Effects of an early-life paraquat exposure on adult resistance to oxidative stress, plumage colour and sperm performance in a wild bird.

Early-life stressful conditions can shape individual phenotypes and ultimately influence fitness. Oxidative stress is a pervasive threat that affects many fitness-related traits and can modulate life-history trade-offs. Yet, the extent to which exposure to oxidative stress during early life can have long-lasting effects on key fitness-related traits remains to be elucidated, particularly in natural populations of vertebrates. Using a wild population of great tits Parus major, we experimentally dosed 11-day-old birds with paraquat, a pro-oxidant molecule, aiming at increasing oxidative stress. One year later, we recaptured 39 of them as adult recruiting breeders and quantified effects of the paraquat exposure on their resistance to oxidative stress, carotenoid-based plumage coloration and male sperm performance. Despite the absence of a short-term effect of paraquat on oxidative stress measured two days later, the pre-fledging exposure to paraquat induced a reduction in individual oxidative damage measured at adulthood. Paraquat-dosed individuals also had brighter plumage, but no effect was observed on male sperm performance. For the first time in a natural population of vertebrates, we experimentally show that an early-life acute exposure to a pro-oxidant has long-lasting effects on individual resistance to oxidative stress at adulthood. Our results are in line with the environmental matching and the hormesis hypotheses but may also reflect selective disappearance of individuals with lower resistance to oxidative stress.

Immune response, oxidative stress and dietary antioxidants in great tit nestlings.

The activation of immune defences counteracts pathogens, but mounting an immune response is costly and can negatively impact life-history traits. Immune activation releases highly reactive species that kill pathogens but can also cause oxidative damage to host tissues, and these negative effects may therefore constrain further investment in immune responses. To offset these toxic effects, animals rely on a complex system of antioxidants. Here, we tested if vitamin E, a dietary antioxidant, can reduce oxidative damage induced by an immune challenge and thus enhance the immune response. In a 2 × 2 experimental design, we supplemented great tit nestlings with either vitamin E or a placebo, and then injected them with either a bacterial lipopolysaccharide (LPS), or a buffer solution (PBS) as a control. LPS-treated nestlings mounted an inflammatory response and increased antioxidant capacity, without any change in ROM (reactive oxygen metabolites), an index of early oxidative damage. These results suggest that the likely transient increase in reactive species of the LPS injection was counteracted by a rise in endogenous antioxidant defences that was independent of supplementary dietary antioxidants. Indeed, vitamin E supplementation neither affected oxidative status nor enhanced the immune response, suggesting that in our experimental condition great tit nestlings were not limited in vitamin E and in antioxidants in general. Overall, our results show that birds can mount an effective antioxidant response to face an immune challenge, and can therefore avoid stress caused by a transient increase in reactive species generated by immune activation.

Differential effects of vitamins E and C and carotenoids on growth, resistance to oxidative stress, fledging success and plumage colouration in wild great tits.

Oxidative stress is the imbalance between the production of reactive species and antioxidants, which causes damage to lipids, proteins and DNA. Antioxidants, like vitamins and carotenoids, can limit oxidative damage and can therefore regulate the trade-off between growth, which is a period of high reactive species production, and self-maintenance. However, the role of carotenoids as antioxidants in vivo has been debated, and it has been suggested that carotenoid-based signals indicate the availability of non-pigmentary antioxidants (e.g. vitamins) that protect carotenoids from oxidation, known as the 'protection hypothesis'. To evaluate the importance of vitamins versus carotenoids as antioxidants during growth and to test the protection hypothesis, we supplemented nestling great tits, Parus major, 3, 5 and 7 days after hatching with a single dose of carotenoids and/or vitamins in a 2×2 full-factorial design. We subsequently measured body condition, antioxidant capacity, oxidative damage, fledging success and plumage reflectance. Vitamins enhanced antioxidant capacity, but did not affect oxidative damage. Vitamin-treated nestlings had higher growth rates and higher probability of fledging. In contrast, carotenoids did not affect any of these traits. Furthermore, carotenoid-based colouration increased over the breeding season in nestlings that received vitamins only. This study shows that vitamins are limiting for growth rate and fledging success, and suggests that vitamins could regulate the trade-off between growth and self-maintenance in favour of the former. Moreover, our results are consistent with the idea that carotenoids are minor antioxidants in birds, but they do not support the protection hypothesis.

Yolk carotenoids increase fledging success in great tit nestlings.

Avian mothers can influence offspring phenotype through the deposition of different compounds into eggs, such as antibodies, hormones and antioxidants. The concentration of carotenoids in yolk is larger than in maternal plasma, suggesting an important role of these compounds for offspring development. Since carotenoids have to be acquired from the diet, they may be available in limiting amounts to the mothers. Here, we investigated the role of egg carotenoids for offspring growth by experimentally increasing the concentration of yolk lutein, the main carotenoid in great tit (Parus major) yolk. We subsequently measured body condition, oxidative stress, immune response, plumage colouration and fledging success. Lutein increased body mass soon after hatching and fledging success, but did not affect tarsus length, oxidative stress, immune response and plumage colouration. The higher content of yolk lutein could have increased body mass by reducing oxidative stress caused by high metabolic rates of rapidly growing embryos or by promoting cell differentiation and proliferation. The positive effect of lutein on fledging success seems to be mediated by its influence on body mass 3 days post-hatch, since these two traits were correlated. The finding that our treatment did not affect traits measured later in the nestling period, except for fledging success, suggests that yolk lutein has short-term effects that are essential to increase survival until fledging. Our study shows the positive effect of yolk lutein on offspring survival in the great tit, and therefore suggests an important role of carotenoid-mediated maternal effects.

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.