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

Defence against the intergenerational cost of reproduction in males: oxidative shielding of the germline

Reproduction is expected to carry an oxidative cost, yet in many species breeders appear to sustain lower levels of oxidative damage compared to non-breeders. This paradox may be explained by considering the intergenerational costs of reproduction. Specifically, a reduction in oxidative damage upon transitioning to a reproductive state may represent a pre-emptive shielding strategy to protect the next generation from intergenerational oxidative damage (IOD) - known as the oxidative shielding hypothesis. Males may be particularly likely to transmit IOD, because sperm are highly susceptible to oxidative damage. Yet, the possibility of male-mediated IOD remains largely uninvestigated. Here, we present a conceptual and methodological framework to assess intergenerational costs of reproduction and oxidative shielding of the germline in males. We discuss variance in reproductive costs and expected payoffs of oxidative shielding according to species' life histories, and the expected impact on offspring fitness. Oxidative shielding presents an opportunity to incorporate intergenerational effects into the advancing field of life-history evolution.

Intertidal limits shape covariation between metabolic plasticity, oxidative stress and telomere dynamics in Pacific oyster (Crassostrea gigas)

In intertidal zones, species such as sessile shellfish exhibit extended phenotypic plasticity to face rapid environmental changes, but whether frequent exposure to intertidal limits of the distribution range impose physiological costs for the animal remains elusive. Here, we explored how phenotypic plasticity varied along foreshore range at multiple organization levels, from molecular to cellular and whole organism acclimatization, in the Pacific oyster (Crassostrea gigas). We exposed 7-month-old individuals for up to 16 months to three foreshore levels covering the vertical range for this species, representing 20, 50 and 80% of the time spent submerged monthly. Individuals at the upper range limit produced energy more efficiently, as seen by steeper metabolic reactive norms and unaltered ATP levels despite reduced mitochondrial density. By spending most of their time emerged, oysters mounted an antioxidant shielding concomitant with lower levels of pro-oxidant proteins and postponed age-related telomere attrition. Instead, individuals exposed at the lower limit range near subtidal conditions showed lower energy efficiencies, greater oxidative stress and shorter telomere length. These results unraveled the extended acclimatization strategies and the physiological costs of living too fast in subtidal conditions for an intertidal species.

Social status influences relationships between hormones and oxidative stress in a cichlid fish

An individual's social environment can have widespread effects on their physiology, including effects on oxidative stress and hormone levels. Many studies have suggested that variation in oxidative stress experienced by individuals of different social statuses might be due to endocrine differences, however, few studies have evaluated this hypothesis. Here, we assessed whether a suite of markers associated with oxidative stress in different tissues (blood/plasma, liver, and gonads) had social status-specific relationships with circulating testosterone or cortisol levels in males of a cichlid fish, Astatotilapia burtoni. Across all fish, blood DNA damage (a global marker of oxidative stress) and gonadal synthesis of reactive oxygen species [as indicated by NADPH-oxidase (NOX) activity] were lower when testosterone was high. However, high DNA damage in both the blood and gonads was associated with high cortisol in subordinates, but low cortisol in dominants. Additionally, high cortisol was associated with greater production of reactive oxygen species (greater NOX activity) in both the gonads (dominants only) and liver (dominants and subordinates). In general, high testosterone was associated with lower oxidative stress across both social statuses, whereas high cortisol was associated with lower oxidative stress in dominants and higher oxidative stress in subordinates. Taken together, our results show that differences in the social environment can lead to contrasting relationships between hormones and oxidative stress.

Oxidative Stress Is a Potential Cost of Synchronous Nesting in Olive Ridley Sea Turtles

Olive ridley sea turtles, Lepidochelys olivacea, exhibit a polymorphic reproductive behavior, nesting solitarily or in mass aggregations termed “arribadas”, where thousands of individuals nest synchronously. Arribada nesting provides fitness benefits including mate finding during nearshore aggregations and predator satiation at the time of hatching, but it is unknown if such benefits come with a physiological cost. We used plasma metabolite profiling, stable isotope analysis, biochemical and endocrine assays to test whether metabolic parameters differ between nesting modes, and if arribada nesting is associated with increased levels of oxidative damage compared to solitary nesting. Arribada nesters were bigger and had higher circulating thyroid hormone levels than solitary nesters. Similarly, pathways related to phospholipid and amino acid metabolism, catabolic processes, and antioxidant defense were enriched in individuals nesting in arribada. Stable isotope signatures in skin samples showed differences in feeding zones with arribada nesters likely feeding on benthic and potentially more productive grounds. Arribada nesters had increased levels of plasma lipid peroxidation and protein oxidation products compared to solitary nesters. These results suggest that metabolic profiles differ between nesting modes and that oxidative stress is a trade-off for the fitness benefits associated with arribada nesting.

Can differential fatty acid composition help migrating birds to limit oxidative lipid damage?

During migratory endurance flights, which are energetically very demanding, migrants have to deal with prolonged elevated generation of reactive oxygen species (ROS). To limit the damaging actions that ROS have on lipids and proteins, migrating birds are known to upregulate their antioxidant defence system. However, there may be additional ways to limit oxidative damage incurred from flying. Migratory endurance flights are fuelled mainly with fatty acids (FAs), and the risk of their peroxidation (resulting in oxidative lipid damage) increases with the number of double bonds in a FA, with polyunsaturated FAs (2 or more double bonds, PUFAs) being most peroxidation-prone. By fuelling their flights with relatively few PUFAs, migratory birds could thus limit oxidative lipid damage. Within migratory birds, there is considerable variation in the length of their flights, with nocturnal migrants making lengthier flight bouts, thus more likely to experience lengthier periods of elevated ROS production, than diurnal migrants. However, whether migrants making lengthier flights incur more oxidative lipid damage is unknown. Neither is it known whether flight length and FA composition are associated. Therefore, we determined plasmatic malondialdehyde level, a marker of oxidative lipid damage, and FA composition of three nocturnal and two diurnal migrant species caught at an autumn stopover site. We found little inter-specific variation in malondialdehyde level, indicating that the amount of oxidative lipid damage was comparable across the species. In contrast, the species strongly differed in their plasmatic FA composition. The nocturnal migrants had significantly lower relative PUFA levels than both diurnal migrants, an effect mainly attributable to linoleic acid, an essential (strictly dietary) FA. Consequently, the susceptibility of plasmatic FAs to lipid peroxidation was significantly lower in the nocturnal than diurnal migrants. Because in birds, energy expenditure during flight decreases with the degree of FA unsaturation, we interpret our observation of lower PUFA levels in nocturnal migrants as support for the idea that utilizing PUFA-poor fuel can help migrating birds to curb oxidative lipid damage.

A meta-analysis of impacts of immune response and infection on oxidative status in vertebrates

Inferring from patterns observed in biomedical research, ecoimmunological theory predicts that oxidative stress is a ubiquitous physiological cost that contributes to generating variation in immune function between individuals or species. This prediction is, however, often challenged by empirical studies testing the relationship between immune response or infection and oxidative status markers. This points out the importance of combining ecological immunology and oxidative stress ecology to further our understanding of the proximate causes and fitness consequences of individual variation in health, and adaptability to natural and anthropogenic environmental changes. I reviewed evidence and performed phylogenetic meta-analyses of changes in oxidative status markers owing to either injection of an antigen or infection in captive and free-living vertebrates (141 studies, 1262 effect sizes, 97 species). The dataset was dominated by studies on fish, birds and mammals, which provided 95.8% of effect sizes. Both antigen injection and parasite exposure were associated with changes of oxidative status. There were significant effects of taxonomic class and experimental environment (captivity vs. wild). In contrast with my predictions, age category (young vs. adult), study design (correlational vs. experimental) and proxies of pace of life (clutch size, litter size, and body mass; for birds and mammals only) were negligible in this dataset. Several methodological aspects (type of immunostimulant, laboratory assay, tissue analysed) showed significant effects on both strength and direction of effect. My results suggest that alterations of oxidative status are a widespread consequence of immune function across vertebrates. However, this work also identified heterogeneity in strength and direction of effect sizes, which suggests that immune function does not necessarily result in oxidative stress. Finally, this work identifies methodological caveats that might be relevant for the interpretation and comparability of results and for the application in conservation programs.

A meta-analytic review of fish antioxidant defense and biotransformation systems following pesticide exposure

Pesticides reach aquatic ecosystems and interact with various targets in cells of fish and other living organisms. Toxicity originates during the metabolization process, which may produce toxic metabolites or reactive oxygen species (ROS). Ethoxyresorufin-O-deethylase (EROD), glutathione S-transferase (GST), catalase (CAT), superoxide dismutase (SOD) activities, and levels of reduced glutathione (GSH) indicate toxicants interacted with drug-metabolizing and antioxidant systems, i.e., they are biomarkers of biotransformation and oxidative stress. We meta-analytically quantified the impact of pesticides on the mean response and variability of these biomarkers. Our goals were to verify (i) the overall effect of pesticides on oxidative stress and biotransformation, and how each biomarker respond to exposure; (ii) how the life stage of fish (juvenile and adult) influence biomarkers variability and mean activity; (iii) to what extent fish sex (male, female or mixed-sex groups) modify pesticides toxicity; (iv) how different classes of pesticides, and the combination of their concentration and time of exposure, affect each biomarker. Overall, pesticides induced oxidative stress and the biotransformation system. Regardless of life stage, EROD mean activity increased significantly. In exposed juveniles, CAT and GST variability decreased and increased, respectively. CAT mean activity was higher in females, while EROD and GST activities increased in males after pesticide exposure. Organophosphorus (OPs) and organochlorine insecticides, along with imidazole and triazole fungicides, affected biomarkers the most, however the combined effect of concentration and time of exposure of OPs was not detected. Notably, imidazoles and triazoles classes increased EROD by more than 100%. Additionally, we identified research gaps, such as the lack of effect estimates of relevant pesticides on EROD (e.g., pyrethroids and neonicotinoids) and the small number of studies evaluating GSH on female fish. Future researchers may use these gaps as a guide towards enhanced experimental designs and, consequently, a better understanding of pesticide toxic effects on fish.

Sexual dimorphism in immune function and oxidative physiology across birds: The role of sexual selection

Sex‐specific physiology is commonly reported in animals, often indicating lower immune indices and higher oxidative stress in males than in females. Sexual selection is argued to explain these differences, but empirical evidence is limited. Here, we explore sex differences in immunity, oxidative physiology and packed cell volume of wild, adult, breeding birds (97 species, 1997 individuals, 14 230 physiological measurements). We show that higher female immune indices are most common across birds (when bias is present), but oxidative physiology shows no general sex‐bias and packed cell volume is generally male‐biased. In contrast with predictions based on sexual selection, male‐biased sexual size dimorphism is associated with male‐biased immune measures. Sexual dichromatism, mating system and parental roles had no effect on sex‐specificity in physiology. Importantly, female‐biased immunity remained after accounting for sexual selection indices. We conclude that cross‐species differences in physiological sex‐bias are largely unrelated to sexual selection and alternative explanations should be explored.

Studying microplastics: Lessons from evaluated literature on animal model organisms and experimental approaches

Although we are witnesses of an increase in the number of studies examining the exposure/effects of microplastics (MPs) on different organisms, there are many unknowns. This review aims to: (i) analyze current studies devoted to investigating the exposure/effects of MPs on animals; (ii) provide some basic knowledge about different model organisms and experimental approaches used in studying MPs; and to (iii) convey directions for future studies. We have summarized data from 500 studies published from January 2011 to May 2020, about different aspects of model organisms (taxonomic group of organisms, type of ecosystem they inhabit, life-stage, sex, tissue and/or organ) and experimental design (laboratory/field, ingestion/bioaccumulation/effect). We also discuss and try to encourage investigation of some less studied organisms (terrestrial and freshwater species, among groups including Annelida, Nematoda, Echinodermata, Cnidaria, Rotifera, birds, amphibians, reptiles), and aspects of MP pollution (long-term field studies, comparative studies examining life stages, sexes, laboratory and field work). We hope that the information presented in this review will serve as a good starting point and will provide useful guidelines for researchers during the process of deciding on the model organism and study designs for investigating MPs.

Controlled expression of the migratory phenotype affects oxidative status in birds

High caloric intake can increase production of reactive oxygen and nitrogen species. We examined whether the emergence of the migratory phenotype, primarily signalled by increased food intake and fuelling, is accompanied by changes in oxidative status. We induced autumn migration followed by a non-migratory wintering phase in common quails (Coturnix coturnix). We compared three markers of oxidative status - oxidative damage to lipids expressed as thiobarbituric acid reactive substances (TBARS); superoxide dismutase (SOD); and glutathione peroxidase (GPx) - between birds sampled during the migratory and non-migratory phase. We found that the emergence of the migratory phenotype was associated with: (i) higher levels of TBARS in the liver; (ii) lower levels of SOD in red blood cells and, marginally, in the liver; (iii) higher levels of GPx in the pectoral muscle; and (iv) sex-specific changes in red blood cells and liver. We found no link between food intake and variation in markers of oxidative status in any of the tissues examined, despite food intake being higher in the migratory birds. However, the increase in body mass was positively correlated with muscle GPx activity as birds entered the pre-migratory fattening phase, while the amount of decrease in body mass was negatively correlated with muscle GPx as birds transitioned to the non-migratory phase. Such correlations were absent in red blood cells and liver. Our work suggests that during the emergence of the migratory phenotype, birds might strategically displace oxidative costs on the liver in order to safeguard the pectoral muscles, which have a fundamental role in successfully completing the migratory flight.

Preliminary study of oxidative stress biomarkers and trace elements in North Sea Harbour Seals

This preliminary study investigated the potential correlations between trace elements (mercury, zinc, cadmium, copper, selenium, lead, nickel, chromium, lithium and vanadium) concentrations, measured in red blood cells, and oxidative stress biomarkers (total thiols, total glutathione, total and selenium-dependent glutathione peroxidases, triglycerides, malondialdehyde) assessed in the respective serum, in males and females P. vitulina, sampled in the Wadden Sea in spring and autumn 2015. Only concentrations of total mercury and zinc showed significant differences by sex, and only lipid peroxidation was different by season. Moreover, significant positive and negative correlations were observed between biomarkers (triglycerides, thiols, malondialdehyde, glutathione) and trace element concentrations (copper, lead, mercury, nickel, zinc). These findings suggest that the studied biomarkers could be useful for the assessment of oxidative stress in harbour seals exposed to trace elements, but further research with larger sample sizes is needed to better understand their specific associations.

Differential oxidative costs of locomotory and genital damage in an orb-weaving spider

In animals that regularly experience tissue loss, physiological responses may have evolved to overcome the related costs. Changes in oxidative status may reflect such self-maintenance mechanisms. Here, we investigated how markers of oxidative status vary in female orb-weaving spiders (Larinia jeskovi) by mimicking two distinct types of tissue loss they may naturally encounter: damage to their locomotory system and damage to their external genital structure (scapus), as inflicted by males during copulation (external female genital mutilation). Damage to the locomotory system resulted in a significant shift in oxidative status, reflecting investment in self-maintenance. In contrast, the loss of the scapus did not result in quantitative changes of oxidative markers. This lack of a physiological response suggests negligible physiological costs of genital mutilation for female spiders. However, not being able to remate with other males might be costly for females.

Sex-Specific Response to Caloric Restriction After Reproductive Investment in Microcebus murinus: An Integrative Approach

In seasonal environments, males and females usually maintain high metabolic activity during the whole summer season, exhausting their energy reserves. In the global warming context, unpredictability of food availability during summer could dramatically challenge the energy budget of individuals. Therefore, one can predict that resilience to environmental stress would be dramatically endangered during summer. Here, we hypothesized that females could have greater capacity to survive harsh conditions than males, considering the temporal shift in their respective reproductive energy investment, which can challenge them differently, as well as enhanced flexibility in females' physiological regulation. We tackled this question on the gray mouse lemur (Microcebus murinus), focusing on the late summer period, after the reproductive effort. We monitored six males and six females before and after a 2-weeks 60% caloric restriction (CR), measuring different physiological and cellular parameters in an integrative and comparative multiscale approach. Before CR, females were heavier than males and mostly characterized by high levels of energy expenditure, a more energetic mitochondrial profile and a downregulation of blood antioxidants. We observed a similar energy balance between sexes due to CR, with a decrease in metabolic activity over time only in males. Oxidative damage to DNA was also reduced by different pathways between sexes, which may reflect variability in their physiological status and life-history traits at the end of summer. Finally, females' mitochondria seemed to exhibit greater flexibility and greater metabolic potential than males in response to CR. Our results showed strong differences between males and females in response to food shortage during late summer, underlining the necessity to consider sex as a factor for population dynamics in climate change models.

Chronic water restriction triggers sex-specific oxidative stress and telomere shortening in lizards

Animals use a variety of strategies to avoid acute dehydration and death. Yet, how chronic exposure to sub-lethal dehydration may entail physiological and fitness costs remains elusive. In this study, we experimentally tested if water restriction causes increased oxidative stress (OS) and telomere length (TL) shortening, two well-described mediators of environment-fitness relationships. We exposed 100 yearling female and male common lizards (Zootoca vivipara) either to a 51-day period of water restriction or to water ad libitum, followed by 45 days in common garden outdoor conditions. We measured the kinetic changes in OS and TL and found that water-restricted males had enhanced antioxidant defences and decreased oxidative damage at day 36, whereas females did not immediately respond. A month and a half after water restriction, both sexes experienced a drop in antioxidant capacity but only males exhibited significant TL shortening. In the following 3 years, we found that lizards with longer initial TL and those who maintained stronger antioxidant defences experienced higher longevity, irrespective of sex and water restriction. Together, these results unravelled sex-specific responses to water restriction, with potential applications in better understanding the physiological costs of increasing summer droughts as a result of global climate change.

Experimental manipulation of reproductive tactics in Seba's short-tailed bats: consequences on sperm quality and oxidative status

To reproduce, males have to fertilize the female's eggs, sometimes in competition with ejaculates of other males. In species where males display alternative reproductive tactics, whereby territorial males secure mating and non-territorial males have to sneak copulations, the latter might be expected to invest relatively more resources towards sperm quality compared with the territorial males. Sperm cells are especially vulnerable to oxidative stress, which reduces male fertility. Therefore, antioxidant resources are expected to modulate sperm quality, and might be allocated differently between reproductive tactics. To test the link between reproductive tactics, redox profile and sperm quality, we experimentally induced changes in the reproductive tactics of 39 captive males Seba's short-tailed bats Carollia perspicillata. We monitored the blood and ejaculate oxidative balance, and the sperm quality before, 7 days and 21 days after the manipulation of reproductive tactic. Although ejaculates' oxidative damage was negatively related to sperm velocity, males exhibited similar blood and ejaculates redox profiles and similar sperm quality, regardless of their reproductive tactic. Possibly, these results arise as a consequence of some constraints having been lifted during the experiment. Our results also suggest that, in Seba's short-tailed bats, the expression of alternative reproductive tactics is not subjected to strong oxidative constraints. Furthermore, our results could reflect an absence of trade-off between pre- and post-copulatory traits in harem males, as they could be selected to invest both in female attraction and sperm quality, as a consequence of their inability to fully monopolize females.

Spatio-temporal variation in oxidative status regulation in a small mammal

Life-history allocation trade-offs are dynamic over time and space according to the ecological and demographical context. Fluctuations in food availability can affect physiological trade-offs like oxidative status regulation, reflecting the balance between pro-oxidant production and antioxidant capacity. Monitoring the spatio-temporal stability of oxidative status in natural settings may help understanding its importance in ecological and evolutionary processes. However, few studies have yet conducted such procedures in wild populations. Here, we monitored individual oxidative status in a wild eastern chipmunk (Tamias striatus) population across the 2017 summer active period and over three study sites. Oxidative damage (MDA: Malondialdehyde levels) and non-enzymatic antioxidant levels (FRAP: Ferric reducing antioxidant power and HASC: Hypochlorous acid shock capacity) were quantified across time and space using assays optimized for small blood volumes. Our results showed an increase in oxidative damage mirrored by a decrease in FRAP throughout the season. We also found different antioxidant levels among our three study sites for both markers. Our results also revealed the effects of sex and body mass on oxidative status. Early in the active season, females and individuals with a greater body mass had higher oxidative damage. Males had higher HASC levels than females throughout the summer. This study shows that oxidative status regulation is a dynamic process that requires a detailed spatial and temporal monitoring to yield a complete picture of possible trade-offs between pro-oxidant production and antioxidant capacity.

Mitochondrial Traits Previously Associated With Species Maximum Lifespan Do Not Correlate With Longevity Across Populations of the Bivalve Arctica islandica

The mitochondrial oxidative stress theory of aging posits that membrane susceptibility to peroxidation and the organization of the electron transport system (ETS) linked with reactive oxygen species (ROS) generation are two main drivers of lifespan. While a clear correlation has been established from species comparative studies, the significance of these characteristics as potential modulators of lifespan divergences among populations of individual species is still to be tested. The bivalve Arctica islandica, the longest-lived non-colonial animal with a record lifespan of 507 years, possesses a lower mitochondrial peroxidation index (PI) and reduced H₂O₂ efflux linked to complexes I and III activities than related species. Taking advantage of the wide variation in maximum reported longevities (MRL) among 6 European populations (36–507 years), we examined whether these two mitochondrial properties could explain differences in longevity. We report no relationship between membrane PI and MRL in populations of A. islandica, as well as a lack of intraspecific relationship between ETS complex activities and MRL. Individuals from brackish sites characterized by wide temperature and salinity windows had, however, markedly lower ETS enzyme activities relative to citrate synthase activity. Our results highlight environment-dependent remodeling of mitochondrial phenotypes.

Formation of an antioxidant profile in the sea trout (Salmo trutta m. trutta L.) from the Slupia River

Using a stage- and sex-based multivariate significance tests on the sea trout Salmo trutta m. trutta L. model, we show dependencies in the balance between lipid peroxidation processes, levels of carbonyl derivatives, and activity of antioxidant enzymes (superoxide dismutase SOD, catalase CAT, glutathione reductase GR, and peroxidase GPx) in the processes of antioxidant profile formation during the fish growing process. The study was aimed at examination of the relationships between the biomarkers of oxidative stress estimated by the total antioxidant status as well as the dependencies between the sex (male, female) and developmental stage of the wild sea trout from the Slupia River and its catchment area rivers. Functioning of the pro/antioxidant balance of the liver tissue reflected the course of the individual developmental stages of the trout and was associated with significant intensification of lipoperoxidation, oxidative modification of proteins, and reduction of the total antioxidant capacity of fish along with age. Formation of a holistic model for the analysis of the involvement of all parameters of antioxidant protection in all stages of development and sex allowed us to obtain the following rank order for the level of lipoperoxidation processes, modified proteins, and antioxidant enzyme complex: CAT > SOD > GPx > GR and TBARS > OMP KD > TAC > OMP AD.

Sex and species differences of stress markers in sympatric cheetahs and leopards in Namibia

Physiological stress markers may provide valuable insight for our understanding of costs of given life-history strategies or of wildlife health condition, most importantly in case of threatened species. In the last decade, there has been growing interest in the ecological relevance of cellular oxidative stress, which would provide complimentary information to that obtained by the classic analyses of glucocorticoid hormones. In this study, we analysed the sex and species variation of five blood-based markers of oxidative status, both molecular oxidative damage and antioxidant protection, in sympatric cheetahs (Acinonyx jubatus) and leopards (Panthera pardus) living on Namibian farmlands. Both these terrestrial carnivores are classified as vulnerable by the International Union for Conservation of Nature. We found that female cheetahs had significantly higher serum reactive oxygen metabolites of non-protein origin and lower glutathione peroxidase activity in whole blood than both male and female leopards and male cheetahs. We also found that cheetahs and leopards differed in the association between the two antioxidant enzymes glutathione peroxidase and superoxide dismutase. Correlations among oxidative status markers were stronger in female cheetahs than leopards or male cheetahs. Our results suggest that female cheetahs are more sensitive to local sources of stress. Our work did not corroborate the assumption that two species with different life histories consistently differ in key physiological traits.

The Comparative Biology of Mitochondrial Function and the Rate of Aging

The mitochondrial hypothesis of aging evolved from the rate-of-living theory. That theory posited that the rate of aging was largely determined by the rate of energy expenditure. The mechanistic link between energy expenditure and aging was hypothesized to be oxidative stress. As both energy expenditure and reactive oxygen species (ROS) centered on the mitochondria that organelle became a central focus of aging research. Until about the turn of the 21st century available evidence largely supported the efficiency of mitochondrial function as a key contributor to aging. However as methods for investigating mitochondrial oxidant production and tissue level oxidative damage improved, evidentiary support for the theory weakened. Recently, direct disruption of mitochondrial function has been shown not to shorten life or health as expected, but in many cases in multiple laboratory species disrupted mitochondrial function has lengthened life, sometimes without apparent tradeoffs. Does this mean that mitochondrial function plays no role in aging as had been posited for many years? One key consideration is that experiments under laboratory conditions can be misleading about physiological processes that occur in the uncertain conditions of nature. Before we discard the mitochondrial hypothesis of aging, more field experiments targeted at that hypothesis need to be performed. Fortunately, emerging technology is making such experiment more possible than ever before.

Physiological costs of infection: herpesvirus replication is linked to blood oxidative stress in equids

Viruses may have a dramatic impact on the health of their animal hosts. The patho-physiological mechanisms underlying viral infections in animals are, however, not well understood. It is increasingly recognized that oxidative stress may be a major physiological cost of viral infections. Here we compare three blood-based markers of oxidative status in herpes positive and negative individuals of the domestic horse (Equus ferus caballus) and of both captive and free-ranging Mongolian khulan (Equus hemionus hemionus) and plains zebra (Equus quagga). Herpes positive free-ranging animals had significantly more protein oxidative damage and lower glutathione peroxidase (antioxidant enzyme) than negative ones, providing correlative support for a link between oxidative stress and herpesvirus infection in free-living equids. Conversely, we found weak evidence for oxidative stress in herpes positive captive animals. Hence our work indicates that environment (captive versus free living) might affect the physiological response of equids to herpesvirus infection. The Mongolian khulan and the plains zebra are currently classified as near threatened by the International Union for Conservation of Nature. Thus, understanding health impacts of pathogens on these species is critical to maintaining viable captive and wild populations.

Migratory flight imposes oxidative stress in bats

Many animal species migrate over long distances, but the physiological challenges of migration are poorly understood. It has recently been suggested that increased molecular oxidative damage might be one important challenge for migratory animals. We tested the hypothesis that autumn migration imposes an oxidative challenge to bats by comparing values of 4 blood-based markers of oxidative status (oxidative damage and both enzymatic and nonenzymatic antioxidants) between Nathusius’ bats Pipistrellus nathusii that were caught during migration flights with those measured in conspecifics after resting for 18 or 24 h. Experiments were carried out at Pape Ornithological Station in Pape (Latvia) in 2016 and 2017. Our results show that flying bats have a blood oxidative status different from that of resting bats due to higher oxidative damage and different expression of both nonenzymatic and enzymatic antioxidants (glutathione peroxidase). The differences in oxidative status markers varied between sampling years and were independent from individual body condition or sex. Our work provides evidence that migratory flight might impose acute oxidative stress to bats and that resting helps animals to recover from oxidative damage accrued en route. Our data suggest that migrating bats and birds might share similar strategies of mitigating and recovering from oxidative stress.