Antioxidant capacity is repeatable across years but does not consistently correlate with a marker of peroxidation in a free-living passerine bird

Blandine Doligez; Charlotte R\'ecapet (ECOBIOP); Mathilde Arriv\'e (IBMP); Pierre Bize

REVIEW 3 major objections 4 minor 3 references

Antioxidant capacity is repeatable across years but does not consistently correlate with a marker of peroxidation in a free-living passerine bird

T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read The relationship between oxidative damage and antioxidant defense is not fixed in wild collared flycatchers, with antioxidant capacity repeatable across years while peroxidation markers are not, and their correlation changing with energy…

desk verdict Solid repeatability data for OXY, but the correlation-modulation headline is ahead of the evidence. read the letter →

arxiv 1908.01488 v1 pith:JOVD3NCN submitted 2019-08-05 q-bio.PE stat.AP

Charlotte Récapet (ECOBIOP) , Mathilde Arrivé (IBMP) , Blandine Doligez , Pierre Bize This is my paper · ORCID

classification q-bio.PEstat.AP

keywords oxidativebalancereactiveoxygenmetabolitesantioxidantcapacityrepeatabilitycollaredflycatcherFicedulaalbicollisenergeticconstraintsfoodsupplementation

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Across 1108 plasma samples from 635 free-living adult collared flycatchers, this paper asks whether two widely used oxidative-balance markers track a stable individual trait or a flexible physiological response. It finds that non-enzymatic antioxidant capacity is repeatable within and between years (repeatability 0.124 to 0.581), while reactive oxygen metabolites—a marker of lipid and protein peroxidation—are not (0 to 0.061). The two markers are positively correlated overall, but the strength of the correlation changes with experimentally increased flight costs and across years, and food supplementation did not change it. The paper's central claim is that the relationship between components of the oxidative balance is not fixed but is modulated by individual and environmental factors, which matters because composite indices of oxidative stress that assume a stable correlation would be misleading.

What carries the argument

The argument is carried by two plasma assays measured repeatedly in the same individuals: the d-ROMs test, which quantifies organic hydroperoxides as a marker of lipid and protein peroxidation, and the OXY test, which measures non-enzymatic antioxidant capacity. The analytical core is a decomposition of the covariance between the two markers into between-individual, within-individual, and total phenotypic components using bivariate mixed-effect models, with repeatability computed from the between-individual variance share. Comparing models that allow the within-individual covariance to differ by experimental treatment or by year is what lets the paper claim that the correlation structure itself is modulated by conditions, rather than merely that the mean levels change.

What would settle it

Measure uric acid, triglycerides, and a haemolysis index on all 1108 plasma samples and re-estimate the repeatabilities and ROM–OXY correlations with these as covariates; if the treatment- and year-dependent differences in the correlation disappear after adjustment, the central claim of flexible oxidative balance would be an artifact of the assays rather than a biological property.

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Extended reading notes

Core claim

The paper's central claim is that the association between oxidative damage and antioxidant defense is context-dependent rather than a fixed physiological constant. In a wild population of collared flycatchers, antioxidant capacity showed significant repeatability across breeding stages and years, whereas reactive oxygen metabolite concentration did not, indicating that circulating peroxidation markers reflect short-term conditions more than stable individual differences. The markers correlated positively at the phenotypic level, but the correlation was stronger in females whose wing load had been increased experimentally and in a year with poor environmental conditions, while supplementary feeding did not alter it. The same population therefore does not have a single ROM–OXY correlation; the covariance structure itself shifts, and the authors argue that interpreting oxidative-stress markers requires flexible, mechanistic models rather than fixed ratios or principal-component scores.

Load-bearing premise

The whole argument rests on the assumption that the d-ROMs and OXY blood tests measure, in collared flycatchers, what they are named for—peroxidation products and non-enzymatic antioxidant capacity—rather than being driven mainly by plasma lipids, uric acid, or sample handling; if the assays mostly capture those non-oxidative factors, the repeatability and correlation estimates would describe assay artifacts.

Editorial extensions

If this is right

Antioxidant capacity can be treated as a moderately stable individual attribute in this species, so a single measure in one breeding season carries information about later seasons; a single ROM measurement does not.
The positive ROM–OXY correlation being stronger under increased wing load and in harsher years means that the same two markers can lead to opposite apparent life-history conclusions depending on when and where the study is done.
Because the correlation changes between years and treatments within one population, published differences in oxidative-stress correlations between species or populations may reflect environmental modulation rather than species biology.
Composite measures that assume a fixed trade-off or ratio between damage and defense—ratios or principal components—will be unreliable unless correlation stability is demonstrated in the study system.
Food supplementation raising antioxidant capacity without changing the correlation suggests that energy and nutrient intake can shift one side of the oxidative balance without necessarily tightening the coupling between the two markers.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The repeatability contrast hints at a practical research implication the authors do not draw: longitudinal studies that want an individual-level oxidative state proxy may get more signal from antioxidant capacity than from circulating peroxidation products.
If energetic constraint is what tightens the correlation, then an acute oxidative challenge given to the same individuals under ad libitum and restricted conditions should reproduce the pattern; this is a testable extension the paper does not run.
The two-marker design limits the scope of flexibility: measuring additional markers of oxidative balance in the same samples would reveal whether the whole redox network is reorganized under constraint or only this particular pair moves together.
The year effect on the correlation, combined with only three breeding seasons, means the paper's environmental interpretation is suggestive; a longer time series with yearly measures of food availability would separate weather effects from other annual differences.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Charlotte, here's my read on the Recapet et al. paper.

The paper's real contribution is the repeatability estimates: antioxidant capacity (OXY) shows moderate repeatability across years in a wild passerine (0.13–0.58), while ROMs are essentially not repeatable at the between-individual level, though the CIs are too wide to make that negative claim confidently (the incubation-stage interval runs 0 to 0.66). That dataset is large—over a thousand samples—and the lab work is careful: they excluded haemolysed and hyperlipemic samples, reported assay repeatability, and tested a small subset for triglyceride confounds. That's genuinely useful for anyone planning longitudinal oxidative-stress studies in birds.

Where I part ways is the second headline: 'relationships are not fixed but may be modulated.' The evidence comes from DIC comparisons of models with homogeneous vs. heterogeneous within-individual covariances. The problem is that the authors admit their inverse-Wishart priors were 'quite informative' on exactly those within-individual correlation parameters. That means the DIC differences (which can look impressive: ΔDIC = -12.6 for wing load, -26.3 for year) could partly reflect prior specification rather than true differences in correlation. The supporting point estimates also don't inspire confidence: the phenotypic correlations for handicapped vs. control females are 0.168 [-0.038, 0.374] and -0.004 [-0.203, 0.186]. Those intervals overlap nearly completely. So 'clearly show' on p.18 is stronger than the posterior evidence justifies. A prior-sensitivity analysis or direct posterior comparison of the heterogeneous covariance parameters would settle this.

Other issues: the data repository link is a placeholder, which needs to be a real URL for reproducibility. And the abstract inconsistently calls the species Ficedula hypoleuca (pied flycatcher) though the title and body say Ficedula albicollis (collared flycatcher). That's just a typo, but it would embarrass the authors in print.

Who should read this? Anyone using the d-ROMs and OXY tests in ecological studies. The repeatability numbers are worth knowing, and the caution about interpreting marker correlations is sensible even if this particular evidence is not as strong as the prose suggests.

I'd send it to peer review—the empirical core deserves a full review—but I'd ask for the data link, a corrected species name, and either a prior-sensitivity analysis or a toned-down claim about modulation. The repeatability findings alone are publishable.

Referee Report

3 major / 4 minor

Summary. This paper investigates the repeatability and correlation of two plasma markers of oxidative balance—reactive oxygen metabolites (d-ROMs) and non-enzymatic antioxidant capacity (OXY)—in free-living collared flycatchers sampled across three breeding seasons. The authors use a large repeated-measures dataset (1108 samples from 635 adults), experimental manipulations of energetic constraints (wing-load increase in 2012–2013, food supplementation in 2014), and univariate plus bivariate mixed models. They report that antioxidant capacity is repeatable within and between years (repeatabilities 0.124–0.581), while ROMs show near-zero repeatability (0–0.061), and that the two markers are globally positively correlated (phenotypic correlation ~0.102), with the strength of this correlation differing between experimental treatments and years according to DIC model comparisons. The paper concludes that the relationship between oxidative damage and antioxidant defense is not fixed but can be modulated by individual and environmental factors.

Significance. If the central claim holds, the paper is valuable: it challenges the common practice of treating ratios or principal components of oxidative-status markers as stable across contexts, and it provides a large, longitudinal, experimentally manipulated dataset from a wild population. The study's strengths include the direct decomposition of between- versus within-individual covariance, the use of repeated measures across years and breeding stages, explicit reporting of credibility intervals, and the inclusion of assay-repeatability information. However, the claim that correlations vary by treatment or year rests on model comparisons that the authors themselves note are sensitive to informative priors, and the supporting posterior intervals overlap substantially. The descriptive findings about repeatability are solid, but the headline conclusion about condition-dependent correlation structure requires additional statistical support before it is treated as established.

major comments (3)

[Methods (p. 11–12), Tables 2–4] The three pieces of evidence for modulated correlations are all DIC comparisons of bivariate mixed models with homogeneous versus heterogeneous within-individual covariances (ΔDIC = −12.6, +4.6, −26.3). The Methods explicitly state that the inverse-Wishart priors used for the covariances 'were quite informative on the posterior distribution of the between- and within-individual correlations, but not on the total phenotypic correlation.' Since the model comparisons are made on exactly these within-individual covariance parameters, the reported DIC differences may reflect prior specification rather than true differences in correlation. Please provide a prior-sensitivity analysis (e.g., varying the inverse-Wishart scale or degrees of belief) or a direct test of posterior differences, such as the probability that the correlation in one group exceeds that in another. Without this, the central conclusion that correlations vary by condition is not yet supported.
[Results (Tables 2–4), Discussion (p. 18)] The point estimates for condition-specific correlations are accompanied by wide, substantially overlapping 95% credibility intervals. For the wing-load comparison, the phenotypic correlations are 0.168 [−0.038, 0.374] in manipulated females versus −0.004 [−0.203, 0.186] in control females; for the year comparison, the estimates are 0.064 [−0.180, 0.282], 0.063 [−0.096, 0.251], and 0.132 [−0.028, 0.325]. These intervals overlap to a large degree, so the phrase 'clearly show' on p. 18 is stronger than the direct posterior evidence. Please report the posterior probability that the correlations differ across groups/years and temper the wording accordingly.
[Results (Table 1)] The conclusion that ROMs 'were not repeatable' is based on estimates with wide confidence intervals that include large values: between-years during incubation r = 0.032 [0, 0.660], and during nestling feeding r = 0.061 [0, 0.257]. The upper bounds do not exclude biologically meaningful repeatability. The qualitative pattern that OXY repeatability exceeds ROM repeatability is plausible, but the wording should state explicitly that high repeatability cannot be ruled out for ROMs given the uncertainty, rather than asserting that ROMs are not repeatable.

minor comments (4)

[Abstract (p. 3)] The abstract identifies the study species as 'Ficedula hypoleuca' (pied flycatcher), which conflicts with the title and the rest of the text, where the species is the collared flycatcher Ficedula albicollis. Please correct this species name.
[Data availability (p. 20)] The data availability statement contains the placeholder '[PERSISTENT WEB LINK TO DATASETS]' rather than an actual DOI or URL. A working link to the figshare repository should be provided.
[Methods (p. 12)] The threshold for 'significantly better' model support (ΔDIC larger than five) is described as a rule of thumb; since DIC is not a formal significance test and can be sensitive to prior specification, it would be helpful to note its heuristic nature and to supplement it with the prior-sensitivity analysis requested above.
[Table 1] For the ROM repeatability estimate of 0.032 with CI [0; 0.660], the lower bound appears to be exactly zero. Please clarify whether this is a rounding artifact of the parametric bootstrap or an exact boundary, and consider reporting additional significant digits for very small estimates.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the paper's repeatability and correlation estimates are empirical outputs, not consequences of how its inputs or priors were defined.

full rationale

This is an empirical measurement study, not a derivation. The central claims—antioxidant capacity repeatability (0.124–0.581), non-repeatability of ROMs (0–0.061), and condition-dependent ROM–OXY correlations—are estimated directly from field samples and laboratory assays, with repeatabilities computed from variance-component ratios and correlations estimated from bivariate mixed models. The hypotheses about whether correlations should be positive or negative are drawn from prior literature but are used only to frame the tests; the estimates themselves are free to come out either way and are not constrained by the hypotheses. The self-citations (Récapet et al. 2016c, 2017) are used to validate the wing-load manipulation via doubly labelled water measures and the food supplementation via parental-visit counts; these are independent empirical validations, not unverified premises that presuppose the paper's conclusions. The appendix checking triglyceride correlations is also an independent auxiliary analysis, and excluding it from the main models does not force any repeatability or correlation estimate. The only methodological caveat is that the inverse-Wishart priors are acknowledged to be informative for the between- and within-individual correlations, and the DIC model comparisons target exactly those covariance parameters; this is a potential prior-sensitivity or inference concern, but it is not a circularity because the priors were not derived from the posterior estimates and the reported correlations could in principle have differed from what was found. No equation is defined in terms of a predicted quantity, no fitted parameter is renamed as a prediction, and no load-bearing argument reduces to a self-citation. The manuscript is self-contained against external benchmarks in the relevant sense: the estimates are data-driven descriptions, not outputs of a derivation whose inputs include the target result.

Assumptions & free parameters 1 free parameters · 3 assumptions · 0 invented entities

The central claims rest on measurement validity and standard statistical assumptions rather than on invented entities. No new particle, mediator, force, or conserved quantity is introduced. The only hand-chosen inputs are the Bayesian prior settings, which the authors acknowledge influenced some reported correlations.

free parameters (1)

MCMC prior covariance settings = Inverse-Wishart with variances set to 1/n_i, null covariances
The reported between- and within-individual correlations partly reflect these chosen priors; the authors state that preliminary analyses showed the priors were informative on the posterior distribution of between- and within-individual correlations.

assumptions (3)

domain assumption Plasma d-ROMs and OXY values reflect systemic oxidative damage and non-enzymatic antioxidant defenses in collared flycatchers.
Used throughout to interpret repeatability and correlations. The paper excludes haemolysed and hyperlipemic samples and tests triglycerides in a small subset (Appendix 1), but does not provide independent validation that the assays isolate oxidative processes.
domain assumption Wing clipping and food supplementation modulate energy expenditure as intended.
The wing-load manipulation is validated in a previous study (Récapet et al. 2016c), and food supplementation is based on reduced parental visits (Récapet et al. 2017). Both validations are self-citations, and the current dataset does not independently confirm the metabolic effect on every sampled individual.
standard math Linear mixed models and bivariate Gaussian models provide unbiased estimates with these sample sizes.
The authors use REML and MCMCglmm, report convergence diagnostics for the MCMC chains, and use parametric bootstraps for repeatability. However, the informative priors weaken the between- and within-individual correlation estimates.

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Cite this review

Pith. "Pith review of Antioxidant capacity is repeatable across years but does not consistently correlate with a marker of peroxidation in a free-living passerine bird." pith.science (2026). https://pith.science/paper/JOVD3NCN

@misc{pith2026190801488,
  author       = {Pith},
  title        = {Pith review of: Antioxidant capacity is repeatable across years but does not consistently correlate with a marker of peroxidation in a free-living passerine bird},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JOVD3NCN}},
  note         = {Machine review of arXiv:1908.01488}
}

read the original abstract

Oxidative stress occurs when reactive oxygen species (ROS) exceed antioxidant defences, which can have deleterious effects on cell function, health and survival. Therefore, organisms are expected to finely regulate pro-oxidant and antioxidant processes. ROS are mainly produced through aerobic metabolism and vary in response to changes in energetic requirements, whereas antioxidants may be enhanced, depleted or show no changes in response to changes in ROS levels. We investigated the repeatability, within-individual variation and correlation across different conditions of two plasmatic markers of the oxidative balance in 1108 samples from 635 free-living adult collared flycatchers (Ficedula albicollis). We sought to manipulate energy constraints by increasing wing load in 2012 and 2013 and by providing additional food in 2014. We then tested the relative importance of within- and between-individual variation on reactive oxygen metabolites (ROMs), a marker of lipid and protein peroxidation, and on non-enzymatic antioxidant defences (OXY test). We also investigated whether the experimental treatments modified the correlation between markers. Antioxidant defences were repeatable (range of repeatability estimates = 0.128--0.581), whereas ROMs were not (0--0.061). Antioxidants varied neither between incubation and nestling feeding nor between sexes. ROMs increased from incubation to nestling feeding in females and were higher in females than males. Antioxidant defences and ROM concentration were globally positively correlated, but the correlation varied between experimental conditions and between years. Hence, the management of oxidative balance in wild animals appears flexible under variable environmental conditions, an observation which should be confirmed over a wider range of markers.

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Works this paper leans on

3 extracted references · 1 canonical work pages

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

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Trends Ecol Evol 25:207–214

doi: 10.1016/j.physbeh.2015.01.017 Wilson AJ, Nussey DH (2009) What is individual quality? An evolutionary perspective. Trends Ecol Evol 25:207–214. doi: 10.1016/j.tree.2009.10.002 Xu Y-C, Yang D-B, Speakman JR, Wang D-H (2014) Oxidative stress in response to natural and 30 experimentally elevated reproductive effort is tissue de pendent. Funct Ecol 28:40...

work page arXiv 2009

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Bioessays 33:255–259

doi: 10.1093/icb/icq049 Speakman JR, Selman C (2011) The free -radical damage theory: Accumulating evidence against a simple link of oxidative stress to ageing and lifespan. Bioessays 33:255–259. doi: 10.1002/bies.201000132 Spiegelhalter DJ, Best NG, Carlin BP, van der Linde A (2002) Bayesian measures of model complexity and fit. J R Stat Soc Ser B 64:583...

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Annu Rev Physiol 75:621–644

doi: 10.1111/j.1365-294X.2011.05331.x Gems D, Partridge L (2013) Genetics of longevity in model organisms: Debates and paradigm shifts. Annu Rev Physiol 75:621–644. doi: 10.1146/annurev-physiol-030212-183712 Giordano M, Costantini D, Pick JL, Tschirren B (2015) Female oxidative status, egg antioxidant protection and eggshell pigmentat ion: a supplemental ...

arXiv 2013