Corticosterone Under Experimental Manipulation of Nutrition and Parasite Burden in a Wild Rodent System

Environmental pressures shape survival and life-history dynamics partly by triggering the release of glucocorticoids, whose short-term benefits but long-term costs make it essential to understand what drives their levels. Here, we used an experimental manipulation of two environmental stressors-food availability and parasite burden-to directly test how they influence fecal corticosterone metabolites (FCMs) in wild wood mice (Apodemus sylvaticus). To do so, we experimentally altered nutrition via high-quality food supplementation and reduced gastrointestinal nematode infection by anthelmintic treatment, using Heligmosomoides polygyrus as an indicator of treatment efficacy. FCM levels were not impacted by either treatment. However, our results may be mediated by variation in resource availability or masked by other factors that affect corticosterone. For example, FCMs declined seasonally, alongside a decline in the number of reproducing individuals. While we expected higher food availability and lower worm burdens to decrease stress, putatively higher rates of reproduction in food-supplemented areas may offset potential declines in corticosterone. Thus, alleviating some environmental stressors in the wild may have unintended consequences on host fitness.