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1.The trade-off between current reproductive effort and survival is a key
concept of life history theory. A variety of studies support the existence
of this trade-off but the underlying physiological mechanisms are not
well-understood. Oxidative stress has been proposed as a potential
mechanism underlying the observed inverse relationship between
reproductive investment and lifespan. Prolonged fasting is associated with
oxidative stress including increases in the production of reactive oxygen
species, oxidative damage and inflammation. 2.Northern elephant seals
(NES) undergo prolonged fasts while maintaining high metabolic rates
during breeding. We investigated NES of both sexes to assess oxidative
stress associated with extended breeding fasts. We measured changes in the
plasma activity or concentrations of markers for oxidative stress in 30
adult male and 33 adult female northern elephant seals across their 1-3
month breeding fasts. Markers assessed included a pro-oxidant enzyme,
several antioxidant enzymes, markers for oxidative damage to lipids,
proteins and DNA, and markers for systemic inflammation. 3.Plasma xanthine
oxidase (XO), a pro-oxidant enzyme that increases production of oxidative
radicals, and several protective antioxidant enzymes increased over
breeding in both sexes. Males showed increased oxidative damage to lipids
and DNA and increased systemic inflammation, while oxidative damage to
proteins declined across breeding. In contrast, females showed no
oxidative damage to lipids or DNA or changes in inflammation, but showed
increases in oxidative damage to proteins. XO activity, antioxidant
enzymes, oxidative damage markers, and inflammatory markers were strongly
correlated in males but these relationships were weaker or non-existent in
females. 4.NES provide evidence for oxidative stress as a physiological
cost of reproduction in a capital breeding mammal. Both sexes strongly
up-regulated antioxidant defenses during breeding. Despite this response,
and in contrast to similar duration non-breeding fasts in previous studies
on conspecifics, there was evidence of oxidative damage to tissues. These
data demonstrate the utility of using plasma markers to examine oxidative
stress but also suggest the necessity of measuring a broad suite of plasma
markers to assess systemic oxidative stress.
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