Data from: Plasma markers of oxidative stress are uncorrelated in a wild mammal

Oxidative stress, which results from an imbalance between the production
of potentially damaging reactive oxygen species versus antioxidant
defenses and repair mechanisms, has been proposed as an important mediator
of life-history trade-offs. A plethora of biomarkers associated with
oxidative stress exist, but few ecological studies have examined the
relationships among different markers in organisms experiencing natural
conditions or tested whether those relationships are stable across
different environments and demographic groups. It is therefore not clear
to what extent studies of different markers can be compared, or whether
studies that focus on a single marker can draw general conclusions
regarding oxidative stress. We measured widely used markers of oxidative
damage (protein carbonyls and malondialdehyde) and antioxidant defense
(superoxide dismutase and total antioxidant capacity) from 706 plasma
samples collected over a 4-year period in a wild population of Soay sheep
on St Kilda. We quantified the correlation structure among these four
markers across the entire sample set and also within separate years, age
groups (lambs and adults), and sexes. We found some moderately strong
correlations between some pairs of markers when data from all 4 years were
pooled. However, these correlations were caused by considerable among-year
variation in mean marker values; correlation coefficients were small and
not significantly different from zero after accounting for among-year
variation. Furthermore, within each year, age, and sex subgroup, the
pairwise correlation coefficients among the four markers were weak,
nonsignificant, and distributed around zero. In addition, principal
component analysis confirmed that the four markers represented four
independent axes of variation. Our results suggest that plasma markers of
oxidative stress may vary dramatically among years, presumably due to
environmental conditions, and that this variation can induce
population-level correlations among markers even in the absence of any
correlations within contemporaneous subgroups. The absence of any
consistent correlations within years or demographic subgroups implies that
care must be taken when generalizing from observed relationships with
oxidative stress markers, as each marker may reflect different and
potentially uncoupled biochemical processes.