Variation of a carotenoid-based trait in relation to oxidative stress and endocrine status during the breeding season in the Eurasian kestrel: a multi-factorial study

Oestradiol reduces female bill colour in a mutually ornamented bird

Carotenoid-based colour signals can be costly to produce and maintain, and trade-offs between signalling and other fitness traits are expected. In mutually ornamented species, trade-offs with reproduction may be stronger for females than males, because females often dedicate more resources to offspring production, which may lead to plastic investment in colour signals and plastic sexual dichromatism. Oestradiol is a candidate mediator of this trade-off because it regulates reproductive physiology and may also influence the expression of coloration. We tested this hypothesis by giving female common waxbills (Estrilda astrild) either oestradiol (17β-oestradiol) or empty implants during the early breeding season and measured spectral reflectance of carotenoid-based bill coloration weekly for two months. Using a model of avian vision, we found that bill colour in oestradiol-implanted females became less saturated, less red in hue and brighter, compared with control females and with unimplanted males. This resulted in a change in bill sexual dichromatism from imperceptible to perceptible. Results support the hypothesis that female reproductive physiology influences investment in coloration through changes in oestradiol and show a form of female-driven plastic sexual dichromatism. Greater sensitivity of female colour to physiological and/or environmental conditions helps explain why differences in sexual dichromatism among species differing in ecology often evolve owing to changes in female rather than male phenotype.

Oxidative stress in birds along a NOx and urbanisation gradient: An interspecific approach

Urbanisation is regarded as one of the most threatening global issues for wildlife, however, measuring its impact is not always straight forward. Oxidative stress physiology has been suggested to be a useful biomarker of health and therefore, a potentially important indicator of the impact that urban environmental stressors, especially air pollution, can have on wildlife. For example, nitrogen oxides (NO_x), released during incomplete combustion of fossil fuels, are highly potent pro-oxidants, thus predicted to affect either the protective antioxidants and/or cause oxidative damage to bio-molecules. To date, epidemiological modelling of the predicted association between oxidative stress and NO_x exposure has not been performed in wild animals. Here, we address this short-coming, by investigating multiple oxidative stress markers in four common passerine bird species, the blue tit (Cyanistes caeruleus), great tit (Parus major), house sparrow (Passer domesticus) and tree sparrow (Passer montanus), living along a gradient of NO_x and urbanisation levels in southern Sweden. First of all, the results revealed that long- and medium-term (one month and one week, respectively) NO_x levels were highly correlated with the level of urbanisation. This confirms that the commonly used urbanisation index is a reliable proxy for urban air pollution. Furthermore, in accordance to our prediction, individuals exposed to higher long- and medium-term NO_x levels/urbanisation had higher plasma antioxidant capacity. However, only tree sparrows showed higher oxidative damage (protein carbonyls) in relation to NO_x levels and this association was absent with urbanisation. Lipid peroxidation, glutathione and superoxide dismutase levels did not co-vary with NO_x/urbanisation. Given that most oxidative stress biomarkers showed strong species-specificity, independent of variation in NO_x/urbanisation, the present study highlights the need to study variation in oxidative stress across contexts, seasons and life-stages in order to understand how the ecology and phylogeny of species interact to affect species resilience to urban environmental stress.

Meta-analysis reveals that reproductive strategies are associated with sexual differences in oxidative balance across vertebrates

Oxidative stress is a key physiological mechanism underlying life-history tradeoffs. Here, I use meta-analytic techniques to test whether sexual differences in oxidative balance are common in vertebrates and to identify which factors are associated with such differences. The dataset included 732 effect size estimates from 100 articles (82 species). Larger unsigned effect size (meaning larger sexual differences in a given marker) occurred in: reptiles and fish; those species that do not provide parental care; and oviparous species. Estimates of signed effect size (positive values meaning higher oxidative stress in males) indicated that females were less resistant to oxidative stress than males in: reptiles while males and females were similar in fish, birds, and mammals; those species that do not provide parental care; and oviparous species. There was no evidence for a significant sexual differentiation in oxidative balance in fish, birds, and mammals. Effect size was not associated with: the number of offspring; whether the experimental animals were reproducing or not; biomarker (oxidative damage, non-enzymatic, or enzymatic antioxidant), the species body mass; the strain (wild vs. domestic); or the study environment (wild vs. captivity). Oxidative stress tended to be higher in females than males across most of the tissues analyzed. Levels of residual heterogeneity were high in all models tested. The findings of this meta-analysis indicate that diversification of reproductive strategies might be associated with sexual differences in oxidative balance. This explorative meta-analysis offers a starting platform for future research to investigate the relationship between sex and oxidative balance further.

Redox changes in the brains of reproductive female rats during aging

Reproduction is a critical and demanding phase of an animal's life. In mammals, females usually invest much more in parental care than males, and lactation is the most energetically demanding period of a female's life. Here, we tested whether oxidative stress is a consequence of reproduction in the brains of female Wistar rats. We evaluated the activities of glutathione peroxidase, glutathione S-transferase, and superoxide dismutase; H₂O₂ consumption; protein carbonylation; NO₂ & NO₃ levels; and total glutathione, as well as sex hormone levels in brain tissue of animals at 3, 6, 12, and 24months of age. Animals were grouped according to reproductive experience: breeders or non-breeders. Most of the studied parameters showed a difference between non-breeders and breeders at 12 and 24months. At 24months of age, breeders showed higher superoxide dismutase activity, H₂O₂ consumption, glutathione peroxidase activity, and carbonyl levels than non-breeders. In 12-month-old non-breeders, we observed a higher level of H₂O₂ consumption and higher superoxide dismutase and glutathione peroxidase activities than breeders. By evaluating the correlation network, we found that there were a larger number of influential nodes and positive links in breeder animals than in non-breeders, indicating a greater number of redox changes in breeder animals. Here, we also demonstrated that the aging process caused higher oxidative damage and higher antioxidant defenses in the brains of breeder female rats at 24months, suggesting that the reproduction process is costly, at least for the female brain. This study shows that there is a strong potential for a link between the cost of reproduction and oxidative stress.

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.

Ecological Immunology through the Lens of Exercise Immunology: New Perspective on the Links between Physical Activity and Immune Function and Disease Susceptibility in Wild Animals

Locomotion and other physical activities by free-living animals may influence immune function and disease susceptibility. This influence may be a consequence of energetic trade-offs or other mechanisms that are often, but not always, inseparably linked to an animal's life history (e.g., flight and migration). Ecological immunology has mainly focused on these life-history trade-offs, overlooking the possible effects of physical activity per se on immune function and disease susceptibility. In this review, we explore the field of exercise immunology, which examines the impact of exercise on immune function and disease susceptibility in humans, with the aim of presenting new perspectives that might be transferable to ecological immunology. First, we explore key concepts in exercise immunology that could be extended to animals. Next, we investigate the concept "exercise" in animals, and propose the use of "physical activity" instead. We briefly discuss methods used in animals to quantify physical activity in terms of energy expenditure and summarize several examples of animals engaging in physical activity. Then, we highlight potential consequences of physical activity on immune function and disease susceptibility in animals, together with an overview of animal studies that examine these links. Finally, we explore and discuss the potential for incorporating perspectives from exercise immunology into ecological immunology. Such integration could help advance our understanding of human and animal health and contribute new ideas to budding "One Health" initiatives.

The influence of diet on nestling body condition of an apex predator: a multi-biomarker approach

Animal body condition refers to the health and physiological state of individuals, and multiple parameters have been proposed to quantify this key concept. Food intake is one of the main determinants of individual body condition and much debate has been generated on how diet relates to body condition. We investigated this relationship in free-living Bonelli's eagle (Aquila fasciata) nestlings sampled at two geographically distant populations in Spain. Nestlings' main prey consumption was estimated by isotopic analyses. A multi-biomarker approach, including morphometric and blood biochemical measures (i.e. hematocrit, plasma biochemistry and oxidative stress biomarkers), enabled us to integrate all the body condition measures taken. A greater consumption of a preferred prey [i.e. the European rabbit (Oryctolagus cuniculus)] improved nestling body condition, as indicated by lower levels of cholesterol in plasma, greater activity of enzymes mediating in protein catabolism, higher levels of tocopherol and glutathione, and less glutathione peroxidase activity, which also suggested lower degree of oxidative stress. On the other hand, increased diet diversity was positively correlated with higher levels of oxidized glutathione, which suggests that these nestlings had poorer body condition than those with a higher frequency of preferred prey consumption. Several factors other than diet [i.e. altitude of nesting areas, nestling sex and age, sampling time (before or after midday) and recent food ingestion] had an effect on certain body condition measures. Our study reveals a measurable effect of diet on a predator's body condition and demonstrates the importance of considering the potential influence of multiple intrinsic and extrinsic factors when assessing animal body condition.

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.

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.

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.

Does reproduction protect against oxidative stress?

A central principle of life-history theory is that parents trade investment into reproduction against that in body maintenance. One physiological cost thought to be important as a modulator of such trade-off is oxidative stress. Experimental support for this hypothesis has, however, proved to be contradictory. In this study, we manipulated the nestling rearing effort of captive canaries (Serinus canaria) soon after the hatching of their nestlings using a brood-size manipulation to test whether an increase in nestling rearing effort translates into an increase in oxidative damage, an increase in ceruloplasmin (which is upregulated in response to oxidative damage) and a decrease in thiol antioxidants. We also compared the blood oxidative stress level of reproducing birds to that of non-reproducing birds, a crucial aspect that most studies have invariably failed to include in tests of the oxidative cost of reproduction. As compared to non-breeding canaries and pre-manipulation values, plasma oxidative damage (reactive oxygen metabolites and protein carbonyls) decreased in breeding canaries irrespective of sex and brood size. In contrast, oxidative damage did not change in non-breeding birds over the experiment. Ceruloplasmin activity in plasma and both non-protein and protein thiols in red blood cells did not change throughout the experiment in both treatment groups. Our results suggest that reproduction may result in decreased rather than increased blood oxidative stress. Our results may explain some of the inconsistencies that have been so far reported in experimental tests of the oxidative cost of reproduction hypothesis.

Constraint and cost of oxidative stress on reproduction: correlative evidence in laboratory mice and review of the literature

Background

One central concept in evolutionary ecology is that current and residual reproductive values are negatively linked by the so-called cost of reproduction. Previous studies examining the nature of this cost suggested a possible involvement of oxidative stress resulting from the imbalance between pro- and anti-oxidant processes. Still, data remain conflictory probably because, although oxidative damage increases during reproduction, high systemic levels of oxidative stress might also constrain parental investment in reproduction. Here, we investigated variation in oxidative balance (i.e. oxidative damage and antioxidant defences) over the course of reproduction by comparing female laboratory mice rearing or not pups.

Results

A significant increase in oxidative damage over time was only observed in females caring for offspring, whereas antioxidant defences increased over time regardless of reproductive status. Interestingly, oxidative damage measured prior to reproduction was negatively associated with litter size at birth (constraint), whereas damage measured after reproduction was positively related to litter size at weaning (cost).

Conclusions

Globally, our correlative results and the review of literature describing the links between reproduction and oxidative stress underline the importance of timing/dynamics when studying and interpreting oxidative balance in relation to reproduction. Our study highlights the duality (constraint and cost) of oxidative stress in life-history trade-offs, thus supporting the theory that oxidative stress plays a key role in life-history evolution.

Sex-related effects of reproduction on biomarkers of oxidative damage in free-living barn swallows (Hirundo rustica)

According to life-history theory, the allocation of limiting resources to one trait has negative consequences for other traits requiring the same resource, resulting in trade-offs among life-history traits, such as reproduction and survival. In vertebrates, oxidative stress is increasingly being considered among the physiological mechanisms forming the currency of life-history trade-offs. In this study of the barn swallow (Hirundo rustica), we focus on the oxidative costs of reproduction, especially egg laying, by investigating the effects of breeding stage (pre- vs. post-laying) and progression of the season on three biomarkers of oxidative damage (OD) to plasma proteins, namely the concentration of malondialdehyde (MDA)-protein adducts and of protein thiol groups (PSH), and the protein carbonyl (PCO) content. Moreover, we investigated whether males and females differed in plasma OD levels, because the inherent sex differences in reproductive roles and physiology may originate sex-specific patterns of OD during breeding. We found that MDA-protein adduct levels were higher in the pre-laying than in the post-laying phase, that males had lower levels of MDA-modified proteins than females, and that the decline of MDA-protein adduct concentration between the pre- and the post-laying phase was more marked for females than males. In addition, MDA-protein adduct levels declined with sampling date, but only during the pre-laying phase. On the other hand, plasma PCO levels increased from the pre- to the post-laying phase in both sexes, and females had higher levels of PCO than males. PSH concentration was unaffected by breeding stage, sex or sampling date. On the whole, our findings indicate that biomarkers of protein oxidation closely track the short-term variation in breeding stage of both male and female barn swallows. Moreover, the higher protein OD levels observed among females compared to males suggest that egg laying entails oxidative costs, which might negatively affect female residual reproductive value.

What does carotenoid-dependent coloration tell? Plasma carotenoid level signals immunocompetence and oxidative stress state in birds-A meta-analysis

Mechanisms maintaining honesty of sexual signals are far from resolved, limiting our understanding of sexual selection and potential important parts of physiology. Carotenoid pigmented visual signals are among the most extensively studied sexual displays, but evidence regarding hypotheses on how carotenoids ensure signal honesty is mixed. Using a phylogenetically controlled meta-analysis of 357 effect sizes across 88 different species of birds, we tested two prominent hypotheses in the field: that carotenoid-dependent coloration signals i) immunocompetence and/or ii) oxidative stress state. Separate meta-analyses were performed for the relationships of trait coloration and circulating carotenoid level with different measures of immunocompetence and oxidative stress state. For immunocompetence we find that carotenoid levels (r = 0.20) and trait color intensity (r = 0.17) are significantly positively related to PHA response. Additionally we find that carotenoids are significantly positively related to antioxidant capacity (r = 0.10), but not significantly related to oxidative damage (r = -0.02). Thus our analyses provide support for both hypotheses, in that at least for some aspects of immunity and oxidative stress state the predicted correlations were found. Furthermore, we tested for differences in effect size between experimental and observational studies; a larger effect in observational studies would indicate that co-variation might not be causal. However, we detected no significant difference, suggesting that the relationships we found are causal. The overall effect sizes we report are modest and we discuss potential factors contributing to this, including differences between species. We suggest complementary mechanisms maintaining honesty rather than the involvement of carotenoids in immune function and oxidative stress and suggest experiments on how to test these.

Interaction between sexual steroids and immune response in affecting oxidative status of birds

One hypothesis explaining the honesty of secondary sexual traits regulated by testosterone (T) is that T can impair the balance between pro-oxidant compounds and antioxidant defences, favouring a status of oxidative stress that only good quality individuals can sustain (oxidative handicap hypothesis). In the present study, we evaluated for the first time the effects of sexual steroids, T and its metabolites 5-α-dihydrotestosterone (DHT) and estradiol (E2) on oxidative damage and plasma non-enzymatic antioxidant capacity, while birds are faced by an oxidative challenge induced by an immune stimulation with sheep red blood cells. We used male and female diamond doves Geopelia cuneata, a species that shows an orange-red periorbital ring, whose size and color are strongly affected by androgens, but not by estrogens. Immunization increased oxidative damage in all groups, regardless of hormone treatment. It also decreased anti-oxidant capacity in all groups, except for testosterone treated birds. The ratio of oxidative damage over anti-oxidant capacity (oxidative stress) was increased in both immunological challenged controls and E2 birds, while challenged birds treated with androgens did not differ from non-challenged birds. The response of males and females to our treatments never differed. Our results undermine the idea that T can induce honest signalling through a pro-oxidant activity.