Marker-dependent associations among oxidative stress, growth and survival during early life in a wild mammal

Oxidative stress is hypothesised to be a key physiological mechanism mediating life-history trade-offs, but evidence from wild populations experiencing natural environmental variation is limited. We tested the hypotheses that increased early-life growth rate increases oxidative stress, and that increased oxidative stress reduces first-winter survival, in wild Soay sheep (Ovis aries) lambs. We measured growth rate and first-winter survival for four consecutive cohorts, and measured two markers of oxidative damage (malondialdehyde, protein carbonyls) and two markers of antioxidant protection (total antioxidant capacity, superoxide dismutase) from blood samples. Faster lamb growth was weakly associated with increased malondialdehyde, but not associated with variation in the other three markers. Lambs with higher superoxide dismutase activity were more likely to survive their first winter, as were male but not female lambs with lower protein carbonyl concentrations. Survival did not vary with malondialdehyde or total antioxidant capacity. Key predictions relating oxidative stress to growth and survival were therefore supported in some oxidative stress markers but not others. This suggests that different markers capture different aspects of the complex relationships between individual oxidative state, physiology and fitness, and that overarching hypotheses relating oxidative stress to life-history variation cannot be supported or refuted by studying individual markers.