Assessing the interconnected behavioral and physiological underpinnings of amphibian responses to fungal infection

Abstract

Pathogens inflict various costs onto their hosts from sublethal changes in physiology and behavior to intense pathology and mortality. The timing of host immune responses and concomitant changes in behavior may be jointly underpinned by an increase in glucocorticoid hormones. The temporal and causal links between these interrelated responses to infection remain equivocal in many host-pathogen systems. Here, using a fungal pathogen implicated in global amphibian declines, Batrachochytrium dendrobatidis (Bd), we examined sublethal consequences of infection in the Cuban tree frog (Osteopilus septentrionalis). Specifically, we tracked changes in the neuroendocrine stress response by measuring urinary corticosterone (CORT) metabolites as well as immunological activation via neutrophil/lymphocyte (N/L) ratios over three consecutive rounds of Bd exposure. Additionally, we measured both exploratory behavior and activity level in open field arenas to evaluate whether infection-mediated changes in behavior and stress physiology coincided. Pathogen-exposed individuals began testing positive for Bd after two rounds of exposure. After the third exposure, Bd-positive frogs had higher CORT levels compared to control frogs. While infection load was not associated with N/L ratio, CORT concentrations showed a negative correlation with N/L ratio, suggesting a link between endocrine activity and immune regulation. Furthermore, there were no differences in exploratory behaviors or activity levels between control and Bd-exposed frogs. The lack of a relationship between N/L ratios and infection load may indicate a lag between neuroendocrine and immunological responses in this host-parasite system. This is further supported by the delayed increase in CORT levels only after three rounds of pathogen exposure. Alternatively, the lack of a relationship may be due to the immunosuppressive capability of Bd. The varied impacts of infection on physiological biomarkers indicates a greater need for researchers to consider simultaneous changes to behavior, neuroendocrine, and immunological measures of stress in future host-pathogen studies.