids and vitamins into yolk. If these molecules play a role in antioxidant defence and immune modulation, our results suggest a possible physiological pathway underlying the age-specific changes in reproductive success of long-lived birds in the wild.

Dietary canthaxanthin reduces xanthophyll uptake and red coloration in adult red-legged partridges

Carotenoids give color to conspicuous animal signals that are often the product of sexual selection. Knowledge of the mechanisms involved in carotenoid-based signaling is critical to understanding how these traits evolve. However, these mechanisms remain only partially understood. Carotenoids are usually viewed as scarce dietary antioxidants whose allocation to ornaments may trade off against health. This trade-off would ensure its reliability as a signal of individual quality. In the case of red (keto)carotenoids, the literature suggests that some species may show constraints in their uptake. Canthaxanthin is one of the most common ketocarotenoids in red ornaments of animals. It is often commercially used as a dietary supplement to obtain redder birds (e.g. poultry). We increased the dietary canthaxanthin levels in captive red-legged partridges (Alectoris rufa). This species shows red non-feathered parts mostly pigmented by another common ketocarotenoid: astaxanthin. We studied the impact on the uptake of carotenoids and vitamins and, finally, on coloration. We also tested the potential protective effect of canthaxanthin when exposing birds to a free radical generator (diquat). Canthaxanthin did not apparently protect birds from oxidative stress, but interfered with the absorption of yellow carotenoids (lutein and zeaxanthin). Zeaxanthin is a precursor of astaxanthin in enzymatic pathways, and their levels in tissues and eggs were lower in canthaxanthin-supplied birds. This led to lower astaxanthin levels in ornaments and paler coloration. As far as we know, this is the first report of a carotenoid supplementation decreasing animal coloration. The results have implications for understanding carotenoid-based signaling evolution, but also for improving husbandry/experimental procedures.

Immune parameters in different age classes of captive male Steller's eiders (Polysticta stelleri)

The immune system is important for host defense against antigens, but little is known about Steller's eider (Polysticta stelleri) immunology. This study compared hematological parameters, serum protein levels, lymphocyte proliferation, heat shock protein levels and oxidative damage in four different age classes of captive male Steller's eiders. The hatch year cohort had significantly higher total white blood cell and lymphocyte counts. The second year cohort had significantly higher albumin, alpha globulins and lymphocyte proliferation, and significantly lower beta globulin levels. The 9 year old males had a significantly higher IgY:IgY(ΔFc) ratio. The oldest eiders in the study, 14 + year old males, had significantly higher serum IgY, pre-albumin and glutathione reductase activity, and the lowest lymphocyte proliferation. This study provided a baseline of immune parameters in captive male Steller's eiders, and the results suggested the parameters were influenced by age-related changes.

A hidden cost of migration? Innate immune function versus antioxidant defense

Migration is energetically demanding and physiologically challenging. Migrating birds, for example, need to boost their antioxidant defenses to defeat the pro‐oxidants produced during high energetic activity. The enhanced antioxidant defense possibly withdraws limited resources (e.g., energy or micronutrients) from other physiological functions, such as immune defense. Such a trade‐off might not occur outside the migration seasons or in resident individuals. Here, we investigate whether there is a negative relationship between innate immune function and antioxidant defense by sampling both migrating and resident blackbirds (Turdus merula) at the same location during the same period of the annual cycle. We show that in migrating blackbirds microbial killing capacity (BKA), an integrative measure of baseline innate immune function was negatively correlated with total nonenzymatic antioxidant capacity. In contrast, in resident conspecifics, sampled at the same time and location, these two physiological measures were not correlated. This suggests that migrating birds trade off innate immune function and antioxidant defense. Furthermore, and likely a consequence of this trade‐off, in migrant blackbirds BKA was positively correlated with oxidative damage to lipids. In resident blackbirds BKA and degree of lipid oxidation were uncorrelated. The mechanism and currencies of the supposed trade‐off are currently unknown, but energetic investments or micronutrients are likely candidates. Future experimental studies could provide more conclusive evidence for this trade‐off; yet, our results open up a new level of thinking about the physiological costs of migration.

Telomere length, sibling competition and development of antioxidant defense in wild house mice

Antioxidants and telomere length are potential biomarkers for individuals' exposure and ability to cope with environmental stressors. However, intraspecific variations in antioxidant alterations due to natural, life cycle related stress, have been rarely estimated. We investigated those changes in wild-derived house mice in a longitudinal study with natural sibling competition as a stressor. Blood was used for telomere length measurements at 8-weeks age and for several selected antioxidants at 8-weeks and 6-months age. Our results show that most of the antioxidants increase during that time, indicating that antioxidant-system continues to develop after early development and sexual maturation. In addition females had higher antioxidant-levels than males. Mice with longer telomeres had also higher superoxide dismutase-activity and more glutathione than mice with shorter telomeres, meaning that long telomeres are associated with better antioxidant defense at maturation and during later life. Sibling competition at early age affected superoxide dismutase-levels at 6-months, but only in females. Females, which were lighter than the average of the litter had low superoxide dismutase -activity in later adulthood, indicating delayed negative effect of sibling competition on antioxidant defense. Our results highlight that sex and developmental stage are crucial in intraspecific comparisons of the antioxidant status and its alterations.

Longevity is associated with relative brain size in birds

Brain size of vertebrates has long been recognized to evolve in close association with basic life-history traits, including lifespan. According to the cognitive buffer hypothesis, large brains facilitate the construction of behavioral responses against novel socioecological challenges through general cognitive processes, which should reduce mortality and increase lifespan. While the occurrence of brain size-lifespan correlation has been well documented in mammals, much less evidence exists for a robust link between brain size and longevity in birds. The aim of this study was to use phylogenetically controlled comparative approach to test for the relationship between brain size and longevity among 384 avian species from 23 orders. We used maximum lifespan and maximum reproductive lifespan as the measures of longevity and accounted for a set of possible confounding effects, such as allometry, sampling effort, geographic patterns, and life-history components (clutch size, incubation length, and mode of development). We found that both measures of longevity positively correlated with relative (residual) brain size. We also showed that major diversification of brain size preceded diversification of longevity in avian evolution. In contrast to previous findings, the effect of brain size on longevity was consistent across lineages with different development patterns, although the relatively low strength of this correlation could likely be attributed to the ubiquity of allomaternal care associated with the altricial mode of development. Our study indicates that the positive relationship between brain size and longevity in birds may be more general than previously thought.

Does baseline innate immunity change with age? A multi-year study in great tits

Throughout their life animals progressively accumulate mostly detrimental changes in cells, tissues and their functions, causing a decrease in individual performance and ultimately an increased risk of death. The latter may be amplified if it also leads to a deterioration of the immune system which forms the most important protection against the permanent threat of pathogens and infectious diseases. Here, we investigated how four baseline innate immune parameters (natural antibodies, complement activity, concentrations of haptoglobin and concentrations of nitric oxide) changed with age in free-living great tits (Parus major). We applied both cross-sectional and longitudinal approaches as birds were sampled for up to three years of their lives. Three out of the four selected innate immune parameters were affected by age. However, the shape of the response curves differed strongly among the innate immune parameters. Natural antibody levels increased during early life until mid-age to decrease thereafter when birds aged. Complement activity was highest in young birds, while levels slightly decreased with increasing age. Haptoglobin levels on the other hand, showed a linear, but highly variable increase with age, while nitric oxide concentrations were unaffected by age. The observed differences among the four studied innate immune traits not only indicate the importance of considering several immune traits at the same time, but also highlight the complexity of innate immunity. Unraveling the functional significance of the observed changes in innate immunity is thus a challenging next step.

Interacting effects of early dietary conditions and reproductive effort on the oxidative costs of reproduction

The hypothesis that oxidative damage accumulation can mediate the trade-off between reproduction and lifespan has recently been questioned. However, in captive conditions, studies reporting no evidence in support of this hypothesis have usually provided easy access to food which may have mitigated the cost of reproduction. Here, I test the hypothesis that greater investment in reproduction should lead to oxidative damage accumulation and telomere loss in domestic zebra finches Taeniopygia guttata. Moreover, since the change or fluctuation in diet composition between early and late postnatal period can impair the ability to produce antioxidant defences in zebra finches, I also tested if early nutritional conditions (constant vs fluctuating early diet) influenced the magnitude of any subsequent costs of reproduction (e.g., oxidative damage and/or telomere shortening). In comparison to pairs with reduced broods, the birds that had to feed enlarged broods showed a higher level of oxidative DNA damage (8-OHdG), but brood size had no effect on telomeres. Fluctuating early diet composition reduced the capacity to maintain the activity of endogenous antioxidants (GPx), particularly when reproductive costs were increased (enlarged brood). The decline in GPx in birds feeding enlarged broods was accompanied by a change in bill colouration. This suggests that birds with lower endogenous antioxidant defences might have strategically increased the mobilization of antioxidants previously stored in other tissues (i.e., bill and liver) and thus, preventing an excessive accumulation of damage during reproduction.

Oxidative ecology of paternal care in wild smallmouth bass, Micropterus dolomieu

Physiologically, oxidative stress is considered a homeostatic imbalance between reactive oxygen species production and absorption. From an ecological perspective, oxidative stress may serve as an important constraint to life history traits such as lifespan, reproduction, and the immune system, and is gaining interest as a potential mechanism underlying life history trade-offs. Of late, there has been much interest in understanding the role of oxidative stress in the ecology of wild animals, particularly during challenging periods such as reproduction. Here, we used a long-term study population of a fish with sole-male parental care, the smallmouth bass, Micropterus dolomieu, to examine the associations among oxidative stress indicators and life history variables in nest-guarding males. In addition, we investigated the potential role of oxidative stress as a physiological mediator of the life history trade-off decision of paternal smallmouth bass to stay with or abandon their brood. We found that oxidative stress was significantly related to the life history of paternal smallmouth bass, such that older, larger fish with greater reproductive experience and larger broods nesting in cooler water temperatures had lower levels of oxidative stress. However, we found no significant correlation between oxidative stress and nesting success, suggesting that oxidative stress may not be involved in the decision of male smallmouth bass to abandon their brood. Wild fish have been relatively understudied in the emerging field of oxidative ecology, and the study presented here makes noteworthy contributions by revealing interesting connections between the life histories of paternal smallmouth bass and their oxidative status.

Oxidative costs of reproduction in mouse strains selected for different levels of food intake and which differ in reproductive performance

Oxidative damage caused by reactive oxygen species has been hypothesised to underpin the trade-off between reproduction and somatic maintenance, i.e., the life-history-oxidative stress theory. Previous tests of this hypothesis have proved equivocal, and it has been suggested that the variation in responses may be related to the tissues measured. Here, we measured oxidative damage (protein carbonyls, 8-OHdG) and antioxidant protection (enzymatic antioxidant activity and serum antioxidant capacity) in multiple tissues of reproductive (R) and non-reproductive (N) mice from two mouse strains selectively bred for high (H) or low (L) food intake, which differ in their reproductive performance, i.e., H mice have increased milk energy output (MEO) and wean larger pups. Levels of oxidative damage were unchanged (liver) or reduced (brain and serum) in R versus N mice, and no differences in multiple measures of oxidative protection were found between H and L mice in liver (except for Glutathione Peroxidase), brain or mammary glands. Also, there were no associations between an individual’s energetic investment (e.g., MEO) and most of the oxidative stress measures detected in various tissues. These data are inconsistent with the oxidative stress theory, but were more supportive of, but not completely consistent, with the ‘oxidative shielding’ hypothesis.

Plasmodium infection and oxidative status in breeding great tits, Parus major

Background

Plasmodium parasites may affect the oxidative status of their hosts, defined as the balance of pro-oxidant compounds and antioxidant defences in an organism. An increased energy requirement, the activation of immune functions or the parasite itself may lead to a higher production of pro-oxidants and/or an antioxidant depletion resulting in a higher oxidative stress and associated damage in infected individuals. Relatively little is known about the mechanisms underlying oxidative processes at play during host-Plasmodium interaction in the wild.

Methods

The effect of Plasmodium infection on host oxidative status was investigated in wild populations of breeding great tits, Parus major, naturally infected by Plasmodium spp. When chicks were 14 days old, the parents were blood-sampled to measure four complementary oxidative status markers: pro-oxidant production as mitochondrial superoxide production in red blood cells (RBC), antioxidant defences as plasma antioxidant capacity and oxidative damage as reactive oxygen metabolites in the plasma and RBC membrane resistance to oxidative attack.

Results

Plasmodium-infected individuals produced more pro-oxidants compared to uninfected ones and pro-oxidant production positively correlated to infection intensity. There was also a conditional effect of reproductive effort on oxidative damage depending on Plasmodium infection status. There was no direct effect of infection on oxidative damage and no effect on antioxidant defences.

Conclusions

The results suggest that Plasmodium parasites may impose a cost in terms of increased oxidative stress possibly mediated via a higher energy requirement in infected hosts. This further suggests that Plasmodium parasites may modify host life history traits via an induction of oxidative stress. This study highlights that measuring several complementary oxidative status markers may enable to capture oxidative processes at play during host-Plasmodium interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1579-9) contains supplementary material, which is available to authorized users.

Specific carotenoid pigments in the diet and a bit of oxidative stress in the recipe for producing red carotenoid-based signals

Colorful ornaments have been the focus of sexual selection studies since the work of Darwin. Yellow to red coloration is often produced by carotenoid pigments. Different hypotheses have been formulated to explain the evolution of these traits as signals of individual quality. Many of these hypotheses involve the existence of a signal production cost. The carotenoids necessary for signaling can only be obtained from food. In this line, carotenoid-based signals could reveal an individual’s capacity to find sufficient dietary pigments. However, the ingested carotenoids are often yellow and became transformed by the organism to produce pigments of more intense color (red ketocarotenoids). Biotransformation should involve oxidation reactions, although the exact mechanism is poorly known. We tested the hypothesis that carotenoid biotransformation could be costly because a certain level of oxidative stress is required to correctly perform the conversion. The carotenoid-based signals could thus reveal the efficiency of the owner in successfully managing this challenge. In a bird with ketocarotenoid-based ornaments (the red-legged partridge; Alectoris rufa), the availability of different carotenoids in the diet (i.e. astaxanthin, zeaxanthin and lutein) and oxidative stress were manipulated. The carotenoid composition was analyzed and quantified in the ornaments, blood, liver and fat. A number of oxidative stress biomarkers were also measured in the same tissues. First, we found that color and pigment levels in the ornaments depended on food levels of those carotenoids used as substrates in biotransformation. Second, we found that birds exposed to mild levels of a free radical generator (diquat) developed redder bills and deposited higher amounts of ketocarotenoids (astaxanthin) in ornaments. Moreover, the same diquat-exposed birds also showed a weaker resistance to hemolysis when their erythrocytes were exposed to free radicals, with females also enduring higher oxidative damage in plasma lipids. Thus, higher color production would be linked to higher oxidative stress, supporting the biotransformation hypothesis. The recent discovery of an avian oxygenase enzyme involved in converting yellow to red carotenoids may support our results. Nonetheless, the effect could also depend on the abundance of specific substrate carotenoids in the diet. Birds fed with proportionally higher levels of zeaxanthin showed the reddest ornaments with the highest astaxanthin concentrations. Moreover, these birds tended to show the strongest diquat-mediated effect. Therefore, in the evolution of carotenoid-based sexual signals, a biotransformation cost derived from maintaining a well-adjusted redox machinery could coexist with a cost linked to carotenoid acquisition and allocation (i.e. a resource allocation trade-off).

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 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.

Low-quality birds do not display high-quality signals: The cysteine-pheomelanin mechanism of honesty

The mechanisms that make that the costs of producing high-quality signals are unaffordable to low-quality signalers are a current issue in animal communication. The size of the melanin-based bib of male house sparrows Passer domesticus honestly signals quality. We induced the development of new bibs while treating males with buthionine-sulfoximine (BSO), a substance that depletes the levels of the antioxidant glutathione (GSH) and the amino acid cysteine, two elements that switch melanogenesis from eumelanin to pheomelanin. Final bib size is negatively related to pheomelanin levels in the bib feathers. BSO reduced cysteine and GSH levels in all birds, but improved phenotypes (bibs larger than controls) were only expressed by high-quality birds (BSO birds with largest bibs initially). Negative associations between final bib size and cysteine levels in erythrocytes, and between pheomelanin and cysteine levels, were observed in high-quality birds only. These findings suggest that a mechanism uncoupling pheomelanin and cysteine levels may have evolved in low-quality birds to avoid producing bibs of size not corresponding to their quality and greater relative costs. Indeed, greater oxidative stress in cells was not observed in low-quality birds. This may represent the first mechanism maintaining signal honesty without producing greater relative costs on low-quality signalers.

A positive correlation between mercury and oxidative stress-related gene expression (GPX3 and GSTM3) is measured in female Double-crested Cormorant blood

Mercury (Hg) is a widespread contaminant that has been shown to induce a wide range of adverse health effects in birds including reproductive, physiological and neurological impairments. Here we explored the relationship between blood total Hg concentrations ([THg]) and oxidative stress gene induction in the aquatic piscivorous Double-crested Cormorants (Phalacrocorax auritus) using a non-lethal technique, i.e., blood gene expression analysis. P. auritus blood was sampled at five sites across the Great Lakes basin, Ontario, Canada and was analyzed for [THg]. To assess cellular stress, the expression of glutathione peroxidases 1 and 3 (GPX1, GPX3), superoxide dismutase 1 (SOD1), heat-shock protein 70 kd-8 (HSP70-8) and glutathione S-transferase µ3 (GSTM3) were measured in whole blood samples using real-time RT-PCR. Results showed a significantly positive correlation between female blood [THg] and both GPX3 and GSTM3 expression. Different levels of oxidative stress experienced by males and females during the breeding season may be influencing the differential oxidative stress responses to blood [THg] observed in this study. Overall, these results suggest that Hg may lead to oxidative stress as some of the cellular stress-related genes were altered in the blood of female P. auritus and that blood gene expression analysis is a successful approach to assess bird health condition.

Superoxide dismutase deficiency impairs olfactory sexual signaling and alters bioenergetic function in mice

Oxidative stress (an overproduction of reactive oxygen species in relation to defense mechanisms) may restrict investment in life history traits, such as growth, reproduction, lifespan, and the production of sexual signals to attract mates. The constraint on sexual signaling by oxidative stress is of particular interest because it has been proposed as a mechanism ensuring that only good-quality males produce the most attractive sexual signals. Despite these predictions, evidence supporting this theory is, at best, equivocal. We used a superoxide dismutase knockout mouse to demonstrate that oxidative stress directly impairs investment in morphological (preputial glands) and molecular (major urinary proteins) components of olfactory signaling essential for mate attraction. By maintaining males in a much more competitive environment than usual for mouse laboratory experiments, we also revealed a range of phenotypes of superoxide dismutase deficiency not observed in previous studies of this mouse model. This range included impaired bioenergetic function, which was undetectable in the control environment of this study. We urge further examination of model organisms in seminatural conditions and more competitive laboratory environments, as important phenotypes can be exposed under these more demanding conditions.

Amelioration of reproduction-associated oxidative stress in a viviparous insect is critical to prevent reproductive senescence

Impact of reproductive processes upon female health has yielded conflicting results; particularly in relation to the role of reproduction-associated stress. We used the viviparous tsetse fly to determine if lactation, birth and involution lead to damage from oxidative stress (OS) that impairs subsequent reproductive cycles. Tsetse females carry an intrauterine larva to full term at each pregnancy cycle, and lactate to nourish them with milk secretions produced by the accessory gland ( = milk gland) organ. Unlike most K-strategists, tsetse females lack an apparent period of reproductive senescence allowing the production of 8-10 progeny over their entire life span. In a lactating female, over 47% of the maternal transcriptome is associated with the generation of milk proteins. The resulting single larval offspring weighs as much as the mother at birth. In studying this process we noted an increase in specific antioxidant enzyme (AOE) transcripts and enzymatic activity at critical times during lactation, birth and involution in the milk gland/fat body organ and the uterus. Suppression of superoxide dismutase (sod) decreased fecundity in subsequent reproductive cycles in young mothers and nearly abolished fecundity in geriatric females. Loss of fecundity was in part due to the inability of the mother to produce adequate milk to support larval growth. Longevity was also impaired after sod knockdown. Generation of OS in virgin females through exogenous treatment with hydrogen peroxide at times corresponding to pregnancy intervals reduced survival, which was exacerbated by sod knockdown. AOE expression may prevent oxidative damage associated with the generation of nutrients by the milk gland, parturition and milk gland breakdown. Our results indicate that prevention of OS is essential for females to meet the growing nutritional demands of juveniles during pregnancy and to repair the damage that occurs at birth. This process is particularly important for females to remain fecund during the latter portion of their lifetime.

Senescence in cell oxidative status in two bird species with contrasting life expectancy

Oxidative stress occurs when the production of reactive oxygen species (ROS) by an organism exceeds its capacity to mitigate the damaging effects of the ROS. Consequently, oxidative stress hypotheses of ageing argue that a decline in fecundity and an increase in the likelihood of death with advancing age reported at the organism level are driven by gradual disruption of the oxidative balance at the cellular level. Here, we measured erythrocyte resistance to oxidative stress in the same individuals over several years in two free-living bird species with contrasting life expectancy, the great tit (known maximum life expectancy is 15.4 years) and the Alpine swift (26 years). In both species, we found evidence for senescence in cell resistance to oxidative stress, with patterns of senescence becoming apparent as subjects get older. In the Alpine swift, there was also evidence for positive selection on cell resistance to oxidative stress, the more resistant subjects being longer lived. The present findings of inter-individual selection and intra-individual deterioration in cell oxidative status at old age in free-living animals support a role for oxidative stress in the ageing of wild animals.

Opposing effects on glutathione and reactive oxygen metabolites of sex, habitat, and spring date, but no effect of increased breeding density in great tits (Parus major)

Oxidative stress (i.e., more oxidants than antioxidants) has been proposed as a proximate currency in life-history trade-offs, which if studied in an ecological setting allow a more realistic perspective on the origin and evolution of trade-offs. Therefore, the aim here was to investigate the impact of ecological and individual factors for variation in markers of oxidative stress using both experimental and correlational data. Total glutathione (tGSH), oxidized glutathione (GSSG), plasma antioxidant capacity (OXY), and plasma-reactive oxygen metabolites (ROM) were measured in more than 700 breeding great tits (Parus major). The main results revealed a pronounced sex difference, with females having lower ROM and OXY, but higher tGSH compared with males. In addition, birds breeding in the evergreen areas had higher tGSH compared with those in the deciduous habitat, but the experimentally manipulated breeding density had no significant effect on any of the redox markers. Independent of the sex differences, the larger the reproductive investment the lower the ROM of both males and females. Taken together, the extracellular markers – ROM and OXY – revealed similar results and were highly correlated. Interestingly, the direction of their effects was in the opposite direction to the endogenously synthesized tGSH and GSSG. This highlights the need to combine extracellular markers with endogenously synthesized antioxidants to understand its implications for the origin and evolution of trade-offs in an ecological setting.

Oxidative stress has been proposed as a proximate currency in life-history trade-offs, which if studied in an ecological setting allow a more realistic perspective on the origin and evolution of trade-offs. Here multiple markers of oxidative stress were analysed in wild great tits. The results reveal that the endogenously synthesized antioxidant glutathione and markers of plasma oxidative stress are affected in opposing directions with regard to sex, habitat type, and spring date. Clutch size was negatively associated with oxidative damage, which suggests that those with high reproductive investment can combat physiological costs linked to oxidative stress. The experimentally manipulated breeding density did not influence oxidative stress physiology. The study highlights the need to measure multiple markers to understand the role of oxidative stress in limiting the expression of life-history traits and trajectories in different ecological contexts.

Explaining variance of avian malaria infection in the wild: the importance of host density, habitat, individual life-history and oxidative stress

Background

Avian malaria (Plasmodium sp.) is globally widespread, but considerable variation exists in infection (presence/absence) patterns at small spatial scales. This variation can be driven by variation in ecology, demography, and phenotypic characters, in particular those that influence the host’s resistance. Generation of reactive oxygen species (ROS) is one of the host’s initial immune responses to combat parasitic invasion. However, long-term ROS exposure can harm the host and the redox response therefore needs to be adjusted according to infection stage and host phenotype. Here we use experimental and correlational approaches to assess the relative importance of host density, habitat composition, individual level variation and redox physiology for Plasmodium infection in a wild population of great tits, Parus major.

Results

We found that 36% of the great tit population was infected with Plasmodium (22% P. relictum and 15% P. circumflexum prevalence) and that patterns of infection were Plasmodium species-specific. First, the infection of P. circumflexum was significantly higher in areas with experimental increased host density, whereas variation in P. relictum infection was mainly attributed to age, sex and reproduction. Second, great tit antioxidant responses – total and oxidizied glutathione - showed age- , sex- and Plasmodium species-specific patterns between infected and uninfected individuals, but reactive oxygen metabolites (ROM) showed only a weak explanatory power for patterns of P. relictum infection. Instead ROM significantly increased with Plasmodium parasitaemia.

Conclusions

These results identify some key factors that influence Plasmodium infection in wild birds, and provide a potential explanation for the underlying physiological basis of recently documented negative effects of chronic avian malaria on survival and reproductive success.