Relationships between hormone levels, metabolism and immune response in toads from a semi-arid region

Steroid hormones (e.g. androgens [AN] and corticosterone [CORT]) modulate complex physiological functions such as reproduction, energy mobilization, metabolism, and immunity. The effects of these steroids on immunocompetence and its metabolic costs can also be affected by fluctuations in environmental resource availability and other factors such as parasitism. To understand these possible interactions, we studied AN and CORT, immune response [swelling response to phytohemagglutinin (PHA) injection and bacterial killing ability (BKA)], parasite load, resting metabolic rate (RMR) and post-immune challenge (PHA injection) oxygen consumption rates during two different phases of the annual cycle of Rhinella jimi toads from the Brazilian semi-arid region (Caatinga), where environmental conditions are highly seasonal. We observed an increase in O₂ consumption rates after both PHA and saline (control) injections, indicating a metabolic response to adverse stimuli rather than the immune challenge. Toads showing higher RMR and VO₂ after the adverse stimuli (PHA/saline injections) had lower field AN and CORT plasma levels, suggesting these hormones might mediate a metabolic energy conservation strategy both at baseline levels and after adverse stimuli. Parasite load appear to constrain the metabolic response to PHA and saline injections. Additionally, individuals with a higher swelling response to PHA had higher field CORT plasma levels (particularly when males are breeding), which opposes the idea of a possible trade-off between reproductive activity and other physiological traits, indicating the immunoenhancing effects of elevated CORT at physiological levels. BKA did not show a seasonal variation or correlation with body condition or hormone levels, indicating that the immune surveillance mediated by the complement remains constant despite ecological and physiological changes.