Nestling erythrocyte resistance to oxidative stress predicts fledging success but not local recruitment in a wild bird

Stressful conditions experienced by individuals during their early development have long-term consequences on various life-history traits such as survival until first reproduction. Oxidative stress has been shown to affect various fitness-related traits and to influence key evolutionary trade-offs but whether an individual''s ability to resist oxidative stress in early life affects its survival has rarely been tested. In the present study, we used four years of data obtained from a free-living great tit population (Parus major; n = 1658 offspring) to test whether pre-fledging resistance to oxidative stress, measured as erythrocyte resistance to oxidative stress and oxidative damage to lipids, predicted fledging success and local recruitment. Fledging success and local recruitment, both major correlates of survival, were primarily influenced by offspring body mass prior to fledging. We found that pre-fledging erythrocyte resistance to oxidative stress predicted fledging success, suggesting that individual resistance to oxidative stress is related to short-term survival. However, local recruitment was not influenced by pre-fledging erythrocyte resistance to oxidative stress or oxidative damage. Our results suggest that an individual ability to resist oxidative stress at the offspring stage predicts short-term survival but does not influence survival later in life.     

Yolk testosterone reduces oxidative damages during postnatal development

Conditions experienced during early life can influence the development of an organism and several physiological traits, even in adulthood. An important factor is the level of oxidative stress experienced during early life. In birds, extra-genomic egg substances, such as the testosterone hormone, may exert a widespread influence over the offspring phenotype. Interestingly, testosterone can also upregulate the bioavailability of certain antioxidants but simultaneously increases the susceptibility to oxidative stress in adulthood. However, little is known about the effects of maternally derived yolk testosterone on oxidative stress in developing birds. Here, we investigated the role of yolk testosterone on oxidative stress of yellow-legged gull chicks during their early development by experimentally increasing yolk testosterone levels. Levels of antioxidants, reactive oxygen species and lipid oxidative damage were determined in plasma during nestlings'' growth. Our results revealed that, contrary to control chicks, birds hatched from testosterone-treated eggs did not show an increase in the levels of oxidative damage during postnatal development. Moreover, the same birds showed a transient increase in plasma antioxidant levels. Our results suggest that yolk testosterone may shape the oxidative stress-resistance phenotype of the chicks during early development owing to an increase in antioxidant defences and repair processes.     

An experimental manipulation of life-history trajectories and resistance to oxidative stress

Optimal investment into life-history traits depends on the environmental conditions that organisms are likely to experience during their life. Evolutionary theory tells us that optimal investment in reproduction versus maintenance is likely to shape the pattern of age-associated decline in performance, also known as aging. The currency that is traded against different vital functions is, however, still debated. Here, we took advantage of a phenotypic manipulation of individual quality in early life to explore (1) long-term consequences on life-history trajectories, and (2) the possible physiological mechanism underlying the life-history adjustments. We manipulated phenotypic quality of a cohort of captive zebra finches (Taeniopygia guttata) by assigning breeding pairs to either an enlarged or a reduced brood. Nestlings raised in enlarged broods were in poorer condition than nestlings raised in reduced broods. Interestingly, the effect of environmental conditions experienced during early life extended to the age at first reproduction. Birds from enlarged broods delayed reproduction. Birds that delayed reproduction produced less offspring but lived longer, although neither fecundity nor longevity were directly affected by the experimental brood size. Using the framework of the life-table response experiment modeling, we also explored the effect of early environmental condition on population growth rate and aging. Birds raised in reduced broods tended to have a higher population growth rate, and a steeper decrease of reproductive value with age than birds reared in enlarged broods. Metabolic resources necessary to fight off the damaging effect of reactive oxygen species (ROS) could be the mechanism underlying the observed results, as (1) birds that engaged in a higher number of breeding events had a weaker red blood cell resistance to oxidative stress, (2) red blood cell resistance to oxidative stress predicted short-term mortality (but not longevity), and (3) was related with a parabolic function to age. Overall, these results highlight that early condition can have long-term effects on life-history trajectories by affecting key life-history traits such as age at first reproduction, and suggest that the trade-off between reproduction and self-maintenance might be mediated by the cumulative deleterious effect of ROS.     

Elevation impacts the balance between growth and oxidative stress in coal tits

The short favorable period of time available for the growth in seasonal environments could constrain the resources allocation between growth and other life-history traits, and the short-term fitness benefits of increased growth rate may prevail over other functions. Accelerated growth rates have been associated with long-term deleterious consequences (e.g., decreased lifespan), and recently oxidative stress (the imbalance between pro-oxidants generation and antioxidant defenses) has been suggested as a mediator of these effects. Here, we examined the impact of elevation on growth rate and self-maintenance parameters (resting metabolism, oxidative damage, and antioxidant defenses) of coal tit chicks (Periparus ater). We predicted that the shorter favorable season at the higher-elevation site could lead to a reallocation of resources towards growth at the expense of self-maintenance processes. We found that chicks at high elevation grew significantly faster in terms of body mass and body size. Chicks from the high-elevation site presented higher resting metabolism, higher oxidative damage level, but similar antioxidant defenses, compared to low-elevation chicks. Interestingly, the chicks exhibiting the better antioxidant defenses at 7 days were also those with the highest resting metabolic rate, and the chicks that grew at the faster rate within the high-elevation site were those with the highest levels of oxidative damage on DNA. Our study supports the idea that increasing elevation leads to a higher growth rate in coal tit chicks, possibly in response to a shorter favorable season. In accordance with life-history theory, a bigger investment in growth was done at the expense of body maintenance, at least in terms of oxidative stress.     

Early growth trajectories affect sexual responsiveness

The trajectory of an animal''s growth in early development has been shown to have long-term effects on a range of life-history traits. Although it is known that individual differences in behaviour may also be related to certain life-history traits, the linkage between early growth or development and individual variation in behaviour has received little attention. We used brief temperature manipulations, independent of food availability, to stimulate compensatory growth in juvenile three-spined sticklebacks Gasterosteus aculeatus. Here, we examine how these manipulated growth trajectories affected the sexual responsiveness of the male fish at the time of sexual maturation, explore associations between reproductive behaviour and investment and lifespan and test whether the perceived time stress (until the onset of the breeding season) influenced such trade-offs. We found a negative impact of growth rate on sexual responsiveness: fish induced (by temperature manipulation) to grow slowest prior to the breeding season were consistently quickest to respond to the presence of a gravid female. This speed of sexual responsiveness was also positively correlated with the rate of development of sexual ornaments and time taken to build a nest. However, after controlling for effects of growth rate, those males that had the greatest sexual responsiveness to females had the shortest lifespan. Moreover, the time available to compensate in size before the onset of the breeding season (time stress) affected the magnitude of these effects. Our results demonstrate that developmental perturbations in early life can influence mating behaviour, with long-term effects on longevity.     

Long-lasting fitness consequences of prenatal sex ratio in a viviparous lizard

Maternal effects and early environmental conditions are important in shaping offspring developmental trajectories. For example, in laboratory mammals, the sex ratio during gestation has been shown to influence fitness-related traits via hormonal interaction between fetuses. Such effects have the potential to shape, or constrain, many important aspects of the organism's life, but their generality and importance in natural populations remain unknown. Using long-term data in a viviparous lizard, Lacerta vivipara, we investigated the relationship between prenatal sex ratio and offspring growth, survival, and reproductive traits as adults. Our results show that females from male-biased clutches grow faster, mature earlier, but have lower fecundity than females from female-biased clutches. Furthermore, male reproduction was also affected by the sex ratio during embryonic development, with males from male-biased clutches being more likely to successfully reproduce at age one than males from female-biased clutches. Thus, the sex ratio experienced during gestation can have profound and long-lasting effects on fitness in natural populations of viviparous animals, with important implications for life-history evolution and sex allocation.     

Quantitative genetic evidence for trade-off between growth and resistance to oxidative stress in a wild bird

Why do animals not grow at their maximal rates? It has been recently proposed that fast growth leads to the accumulation of cellular damages due to oxidative stress, influencing subsequent performances and life span. Therefore, the trade-off between fast growth and oxidative stress may potentially function as an important constraint in the evolution of growth trajectories. We test this by examining a potential antagonistic pleiotropy between growth and blood resistance to controlled free radical attack in a wild bird using a cross-fostering design and robust quantitative genetic analyses. In the yellow-legged gull Larus michahellis, decreased resistance to oxidative stress at age 8 days was associated with faster growth in mass, across the first 8 days of life, suggesting a trade-off between mass growth and oxidative-stress-related somatic maintenance. We found a negative genetic correlation between chick growth and resistance to oxidative stress, supporting the presence of the genetic trade-off between the two traits. Therefore, investment of somatic resources in growth could be constrained by resistance to oxidative stress in phenotypic and genetic levels. Our results provide first evidence for a potential genetic trade-off between life-history and underlying physiological traits in a wild vertebrate. Future studies should explore genetic trade-offs between life-history traits and other oxidative-stress-related traits.     

Prey life stages modulate effects of predation stress on prey lifespan,development, and reproduction in mites

The non-consumptive effects of predator-induced stress can influence a variety of life-history traits. Many previous studies focused only on short-term effects such as development and reproductive rates. Recent studies have showed that long-term predation stress (given during the whole life of the prey) and short-term predation stress (provided during the immature stage of the prey) could generate completely opposite results: the former could decrease lifespan, whereas the later could increase lifespan. However, it is still unclear whether the advantage is because of the short duration of exposure or the early stage of life during which exposure was exerted. Thus, in this study, the prey (Tyrophagus putrescentiae) was exposed to predation stress from the predator (Neoseiulus cucumeris) during different life stages (larva, protonymph, tritonymph, first 5 d of oviposition, the full lifespan or none of the above). The results showed that the predation stress supplied during larval and protonymphal stage delayed development, reduced fecundity and prolonged lifespan of the prey, while the stress given during tritonymphal stage only reduced lifespan slightly and the stress given during the first 5 d of oviposition did not change lifespan but reduced fecundity. This study indicated that the effects of predation stress are dependent on prey life stage and the predation stress experienced in the early life stages is important to lifespan modulation.     

Maternal developmental stress reduces reproductive success of female offspring in zebra finches

Environmental factors play a key role in the expression of phenotypic traits and life-history decisions, specifically when they act during early development. In birds, brood size is a main environmental factor affecting development. Experimental manipulation of brood sizes can result in reduced offspring condition, indicating that developmental deficits in enlarged broods have consequences within the affected generation. Yet, it is unclear whether stress during early development can have fitness consequences projecting into the next generation. To study such trans-generational fitness effects, we bred female zebra finches, Taeniopygia guttata, whose mothers had been raised in different experimental brood sizes. We found that adult females were increasingly smaller with increasing experimental brood size in which their mother had been raised. Furthermore, reproductive success at hatching and fledging covaried negatively with the experimental brood size in which their mothers were raised. These results illustrate that early developmental stress can have long-lasting effects affecting reproductive success of future generations. Such trans-generational effects can be life-history responses adapted to environmental conditions experienced early in life.     

Does artificial selection on diapause incidence cause correlational changes in other life‐history traits? Case study in a spider mite population

Theoretical studies suggest that the timing of entering hibernation by arthropods has large effects on long-term fitness, incurring strong selection pressure on diapause attributes every year. On the other hand, diapause attributes are often genetically correlated with other important life-history traits such as fecundity or development time. To understand the evolutionary process of life cycle formation, there is a need to investigate not only diapause attributes themselves but also their genetic association with other life-history traits. The Kanzawa spider mite, Tetranychus kanzawai Kishida (Acari: Tetranychidae), is a small herbivore that lives on the undersurface of host plant leaves. This mite has been investigated for the mode of inheritance of diapause attributes, but scarcely for genetic correlations with other life-history traits. Here, I investigated whether diapause proneness, measured as the proportion of diapausing females under short-day conditions, is genetically correlated with fecundity or development time under long-day conditions using artificial selection experiments. Diapause incidence responded to the selection for both increasing and decreasing directions, suggesting that high genetic variance in diapause proneness is maintained in the study population. However, the change in proportion of diapausing females during the selection period was not associated with responses in fecundity or development time. These results suggest that diapause proneness and other life-history traits have different genetic backgrounds, and thus diapause proneness may freely evolve without being constrained by changes in other life-history traits.     

Development time mediates the effect of larval diet on ageing and mating success of male antler flies in the wild

High-quality developmental environments often improve individual performance into adulthood, but allocating toward early life traits, such as growth, development rate and reproduction, may lead to trade-offs with late-life performance. It is, therefore, uncertain how a rich developmental environment will affect the ageing process (senescence), particularly in wild insects. To investigate the effects of early life environmental quality on insect life-history traits, including senescence, we reared larval antler flies (Protopiophila litigata) on four diets of varying nutrient concentration, then recorded survival and mating success of adult males released in the wild. Declining diet quality was associated with slower development, but had no effect on other life-history traits once development time was accounted for. Fast-developing males were larger and lived longer, but experienced more rapid senescence in survival and lower average mating rate compared to slow developers. Ultimately, larval diet, development time and body size did not predict lifetime mating success. Thus, a rich environment led to a mixture of apparent benefits and costs, mediated by development time. Our results indicate that ‘silver spoon'' effects can be complex and that development time mediates the response of adult life-history traits to early life environmental quality.     

Context-dependent effects of ranaviral infection on northern leopard frog life history traits

Pathogens have important effects on host life-history traits, but the magnitude of these effects is often strongly context-dependent. The outcome of an interaction between a host and an infectious agent is often associated with the level of stress experienced by the host. Ranavirus causes disease and mortality in amphibian populations in various locations around the world, but most known cases of ranaviral infection have occurred in North America and the United Kingdom. While Ranavirus virulence has been investigated, the outcome of Ranavirus infection has seldom been related to the host environment. In a factorial experiment, we exposed Northern leopard frog (Lithobates pipiens, formerly Rana pipiens) tadpoles to different concentrations of Ranavirus and investigated the effect of host density on certain life-history traits, namely survival, growth rate, developmental stage and number of days from virus exposure to death. Our results suggest a prominent role of density in driving the direction of the interaction between L. pipiens tadpoles and Ranavirus. We showed that increasing animal holding density is detrimental for host fitness as mortality rate is higher, day of death earlier, development longer and growth rate significantly lower in high-density tanks. We observed a linear increase of detrimental effects when Ranavirus doses increased in low-density conditions, with control tadpoles having a significantly higher overall relative fitness. However, this pattern was no longer observed in high-density conditions, where the effects of increasing Ranavirus dose were limited. Infected and control animals fitness were consequently similar. We speculate that the host may eventually diverts the energy required for a metabolic/immune response triggered by the infection (i.e., direct costs of the infection) to better cope with the increase in environmental "stress" associated with high density (i.e., indirect benefits of the infection). Our results illustrate how the net fitness of organisms may be shaped by ecological context and emphasize the necessity of examining the direct/indirect costs and benefits balance to fully understand host-pathogen interactions.     

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.

     

Diverse population trajectories among coexisting species of subarctic forest moths

Records of 232 moth species spanning 26 years (total catch of ca. 230,000 specimens), obtained by continuous light-trapping in Kevo, northernmost subarctic Finland, were used to examine the hypothesis that life-history traits and taxonomic position contribute to both relative abundance and temporal variability of Lepidoptera. Species with detritophagous or moss-feeding larvae, species hibernating in the larval stage, and species pupating during the first half of the growing season were over-represented among 42 species classified as abundant during the entire sampling period. The coefficients of variation in annual catches of species hibernating as eggs averaged 1.7 times higher than those of species hibernating as larvae or pupae. Time-series analysis demonstrated that periodicity in fluctuations of annual catches is generally independent of life-history traits and taxonomic affinities of the species. Moreover, closely related species with similar life-history traits often show different population dynamics, undermining the phylogenetic constraints hypothesis. Species with the shortest (1 year) time lag in the action of negative feedback processes on population growth exhibit the largest magnitude of fluctuations. Our analyses revealed that only a few consistent patterns in the population dynamics of herbivorous moths can be deduced from life-history characteristics of the species. Moreover, the diversity of population behaviour in one moth assemblage challenges any conventional wisdom suggesting predictable patterns. Our results raise several questions about perceptions and paradigms in insect population dynamics and stress the need for research on detritivorous insect population dynamics, as well as the need for more assemblage-wide studies using common trapping methods to provide comparative data on related and unrelated species with different life-history traits.     

Oxidative stress as a mediator of life history trade-offs: mechanisms, measurements and interpretation

The concept of trade-offs is central to our understanding of life-history evolution. The underlying mechanisms, however, have been little studied. Oxidative stress results from a mismatch between the production of damaging reactive oxygen species (ROS) and the organism's capacity to mitigate their damaging effects. Managing oxidative stress is likely to be a major determinant of life histories, as virtually all activities generate ROS. There is a recent burgeoning of interest in how oxidative stress is related to different components of animal performance. The emphasis to date has been on immediate or short-term effects, but there is an increasing realization that oxidative stress will influence life histories over longer time scales. The concept of oxidative stress is currently used somewhat loosely by many ecologists, and the erroneous assumption often made that dietary antioxidants are necessarily the major line of defence against ROS-induced damage. We summarize current knowledge on how oxidative stress occurs and the different methods for measuring it, and highlight where ecologists can be too simplistic in their approach. We critically review the potential role of oxidative stress in mediating life-history trade-offs, and present a framework for formulating appropriate hypotheses and guiding experimental design. We indicate throughout potentially fruitful areas for further research.     

Immune Activation Reduces Sperm Quality in the Great Tit

Mounting an immune response against pathogens incurs costs to organisms by its effects on important life-history traits, such as reproductive investment and survival. As shown recently, immune activation produces large amounts of reactive species and is suggested to induce oxidative stress. Sperm are highly susceptible to oxidative stress, which can negatively impact sperm function and ultimately male fertilizing efficiency. Here we address the question as to whether mounting an immune response affects sperm quality through the damaging effects of oxidative stress. It has been demonstrated recently in birds that carotenoid-based ornaments can be reliable signals of a male''s ability to protect sperm from oxidative damage. In a full-factorial design, we immune-challenged great tit males while simultaneously increasing their vitamin E availability, and assessed the effect on sperm quality and oxidative damage. We conducted this experiment in a natural population and tested the males'' response to the experimental treatment in relation to their carotenoid-based breast coloration, a condition-dependent trait. Immune activation induced a steeper decline in sperm swimming velocity, thus highlighting the potential costs of an induced immune response on sperm competitive ability and fertilizing efficiency. We found sperm oxidative damage to be negatively correlated with sperm swimming velocity. However, blood resistance to a free-radical attack (a measure of somatic antioxidant capacity) as well as plasma and sperm levels of oxidative damage (lipid peroxidation) remained unaffected, thus suggesting that the observed effect did not arise through oxidative stress. Towards the end of their breeding cycle, swimming velocity of sperm of more intensely colored males was higher, which has important implications for the evolution of mate choice and multiple mating in females because females may accrue both direct and indirect benefits by mating with males having better quality sperm.     

Higher in vitro resistance to oxidative stress in extra-pair offspring

Oxidative stress is considered to act as a universal physiological constraint in life-history evolution of animals. This should be of interest for extra-pair paternity behaviour, and we tested here the prediction that offspring arising from extra-pair matings of female great tits show higher resistance to oxidative stress than within-pair offspring. Resistance to oxidative stress, measured as the whole blood resistance to a controlled free-radical attack, was significantly higher for extra-pair offspring as predicted although these were not heavier or in better body condition than within-pair offspring. Since resistance to oxidative stress has been suggested to enhance survival and reproductive rates, extra-pair offspring with superior resistance to oxidative stress, be it through maternal effects or paternal inheritance, may achieve higher fitness and thus provide significant indirect fitness benefits to their mothers. In addition, because oxidative stress affects colour signals and sperm traits, females may also gain fitness benefits by producing sons that are more attractive (sexy-sons hypothesis) and have sperm of superior quality (sexy-sperm hypothesis). Heritability of resistance to oxidative stress as well as maternal effects may both act as proximate mechanisms for the observed result. Disentangling these two mechanisms would require an experimental approach. Future long-term studies should also aim at experimentally testing whether higher resistance to oxidative stress of EP nestlings indeed translates into fitness benefits to females.     

Mapping the adaptive landscape of a major agricultural pathogen reveals evolutionary constraints across heterogeneous environments

The adaptive potential of pathogens in novel or heterogeneous environments underpins the risk of disease epidemics. Antagonistic pleiotropy or differential resource allocation among life-history traits can constrain pathogen adaptation. However, we lack understanding of how the genetic architecture of individual traits can generate trade-offs. Here, we report a large-scale study based on 145 global strains of the fungal wheat pathogen Zymoseptoria tritici from four continents. We measured 50 life-history traits, including virulence and reproduction on 12 different wheat hosts and growth responses to several abiotic stressors. To elucidate the genetic basis of adaptation, we used genome-wide association mapping coupled with genetic correlation analyses. We show that most traits are governed by polygenic architectures and are highly heritable suggesting that adaptation proceeds mainly through allele frequency shifts at many loci. We identified negative genetic correlations among traits related to host colonization and survival in stressful environments. Such genetic constraints indicate that pleiotropic effects could limit the pathogen’s ability to cause host damage. In contrast, adaptation to abiotic stress factors was likely facilitated by synergistic pleiotropy. Our study illustrates how comprehensive mapping of life-history trait architectures across diverse environments allows to predict evolutionary trajectories of pathogens confronted with environmental perturbations.Subject terms: Population genetics, Plant sciences, Molecular evolution, Fungi     

Effects of sibling competition on growth, oxidative stress, and humoral immunity: a two-year brood-size manipulation

We investigated the effects of ecological context (by comparing data from two consecutive years) and experimentally manipulated nestling developmental conditions (large vs. small brood size) on immune function (immunoglobulin Y [IgY]) and oxidative stress in nestling European starlings Sturnus vulgaris. On the basis of annual differences in chicks' morphological traits and body masses close to fledging, we established that 2007 was a relative low-quality year and 2008 was a relatively high-quality year. Total antioxidant capacity (TAC) was significantly lower in experimentally enlarged broods, but only in the low-quality year (2007). Total oxidant status (TOS) was independent of brood size in both years but was 45% higher in the low-quality year. Consequently, plasma oxidative status (the ratio between TOS and TAC) was higher in 2007. In contrast, plasma IgY levels were higher in the experimentally enlarged broods and in the high-quality year (2008). Thus, immune function and oxidative stress showed inverse relationships with developmental conditions and annual variation in year quality. Finally, TOS and TAC were positively correlated, but only in the low-quality year (2007), and there was no relationship observed between IgY and markers of oxidative stress. Our results demonstrate the importance of taking into account year effects or ecological context when assessing environmental effects on physiological mechanisms underlying the life-history traits of chicks, such as oxidative stress.     

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.