Oxidative physiology is weakly associated with pigmentation in birds

Abstract

The mechanistic link between avian oxidative physiology and plumage coloration has attracted considerable attention in past decades. Hence, multiple proximal hypotheses were proposed to explain how oxidative state might covary with the production of melanin and carotenoid pigments. Some hypotheses underscore that these pigments (or their precursors, e.g., glutathione) have antioxidant capacities or function as molecules storing the toxic excess of intracellular compounds, while others highlight that these pigments can act as pro‐oxidants under specific conditions. Most studies addressing these associations are at the intraspecific level, while phylogenetic comparative studies are still scarce, though needed to assess the generality of these associations. Here, we tested whether plumage and bare part coloration were related to oxidative physiology at an interspecific level by measuring five oxidative physiology markers (three nonenzymatic antioxidants and two markers of lipid peroxidative damage) in 1387 individuals of 104 European bird species sampled during the breeding season, and by scoring plumage eumelanin, pheomelanin, and carotenoid content for each sex and species. Only the plasma level of reactive oxygen metabolites was related to melanin coloration, being positively associated with eumelanin score and negatively with pheomelanin score. Thus, our results do not support the role of antioxidant glutathione in driving variation in melanin synthesis across species. Furthermore, the carotenoid scores of feathers and bare parts were unrelated to the measured oxidative physiology parameters, further suggesting that the marked differences in pigmentation across birds does not influence their oxidative state.

Songbirds avoid the oxidative stress costs of high blood glucose levels

“…These findings agree with our previous results based on a considerably smaller oxidative physiology dataset (Vágási et al, 2016). We have previously shown that all blood glucose (G 0 , G 30 and ΔG) and oxidative state variables measured here are suitable for among-species comparative analyses because they fulfil the criterion of being species-specific, as demonstrated by their significant repeatability within species (i.e., conspecifics resemble each other more than members of other species; see details in (Marton et al, 2022;Tomášek et al, 2022)).…”

“…Diet might also confound the results because more AGEs are formed at higher protein and amino acid concentrations (Cerami, 1985). Our previous results support this notion as we showed that carnivorous birds have higher MDA and lower tGSH levels as compared with herbivorous ones (Marton et al, 2022). However, in the present study we could not assess whether the interaction between diet and glucose variables influences markers of oxidative state because our samples only covered songbirds (see (Tomášek et al, 2019)), a group with less variable diet than in the study by (Marton et al, 2022).…”

“…Our previous results support this notion as we showed that carnivorous birds have higher MDA and lower tGSH levels as compared with herbivorous ones (Marton et al, 2022). However, in the present study we could not assess whether the interaction between diet and glucose variables influences markers of oxidative state because our samples only covered songbirds (see (Tomášek et al, 2019)), a group with less variable diet than in the study by (Marton et al, 2022). Nevertheless, diet probably has little effect on our results because a recent analysis on a much larger dataset including 160 temperate and tropical songbird species, showed that only nectarivory/frugivory elevates blood glucose level (Tomášek et al, 2022), but this diet type is missing in our dataset.…”

Correlated evolution of oxidative physiology and MHC-based immunosurveillance in birds

“…Nonetheless, our analyses provided no support for this scenario, since most associations between oxidative physiology and the MHC remained significant after controlling for basic life history traits. These associations were also robust to variation in diet, which has been previously shown to drive the evolution of antioxidant defences in birds [37,92]. Nevertheless, associations between oxidative physiology and the MHC can be mediated by metabolic function.…”

Pigment molecular composition reveals significant information for visual communication

“…However, the association between melanin‐based phenotypes and oxidative stress is still not fully understood. This is exemplified by a recent comparative study neglecting any association between pigmentation and oxidative status in birds after comparing many species whose pigment phenotypic expression was assessed by asigning scores from 0 to 5 obtained from the examination of book plates (Marton et al, 2022). While these methods for determining phenotypic expression have been useful in the past, the present study stresses that the ecological implications of pigmentation will be unveiled only by the study of pigment traits at the molecular level.…”