The physical “costs” of reproduction to an organism, particularly under less than ideal conditions, have been the subject of many studies in recent years. Theories suggest the physical investment in a pregnancy may sometimes come at the cost of either individual survival or future reproductive events. There is still much debate as to how these reproductive costs might manifest themselves physiologically, but one potential avenue is oxidative stress. Studies have shown pregnancy increases oxidative stress levels in humans and other mammals, but also many viviparous species also increase antioxidant levels possibly to reduce the chance of damage to their developing embryos from exposure to oxidative stress, a concept known as oxidative shielding.
Many researchers have examined the impact that limitations in food or energy can have on oxidative stress levels during pregnancy, but a recent paper from Stier et al examined the effects of limited water availability on pregnant aspic vipers. Previous studies from the same group had already demonstrated lack of access to water during mid-gestation caused dehydration and physiological stress (as measured by corticosterone levels) but did not impair reproductive outcome or affect offspring phenotype. In the current study these conditions were repeated, and at the end blood samples were taken from both control and water deprived snakes. Eight separate makers of oxidative stress were assessed in either plasma or RBCs. Note, red blood cells in non-mammalian vertebrates have both a nucleus and mitochondria unlike mammalian RBCs. These cells can therefore be used to gather information on things like cellular oxidative stress and mitochondrial function that would not be measured in mammalian RBCs.
– Non-enzymatic antioxidant capacity was measured in plasma and RBCs based on mmols of hypochlorous acid (HClO) neutralized. In this test pregnant snakes showed lower antioxidant capacity than control females that were not pregnant, but water deprived snakes showed an increase in antioxidant capacity over control snakes with free access to water.
– Total and oxidized glutathione were measured in lysed RBCs. Total glutathione is an indicator of antioxidant protection, and the ratio of oxidized glutathione to total glutathione is an indication of the amount of pro-oxidant power being buffered by the glutathione system in the sample. Total glutathione was observed to be higher in the water deprived snakes as compared to the snakes with free access to water, but no statistically significant differences were observed between pregnant and non-pregnant snakes regardless of water variability.
-Glutathione peroxidase and superoxide dismutase activities are indicators of the function of anti-oxidant scavenging enzyme cascades within the RBC lysate samples. Increased superoxide dismutase activity was found in pregnant snakes vs non-pregnant controls, but no significant difference was observed between the water deprived snakes and those with free access to water. Glutathione peroxidase activity was not statistically different across any of the four experimental groups of snakes.
-Protein carbonylation occurs when proteins react with free radicals or lipid peroxidation products and can therefore be used as an indication of the level of oxidative stress on the organism or sample. The plasma samples showed higher levels of carbonylated proteins in pregnant snakes over non-pregnant snakes. Interestingly, water deprived snakes had lower levels of protein carbonylation than the snakes with free access to water.
Overall, as expected, Stier et al. found many of the oxidative stress markers were increased in the pregnant snakes, indicating for this species, pregnancy does carry a cost in terms of potential oxidative damage. However, no cumulative effects were seen between reproduction and water deprivation, suggesting that these snakes, which are naturally found in a dry desert habitat have developed means of coping with dehydration without a significant increase in oxidative stress. It is important to understand the physical costs of pregnancy in animal species within their environment in order to understand what help might be needed to increase positive maternal and offspring outcomes for endangered or protected species.
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