Seasonal changes of faecal cortisol metabolite levels in Gracilinanus agilis (Didelphimorphia: Didelphidae) and its association to life histories variables and parasite loads

The aim of this study was to evaluate the role of environmental (dry versus wet season) and individual (sex, body mass and reproductive status) factors in the levels of faecal cortisol metabolites (FGCs) in Gracilinanus agilis faecal samples as an index of stress levels in this species; as well as its association with abundance of Eimeria spp, as an indicator of immunocompetence against parasites. Our study found that FGCFGCs are a reliable indicator of adrenal activity in G. agilis. We found that FGCFGCs increase considerably by environmental stressors like the dry season. Moreover, the observed positive association between FGCs and body mass is the result of the effect of season and reproduction in both variables. We also demonstrated that an increase in FGC levels among G. agilis during the dry season is associated with a rise in the probability of being infected by Eimeria spp. Hence, our finding supports the corticosteroid-fitness hypothesis, which predicts that increased glucocorticoids as a response to stressors usually results in decreased fitness of individuals, translated into low future survival and reproductive success, and higher parasite infection. To our knowledge, this is the first study that integrates environmental changes, hormone responses and parasite loads in a US marsupial in both empirical and experimental approaches.