Effects of dietary antioxidants and environmental stressors on immune function and condition in Lithobates (Rana) sylvaticus

The negative effects of abiotic and biotic stressors on animal condition, physiology, behaviour, and fitness have become increasingly recognized, making it critical to understand how these may be mitigated, such as through diet. When consumed, dietary antioxidants (secondary metabolites produced by plants) provide protection from oxidative damage that can result from exposure to stressors. We examined the effects of dietary antioxidants (β-carotene and vitamin E) on the ability of Lithobates sylvaticus (wood frog) tadpoles to overcome the detrimental effects imposed by two common environmental stressors - exposure to the herbicide atrazine or infection by a pathogenic helminth parasite (Echinostoma trivolvis). Tadpoles which were fed high concentrations of either vitamin E or β-carotene had a greater immune response compared to those given trace diets of either compound as measured by circulating white blood cells and a phytohemaglutination assay (PHA). Similarly, tadpoles in the herbicide and parasitism treatments that were given trace antioxidant diets had significant weight loss, whereas those consuming high levels of vitamin E or β-carotene did not. Our results suggest that dietary antioxidants, specifically β-carotene and vitamin E, have a positive effect on tadpole immune systems and overall condition that likely allows them to better cope with natural stressors, with potential implications for their foraging behaviour.