Reproduction is not costly in terms of oxidative stress

One of the core assumptions of life-history theory is the negative trade-off between current and future reproduction. Investment in current reproduction is expected to decrease future reproductive success or survival, but the physiological mechanisms underlying these costs are still obscure. To test for a role of oxidative stress, we measured oxidative damage to lipids and proteins in liver, heart, kidneys and muscles, as well as the level of antioxidants (total glutathione and catalase), in breeding and non-breeding bank voles. We used females from lines selected for high aerobic metabolism and non-selected control lines and manipulated their reproductive investment by decreasing or increasing litter size. Unlike in most previous studies, the females reared four consecutive litters (the maximum possible during a breeding season). Contrary to predictions, oxidative damage in reproducing females was decreased or not changed, and did not differ between the selected and control lines. Oxidative damage to lipids and proteins in the liver was lower in females that weaned enlarged litters than in non-breeding ones, and was intermediate in those with reduced litters. Oxidative damage to proteins in the heart also tended to be lower in breeding females than in non-breeding ones. A negative relationship between the level of oxidative damage and activity of catalase in kidneys indicated a protective action of antioxidants. In conclusion, our study falsified the hypothesis that oxidative stress is a part of the proximate physiological mechanism underlying the fundamental life-history trade-off between current and future reproduction.