David Costantini

Oxidative stress in ecology and evolution: lessons from avian studies

Although oxidative stress is a central topic in biochemical and medical research, the number of reports on its relevance in life-history studies of non-human animals is still low. Information about oxidative stress in wild birds may help describe functional interactions among the components of life-history traits. Currently available evidence suggests that oxidative stress may impart an important physiological cost on longevity, reproduction, immune response or intense physical activity. Given the gaps in our present knowledge, it is still premature to attempt to draw definitive conclusions and basic questions (e.g. how is oxidative stress generated and how do organisms cope with it?) have yet to be fully explored under natural conditions. Therefore, caution is needed in developing hypotheses or drawing general conclusions until additional data become available to perform more rigorous comparative analyses.

Does immune response cause oxidative stress in birds? A meta-analysis

In recent years ecological research has focused on the relevance of antioxidants and oxidative stress in the evolution of life-history strategies and physiological trade-off in birds. Some studies sought to evaluate whether a consequence of immune response is oxidative stress. In a meta-analysis of 16 studies of ten species of birds including 49 estimates of effect size from experimental studies, we show that induction of an immune response in a diverse group of bird species may determine oxidative stress (variance explained: 4.1%), but, most notably, may determine changes in oxidative stress markers (variance explained: 15.0%). These conclusions were robust to control for sampling effort and publication bias. Finally, this finding suggests that (1) oxidative stress may be a physiological cost associated with the immune response and (2) an important role of antioxidants in birds is to control the potentially negative effects of such oxidative stress to prevent immuno-pathological damage to host tissues.

Correlates of oxidative stress in wild kestrel nestlings (Falco tinnunculus)

The fitness of an organism can be affected by conditions experienced during early development. In light of the impact that oxidative stress can have on the health and ageing of a bird species, this study evaluated factors accounting for the variation in oxidative stress levels in nestlings of the Eurasian kestrel (Falco tinnunculus) by measuring the serum concentration of reactive oxygen metabolites and the serum antioxidant barrier against hypochlorite-induced oxidation. The ratio between these two variables was considered as an index of oxidative stress, with higher values meaning higher oxidative damage. Six-chick broods showed the highest level of oxidative stress, while no effect of sex was found. Age showed an inverse relationship with the oxidants and the levels of oxidative stress, with younger birds having higher levels. Hatching date, body condition, body mass and carotenoid concentration did not show any relationship with oxidants, antioxidants or degree of oxidative stress. These findings suggest that intrabrood sibling competition could play a role in determining oxidative stress, and that in carnivorous birds other antioxidant molecules could be more important than carotenoids to reduce oxidative stress.

Long flights and age affect oxidative status of homing pigeons (Columba livia)

SUMMARY Flying is an energy demanding activity that imposes several physiological challenges on birds, such as increase in energy expenditure. Evidence from sports medicine shows that exhausting exercise may cause oxidative stress. Studies on avian flight have so far considered several blood parameters, such as uric acid, corticosteroids, or circulating free fatty acids, but only one study has analysed markers of oxidative stress in flying birds. In this study, we evaluated, for the first time, how different flight efforts affect the oxidative status using homing pigeons (Columba livia) as a model species. Two groups of pigeons flew for around 60 and 200 km, respectively. Pigeons that flew for 200 km had a 54% increase in oxidative damage as measured by serum reactive oxygen metabolites (ROMs), a 19% drop in total serum antioxidant capacity (OXY) and an 86% increase of oxidative stress (ROMs/OXY×1000). Older pigeons depleted more serum antioxidants regardless of the release distance. Among pigeons that flew the longer distance, heavier ones depleted less serum antioxidants. The results of the study suggest that long flights may cause oxidative stress, and that older individuals may experience higher physiological demands.

Oxidative Stress and Hormesis in Evolutionary Ecology and Physiology

The transition from a reducing to an oxidising chemistry in the atmosphere and oceans paved the way for the diversification of life. Oxygen expanded metabolic and biochemical capacities of organisms. Over the incipient stages of evolution of oxidative metabolism, organisms also needed to develop mechanisms to mitigate the toxic effects of oxygen derivatives, such as free radicals and nonradical reactive species. This chapter provides a general historical background, with definitions and information of free radicals, antioxidants and oxidative stress. This chapter also examines how mild doses of stress can have stimulatory effects on organismal performance through hormetic mechanisms and that this may significantly relate to evolutionary fitness and to the ecology of species. Finally, the chapter explains the concept of life-history trade-offs and highlights how the need to manage oxidative stress in an optimal way may be an important mechanism driving the outcome of many of these trade-offs. 1.1 The Great Oxidation Event: From a Reducing to an Oxidising World The planet Earth is approximately 4.5 billion years old. The atmosphere of the primeval Earth was quite different from what we observe nowadays. It was mildly reducing, with large proportions of methane, ammonia and hydrogen and a low concentration of oxygen (Schopf and Klein 1992; Sessions et al. 2009). Around 2.45 billion years ago, atmospheric oxygen rose suddenly in what is now termed the Great Oxidation Event (Sessions et al. 2009). A second significant increase in atmospheric oxygen occurred at around 600–800 million years ago and was accompanied by the oxygenation of the deep oceans and emergence of multicellular animals (Sessions et al. 2009). The increase in oxygen concentration in the atmosphere and oceans paved the way for the diversification of life (Fig. 1.1). D. Costantini, Oxidative Stress and Hormesis in Evolutionary Ecology and Physiology, DOI: 10.1007/978-3-642-54663-1_1, Springer-Verlag Berlin Heidelberg 2014 1 The transition from a reducing to an oxidising atmosphere was characterised by the evolution of metabolic networks of increasing complexity (Raymond and Segrè 2006). Adaptation to molecular oxygen has also likely taken place independently in species from diverse lineages, even if it is unclear whether it contributed to shaping taxonomical diversity (Raymond and Segrè 2006). Certainly, oxygen expanded metabolic and biochemical capacities of organisms. The stimulatory effect of oxygen on the evolution of metabolic networks was not cost-free. Beyond diversification of mechanisms using oxygen to produce energy, organisms also needed to evolve mechanisms to mitigate the toxic effects of oxygen derivatives, such as free radicals and non-radical reactive species. 1.2 Reactive Species, Antioxidants and Oxidative Stress 1.2.1 On the Nature of Free Radicals and Other Reactive Species The discovery of organic free radicals dates back to over a century ago, when the scientist Gomberg (1900) at the University of Michigan identified the triphenylmethyl The primeval Earth’s atmosphere was mildly reducing. Photochemical reactions between simple gas elements 2H2 + CO2 → H2CO + H2O Evolution of anaerobic bacteria H2S + CO2 → (H2CO)n + S Evolution of photosynthetic organisms H2O + CO2 → (H2CO)n + O2 Evolution of aerobic eukaryotes; aerobic pathways produce much more energy than anaerobic pathways O2 + (H2CO)n → H2O + CO2 Aerobic pathways generate oxygen free radicals and non-radical species. Hence, evolution of antioxidant mechanisms to cope with oxidative stress. Fig. 1.1 Sequence of main transitions in energetic metabolism induced by changes in atmosphere and ocean chemistry (see Falkowski 2006) 2 1 Historical and Contemporary Issues of Oxidative Stress

Environmental and genetic components of oxidative stress in wild kestrel nestlings ( Falco tinnunculus )

In this study, we estimated the environmental and genetic components of two variables related to avian oxidative stress using wild nestlings of the Eurasian kestrel (Falco tinnunculus). The study was carried out during two breeding seasons. In the first season, we assessed the between- and within-nest resemblance in serum reactive oxygen metabolites (ROMs) and total serum antioxidant barrier (OXY). In the second season, we carried out a cross-fostering experiment to determine the importance of environmental and genetic factors on ROMs and OXY. The 23.5% of ROMs variance was explained by the nest of origin, indicating a main genetic component. In contrast, the 52.8% of OXY variance was explained by the nest of rearing, indicating that this variable was more influenced by environmental components. These findings suggest that variations in ROMs and OXY could reflect, respectively, the expression of different genetic polymorphisms and differences in dietary uptake of antioxidants.

Carotenoid availability does not limit the capability of nestling kestrels (Falco tinnunculus) to cope with oxidative stress.

It is recognized that carotenoids are useful anti-oxidants in embryo and hatchling avian models. However, recent evidence suggests that the anti-oxidant role of carotenoids in nestling or adult birds may not be as important as previously thought. The aim of the present work was to investigate if supplemental carotenoids decreased the level of oxidative damage (by reactive oxygen metabolites, ROMs) and increased the serum anti-oxidant capacity (OXY) in nestling Eurasian kestrels Falco tinnunculus. Circulating carotenoids in supplemented nestlings increased about 1.5-fold compared to the control and pre-treatment levels at the end of the supplementation period. There was no effect on ROMs, OXY or the level of oxidative stress (ratio between ROMs and OXY), however, or on body mass or body condition of nestlings. ROMs and OXY decreased with age, but this pattern varied across the nests. Our results show that (i) in general, younger nestlings actually have to cope with a high free radical production, and (ii) the ability of wild nestling kestrels to cope with oxidative stress is not affected by carotenoid availability.

Early life experience primes resistance to oxidative stress

SUMMARY The extent to which early stress exposure is detrimental to Darwinian fitness may depend on its severity, with mild stress exposure actually having a stimulatory and, possibly, beneficial effect through a hormetic response to the stressful stimulus. We need to understand such hormetic processes to determine how the early environment can help shape a phenotype adapted to the conditions the organism is most likely to experience in its adult environment. Using the zebra finch (Taeniopygia guttata), we tested the hypothesis that individuals exposed to mild heat stress earlier in life will suffer less oxidative stress when faced with high heat stress in adulthood than will individuals either not pre-exposed to heat stress or exposed to high heat stress earlier in life. Our findings demonstrate that early life exposure to mild heat stress primes the system to better withstand oxidative stress when encountering heat stress as an adult. These findings point to a potential mechanism linking early life experiences to future Darwinian fitness.

Consistent differences in feeding habits between neighbouring breeding kestrels

In this study, we analysed the diet of breeding kestrels (Falco tinnunculus) in a Mediterranean area with the aim to evaluate the relative importance of both hunting area and individual feeding behaviour as factors affecting prey selection. Differently from the populations from middle and northern Europe which primarily feed on voles, the kestrels breeding in the Mediterranean region showed a wider diet composition. As expected, hunting area features influenced the diet composition and, in general, the kestrels were feeding on what was locally more abundant. However, we detected consistent differences in the diet composition between neighbouring breeding pairs which were also maintained in subsequent years. Since the neighbouring birds were sharing the same hunting grounds, the differences observed were likely to reflect individual preferences or capabilities in catching some prey type regardless of their actual availability. The presence of differences in diet composition between neighbouring pairs and their temporal consistency suggests that the hunting skills, and in general the feeding behaviour of kestrels, is likely to represent a trait characterising a behavioural type.

Biochemical integration of blood redox state in captive zebra finches (Taeniopygia guttata)

SUMMARY Integration is a property of biological systems that refers to the extent to which their components are correlated through functional, structural, developmental or evolutionary interdependency. This study examined patterns of functional integration among different molecular components of the blood redox system (both plasma and red blood cells) in zebra finches (Taeniopygia guttata). Our results show a two cluster organization of the six measured variables: one cluster includes glutathione peroxidase in both red blood cells and plasma, thiol concentration in red blood cells and plasma hydroperoxides; the other cluster comprises a measure of the non-enzymatic antioxidant capacity in red blood cells and plasma. The interaction network amongst these variables shows (i) a strong positive connectivity among hydroperoxides, glutathione peroxidase and thiols, and (ii) negative connectivity between non-enzymatic and enzymatic antioxidants. Overall, our results also suggest strong and significant integration between the oxidative state of red blood cells and plasma.