Oxidative stress physiology in relation to life history traits of a free-living vertebrate: the spotted snow skink, Niveoscincus ocellatus

An evolutionary perspective on stress responses, damage and repair

Variation in stress responses has been investigated in relation to environmental factors, species ecology, life history and fitness. Moreover, mechanistic studies have unravelled molecular mechanisms of how acute and chronic stress responses cause physiological impacts ('damage'), and how this damage can be repaired. However, it is not yet understood how the fitness effects of damage and repair influence stress response evolution. Here we study the evolution of hormone levels as a function of stressor occurrence, damage and the efficiency of repair. We hypothesise that the evolution of stress responses depends on the fitness consequences of damage and the ability to repair that damage. To obtain some general insights, we model a simplified scenario in which an organism repeatedly encounters a stressor with a certain frequency and predictability (temporal autocorrelation). The organism can defend itself by mounting a stress response (elevated hormone level), but this causes damage that takes time to repair. We identify optimal strategies in this scenario and then investigate how those strategies respond to acute and chronic exposures to the stressor. We find that for higher repair rates, baseline and peak hormone levels are higher. This typically means that the organism experiences higher levels of damage, which it can afford because that damage is repaired more quickly, but for very high repair rates the damage does not build up. With increasing predictability of the stressor, stress responses are sustained for longer, because the animal expects the stressor to persist, and thus damage builds up. This can result in very high (and potentially fatal) levels of damage when organisms are exposed to chronic stressors to which they are not evolutionarily adapted. Overall, our results highlight that at least three factors need to be considered jointly to advance our understanding of how stress physiology has evolved: (i) temporal dynamics of stressor occurrence; (ii) relative mortality risk imposed by the stressor itself versus damage caused by the stress response; and (iii) the efficiency of repair mechanisms.

Individual telomere dynamics and their links to life history in a viviparous lizard

Emerging patterns suggest telomere dynamics and life history are fundamentally linked in endotherms through life-history traits that mediate the processes underlying telomere attrition. Unlike endotherms, ectotherms maintain the ability to lengthen somatic telomeres throughout life and the link between life-history strategies and ectotherm telomere dynamics is unknown. In a well-characterized model system (Niveoscincus ocellatus), we used long-term longitudinal data to study telomere dynamics across climatically divergent populations. We found longer telomeres in individuals from the cool highlands than those from the warm lowlands at birth and as adults. The key determinant of adult telomere length across populations was telomere length at birth, with population-specific effects of age and growth on adult telomere length. The reproductive effort had no proximate effect on telomere length in either population. Maternal factors influenced telomere length at birth in the warm lowlands but not the cool highlands. Our results demonstrate that life-history traits can have pervasive and context-dependent effects on telomere dynamics in ectotherms both within and between populations. We argue that these telomere dynamics may reflect the populations' different life histories, with the slow-growing cool highland population investing more into telomere lengthening compared to the earlier-maturing warm lowland population.

Effects of a Bacterial Infection on Mitochondrial Function and Oxidative Stress in a Songbird

AbstractAs a major physiological mechanism involved in cellular renewal and repair, immune function is vital to the body's capacity to support tissue maintenance and organismal survival. Because immune defenses can be energetically expensive, the activities of metabolically active organs, such as the liver, are predicted to increase during infection by most pathogens. However, some pathogens are immunosuppressive, which might reduce the metabolic capacities of select organs to suppress immune response. Mycoplasma gallisepticum (MG) is a well-known immunosuppressive bacterium that infects domestic chickens and turkeys as well as songbirds. In the house finch (Haemorhous mexicanus), which is the primary host for MG among songbird species, MG infects both the respiratory system and the conjunctiva of the eye, causing conspicuous swelling. To study the effect of a systemic bacterial infection on cellular respiration and oxidative damage in the house finch, we measured mitochondrial respiration, mitochondrial membrane potential, reactive oxygen species production, and oxidative damage in the livers of house finches that were wild caught and either infected with MG, as indicated by genetic screening for the pathogen, or free of MG infection. We observed that MG-infected house finches showed significantly lower oxidative lipid and protein damage in liver tissue compared with their uninfected counterparts. Moreover, using complex II substrates, we documented a nonsignificant trend for lower state 3 respiration of liver mitochondria in MG-infected house finches compared with uninfected house finches (P=0.07). These results are consistent with the hypothesis that MG suppresses organ function in susceptible hosts.

Low-dose immune challenges result in detectable levels of oxidative damage

Infection can result in substantial costs to animals, so they frequently respond by removing infectious agents with an immune response. However, immune responses entail their own costs, including upregulation of processes that destroy pathogens (e.g. the production of reactive oxygen species) and processes that limit the extent of self-damage during the immune response (e.g. production of anti-inflammatory proteins such as haptoglobin). Here, we simulated bacterial infection across a 1000-fold range using lipopolysaccharide (LPS) administered to northern bobwhite quail (Colinus virginianus), and quantified metrics related to pro-inflammatory conditions [i.e. generation of oxidative damage (d-ROMs), depletion of antioxidant capacity], anti-inflammatory mechanisms (i.e. production of haptoglobin, expression of the enzyme heme oxygenase, production of the organic molecule biliverdin) and nutritional physiology (e.g. circulating triglyceride levels, maintenance of body mass). We detected increases in levels of haptoglobin and d-ROMs even at LPS doses that are 1/1000th the concentration of doses frequently used in ecoimmunological studies, while loss of body mass and decreases in circulating triglycerides manifested only in individuals receiving the highest dose of LPS (1 mg LPS kg-1 body mass), highlighting variation among dose-dependent responses. Additionally, individuals that lost body mass during the course of the experiment had lower levels of circulating triglycerides, and those with more oxidative damage had greater levels of heme oxygenase expression, which highlights the complex interplay between pro- and anti-inflammatory processes. Because low doses of LPS may simulate natural infection levels, variation in dose-dependent physiological responses may be particularly important in modeling how free-living animals navigate immune challenges.

The untapped potential of reptile biodiversity for understanding how and why animals age

1. The field of comparative aging biology has greatly expanded in the past 20 years. Longitudinal studies of populations of reptiles with a range of maximum lifespans have accumulated and been analyzed for evidence of mortality senescence and reproductive decline. While not as well represented in studies of amniote senescence, reptiles have been the subjects of many recent demographic and mechanistic studies of the biology of aging. 2. We review recent literature on reptile demographic senescence, mechanisms of senescence, and identify unanswered questions. Given the ecophysiological and demographic diversity of reptiles, what is the expected range of reptile senescence rates? Are known mechanisms of aging in reptiles consistent with canonical hallmarks of aging in model systems? What are the knowledge gaps in our understanding of reptile aging? 3. We find ample evidence of increasing mortality with advancing age in many reptiles. Testudines stand out as slower aging than other orders, but data on crocodilians and tuatara are sparse. Sex-specific analyses are generally not available. Studies of female reproduction suggest that reptiles are less likely to have reproductive decline with advancing age than mammals. 4. Reptiles share many physiological and molecular pathways of aging with mammals, birds, and laboratory model organisms. Adaptations related to stress physiology coupled with reptilian ectothermy suggest novel comparisons and contrasts that can be made with canonical aging phenotypes in mammals. These include stem cell and regeneration biology, homeostatic mechanisms, IIS/TOR signaling, and DNA repair. 5. To overcome challenges to the study of reptile aging, we recommend extending and expanding long-term monitoring of reptile populations, developing reptile cell lines to aid cellular biology, conducting more comparative studies of reptile morphology and physiology sampled along relevant life-history axes, and sequencing more reptile genomes for comparative genomics. Given the diversity of reptile life histories and adaptations, achieving these directives will likely greatly benefit all aging biology.

Immune response declines with age in a wild lizard

The immune system allows animals to survive the constant attacks of pathogens. Although it is essential for survival and fitness, the performance of the immune system can decline with age (a process known as immunosenescence), which may be responsible for the increased mortality of older individuals. Only a few studies have investigated the effect of age on the immune system in reptiles, and no evidence of immunosenescence has yet been reported in lizards. In a wild population of Dickerson’s collared lizards (Crotaphytus dickersonae), we evaluated the effect of age (estimated by skeletochronology) on the male’s immune response to a novel antigen (phytohaemagglutinin). We found a strong negative relationship between immune response and age, the first evidence of immunosenescence in a lizard. Our results show that immunosenescence can occur even in relatively short-lived reptiles and underline the importance of age on performance in fitness-related traits.

Temperature and telomeres: thermal treatment influences telomere dynamics through a complex interplay of cellular processes in a cold-climate skink

Telomere dynamics vary fundamentally between endothermic populations and species as a result of differences in life history, yet we know little about these patterns in ectotherms. In ectotherms, the relationships between climate, metabolism and life history suggest that telomere attrition should be higher at relatively high environmental temperatures compared to relatively low environmental temperatures, but these effects may vary between populations due to local adaptation. To address this hypothesis, we sampled reactive oxygen species (ROS) and telomere length of lizards from warm lowland and cool highland populations of a climatically widespread lizard species that we exposed to hot or cold basking treatments. The hot treatment increased relative telomere length compared to the cold treatment independent of climatic origin or ROS levels. Lizards from the cool highland region had lower ROS levels than those from the warm lowland region. Within the highland lizards, ROS increased more in the cold basking treatment than the hot basking treatment. These results are in the opposite direction to those predicted, suggesting that the relationships between temperature, metabolism, ROS and telomere dynamics are not straightforward. Future work incorporating detailed understanding of the thermal reaction norms of these and other linked traits is needed to fully understand these processes.

Reproduction and maternal care increase oxidative stress in a mouthbrooding cichlid fish

Investment in reproduction and postzygotic parental care is an energetically costly yet fundamental aspect of the life-history strategies in many species. Recently, oxidative stress has received attention as a potential mediator in the trade-off between reproduction, growth, and survival. During activities that increase metabolic activity, such as providing offspring care, an overproduction of reactive oxygen species can occur that cannot be counteracted by antioxidants, leading to oxidative stress and tissue damage. Here, we investigated the oxidative costs of reproduction and maternal care over the course of the reproductive cycle in a mouthbrooding cichlid fish within socially stable and unstable environments. We manipulated social stability by disrupting the habitat in socially unstable tanks. We expected to see an increase in the burden of maternal care within unstable environments due to increased male harassment of females as a byproduct of increased male–male aggression. We found that brooding females have higher levels of oxidative stress than nonbrooding females and oxidative stress fluctuates throughout the reproductive cycle. These fluctuations were driven by a spike in reactive oxygen metabolites at the beginning of brood care followed by an increase in antioxidant defense. Surprisingly, the link between reproduction and oxidative stress was not different between females from stable or unstable environments. Our study illustrates a more complete picture of the physiological costs of reproduction and parental care throughout different stages of care rather than a simplistic end-point observation of how reproduction and parental care affect an individual.

Understanding diversity in oxidative status and oxidative stress: the opportunities and challenges ahead

Oxidative stress may be of profound biological relevance. In this Commentary, I discuss some key issues faced by the emerging field of oxidative stress ecology, and seek to provide interpretations and solutions. First, I show that the way in which we define oxidative stress has far-reaching implications for the interpretation of results, and that we need to distinguish between (1) a biochemical definition in terms of the molecular outcomes of oxidative stress (e.g. generation of oxidative damage) and (2) a biological definition in terms of the fitness consequences for the organism (e.g. effects on fertility). Second, I discuss the dangers of comparing different tissues and markers. Third, I highlight the need to pay more attention to the cross-talk between oxidative stress and other important physiological costs and functions; this will allow us to better understand the mechanistic basis of fitness costs. Fourth, I propose the 'redox signalling hypothesis' of life history to complement the current 'oxidative stress hypothesis' of life history. The latter states that oxidative damage underlies trade-offs because it affects traits like growth, reproduction or cell senescence. By contrast, the redox signalling hypothesis states that a trade-off between signalling and biochemical oxidative stress underlies the regulation of reactive oxygen species production and their subsequent control. Finally, I critically appraise our current knowledge of oxidative stress ecology, highlighting key research themes and providing an optimistic overview of future opportunities for the discipline to yield considerable insight into the ecological and evolutionary meaning of oxidative stress.

Transplanting gravid lizards to high elevation alters maternal and embryonic oxygen physiology, but not reproductive success or hatchling phenotype

Increased global temperatures have opened previously inhospitable habitats, such as at higher elevations. However, the reduction of oxygen partial pressure with increase in elevation represents an important physiological constraint that may limit colonization of such habitats, even if the thermal niche is appropriate. To test the mechanisms underlying the response to ecologically-relevant levels of hypoxia, we performed a translocation experiment with the common wall lizard (Podarcis muralis), a widespread European lizard amenable to establishing populations outside its natural range. We investigated the impacts of hypoxia on the oxygen physiology and reproductive output of gravid common wall lizards and the subsequent development and morphology of their offspring. Lowland females transplanted to high elevations increased their haematocrit and haemoglobin concentration within days and maintained routine metabolism compared to lizards kept at native elevations. However, transplanted lizards suffered from increased reactive oxygen metabolite production near the oviposition date, suggesting a cost of reproduction at high elevation. Transplanted females and females native to different elevations did not differ in reproductive output (clutch size, egg mass, relative clutch mass, or embryonic stage at oviposition) or in post-oviposition body condition. Developing embryos reduced heart rates and prolonged incubation times at high elevations within the native range and at extreme high elevations beyond the current range, but this reduced oxygen availability did not affect metabolic rate, hatching success, or hatchling size. These results suggest that this opportunistic colonizer is capable of successfully responding to novel environmental constraints in these important life-history stages.

Cellular metabolic rates and oxidative stress profiles in primary fibroblast cells isolated from virgin females, reproductively experienced females, and male Sprague-Dawley rats

Life-history theory posits that differences in reproductive strategies may dictate lifespans of organisms. Animals that have higher investments in reproduction in terms of litter size and frequency of litters tend to have shorter lifespans. The accumulation of oxidative stress damage has been proposed to be a cost of reproduction and a mediator of life-histories among animals, however, the implications of reproduction on oxidative stress still remain unclear. We tested physiological consequences of reproduction on metabolism and oxidative stress of Sprague-Dawley Rats (Rattus norvegicus) with various reproductive experiences at the cell level. We grew primary dermal fibroblasts from Sprague-Dawley rats which have the potential of having large litters frequently. Cells were isolated from virgin females, primiparous females, multiparous females, and reproductively-experienced males. We measured basal oxygen consumption (OCR), proton leak, ATP production, spare respiratory capacity, coupling efficiency and glycolysis using a Seahorse XF96 oxygen flux analyzer. Additionally, we measured rates of RS (reactive species) production, reduced glutathione (GSH), mitochondrial content, and lipid peroxidation (LPO) damage to quantify oxidative stress. There were no significant differences in any OCR or glycolytic parameters across any of our groups. However, reproductively-experienced females had significantly lower rates of LPO damage as compared with virgin females and males, as well as nonsignificant decreases in GSH concentration. Decreases in LPO damage and GSH indicate that reproductively-experienced females potentially use their endogenous antioxidant system to combat delirious effects of increased metabolism during reproduction. Our results suggest that reproduction may, in fact, have a protective effect in females.

Oxidative damage and antioxidant defense are assay and tissue-dependent both in captive and wild-caught bank voles (Myodes glareolus) before and after reproduction

Reproduction is costly and life-history theory predicts that current parental investment will result in lower survival or decreased future reproduction. The physiological mechanisms mediating the link between reproduction and survival are still under debate and elevated oxidative damage during reproduction has been proposed as a plausible candidate. Previous studies of oxidative stress during reproduction in animals under natural conditions have been restricted to analyses of blood. Herein, we measured the level of oxidative damage to lipids (tiobarbituric-acid-reactive substances) and proteins (carbonyls) in the liver, kidneys, heart and skeletal muscles in free-living bank vole females from spring and autumn generations, before and after reproduction. Antioxidant defense in the liver and kidneys was also determined. We expected oxidative damage to tissues and hypothesized that the damage would be more uniform between tissues in wild animals compared to those breeding under laboratory conditions. Considering all combinations of markers/tissues/generations, oxidative damage in females did not differ before and after reproduction in 12 comparisons, was lower after reproduction in three comparisons, and was higher after breeding in one comparison. The total glutathione was significantly increased after reproduction only in the liver of the autumn generation and there was no change in catalase activity. Our results confirm-for the first time in the field-previous observations from laboratory studies that there is no simple link between oxidative stress and reproduction and that patterns depend on the tissue and marker being studied. Overall, however, our study does not support the hypothesis that the cost of reproduction in bank voles is mediated by oxidative stress in these tissues.

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.

All or nothing: Survival, reproduction and oxidative balance in Spotted Wing Drosophila (Drosophila suzukii) in response to cold

Winter severity and overwintering capacity are key ecological factors in successful invasions, especially in ectotherms. The integration of physiological approaches into the study of invasion processes is emerging and promising. Physiological information describes the mechanisms underlying observed survival and reproductive capacities, and it can be used to predict an organism's response to environmental perturbations such as cold temperatures. We investigated the effects of various cold treatments on life history and physiological traits of an invasive pest species, Drosophila suzukii, such as survival, fertility and oxidative balance. This species, a native of temperate Asian areas, is known to survive where cold temperatures are particularly harsh and has been recently introduced into Europe and North America. We found that cold treatments had a strong impact on adult survival but no effect on female's fertility. Although only minor changes were observed after cold treatment on studied physiological traits, a strong sex-based difference was observed in both survival and physiological markers (antioxidant defences and oxidative markers). Females exhibited higher survival, reduced oxidative defences, less damage to nucleic acids, and more damage to lipids. These results suggest that D. suzukii relies on a pathway other than oxidative balance to resist cold injury. Altogether, our results provide information concerning the mechanisms of successful invasion by D. suzukii. These findings may assist in the development of population models that predict the current and future geographic ranges of this species.

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.

Energy expenditure of the spotted snow skink, Niveoscincus ocellatus, at two climatic extremes of its distribution range

The study of energy expenditure between populations of a wide ranging ectothermic species may provide an insight into how organisms respond to variation in environmental conditions. In this study, the energy expenditure of male spotted snow skinks, Niveoscincus ocellatus, living at the two extremes of the species' distribution range (warm lowland versus cold alpine site) was measured using the doubly labelled water method. Males at the cold alpine site expended more energy per gram per hour compared to their counterparts living at the warm lowland site. Lizards living at high altitude were active at lower temperatures compared with those at the low altitude site, which resulted in a longer activity time for the highland population. However, the differences in energy expenditure cannot be explained only by these differences in activity time. We further suggest that at the cold alpine site, lizards compensated for the low temperatures by elevating their metabolism which subsequently increased their energy expenditure. An elevated metabolic rate combined with modified thermoregulatory behaviour is likely an important mechanism allowing N. ocellatus to cope with the cold environments at high altitude sites.

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.

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.

Relationships between hair melanization, glutathione levels, and senescence in wild boars

The synthesis of melanins, which are the most common animal pigments, is influenced by glutathione (GSH), a key intracellular antioxidant. At high GSH levels, pheomelanin (the lightest melanin form) is produced, whereas production of eumelanin (the darkest melanin form) does not require GSH. Oxidative damage typically increases with age, and age-related decreases in GSH have accordingly been found in diverse organisms. Therefore, there should be positive associations between the capacity to produce eumelanic traits, GSH levels, and senescence, whereas there should be negative associations between the capacity to produce pheomelanic traits, GSH levels, and senescence. We explored this hypothesis in a free-ranging population of wild boars Sus scrofa of different ages. As expected from the fact that pheomelanogenesis consumes GSH, levels of this antioxidant in muscle tended to be negatively related to pheomelanization and positively related to eumelanization in pelage, and the degree of pelage pheomelanization was positively related to oxidative damage as reflected by levels of thiobarbituric-acid-reactive substances (TBARS), which is consistent with the hypothesis that pheomelanin synthesis has physiological costs. In our cross-sectional sample, GSH levels did not show senescence effects, and we did not detect senescence effects in pelage melanization. Prime body condition and low TBARS levels were also associated with hair graying, which is attributable to a loss of melanin produced by oxidative stress, thus raising the possibility that hair graying constitutes a signal of resistance to oxidative stress in wild boars. Our results suggest that the degree of melanization is linked to GSH levels in wild boars and that their antioxidant damage shows senescence effects.