Abstract
According to life-history theory, the allocation of limiting resources to one trait has negative consequences for other traits requiring the same resource, resulting in trade-offs among life-history traits, such as reproduction and survival. In vertebrates, oxidative stress is increasingly being considered among the physiological mechanisms forming the currency of life-history trade-offs. In this study of the barn swallow (Hirundo rustica), we focus on the oxidative costs of reproduction, especially egg laying, by investigating the effects of breeding stage (pre- vs. post-laying) and progression of the season on three biomarkers of oxidative damage (OD) to plasma proteins, namely the concentration of malondialdehyde (MDA)-protein adducts and of protein thiol groups (PSH), and the protein carbonyl (PCO) content. Moreover, we investigated whether males and females differed in plasma OD levels, because the inherent sex differences in reproductive roles and physiology may originate sex-specific patterns of OD during breeding. We found that MDA-protein adduct levels were higher in the pre-laying than in the post-laying phase, that males had lower levels of MDA-modified proteins than females, and that the decline of MDA-protein adduct concentration between the pre- and the post-laying phase was more marked for females than males. In addition, MDA-protein adduct levels declined with sampling date, but only during the pre-laying phase. On the other hand, plasma PCO levels increased from the pre- to the post-laying phase in both sexes, and females had higher levels of PCO than males. PSH concentration was unaffected by breeding stage, sex or sampling date. On the whole, our findings indicate that biomarkers of protein oxidation closely track the short-term variation in breeding stage of both male and female barn swallows. Moreover, the higher protein OD levels observed among females compared to males suggest that egg laying entails oxidative costs, which might negatively affect female residual reproductive value.
Highlights
Life-history theory posits that organisms trade investment in current reproduction against future reproductive prospects and survival [1,2,3]
Levels of MDA-modified proteins markedly declined from the pre-laying to the post-laying period in both sexes, but the decline was more marked among females than males (Fig. 2a)
The concentration of PSH was unaffected by any of the considered factors or their interaction (Table 1), whereas we found a significantly higher protein carbonyl (PCO) content in females compared to males (Table 1, Fig. 3), and a significant increase of PCO content from the pre- to the post-laying phase, which was similar in the two sexes (Table 1, Fig. 3)
Summary
Life-history theory posits that organisms trade investment in current reproduction against future reproductive prospects and survival [1,2,3]. Oxidative stress results from an imbalance between the production of pro-oxidant substances, so called ‘reactive species’ (RS, sensu [21]), originating as by-products of oxidative metabolism and different physiological and pathophysiological functions, and the wide array of antioxidant defences and repair mechanisms that organisms have evolved to counteract the negative effects of RS. It is becoming increasingly recognized that some RS play a key role in cell signalling [23], oxidative stress is known to impair physiological functions [22], may accelerate ageing, and cause higher susceptibility to environmental stress or pathogens [19,24], thereby resulting in reduced fitness
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