Lungworm parasite intensity and its association with the endocrine and immune systems and energy reserves in male cane toads

Host-parasite interactions can be harmful or harmless depending on the fitness and stress levels of the organisms. Interactions between the endocrine system, the immune system and energy expenditure are critical in the maintenance of homeostasis during stressful periods in vertebrates. We examined whether lungworm loads (Rhabdias spp.) of male cane toad (Rhinella marina) sampled during the wet season from three different populations, are associated with hormone levels, immune competence, and metabolic status. We found evidence of direct and strong associations between parasite burdens and variables related to hormonal levels, metabolic status and immune competence. We also found that interrelationships among independent variables can change the strength of the association with lungworm loads. These findings suggest that high levels of corticosterone affect immune competence against parasites, both directly and by inducing changes in the metabolic status of the animals. Additionally, high testosterone levels during the reproductive season, influenced variables related to metabolic status, which will also lead to higher parasite loads. Overall, we conclude that the endocrine system and metabolic status in male cane toads are critical to the immune competence against parasites during the reproductive season.