Acute stress, steroid plasma levels, and innate immunity in Brazilian toads

Stress from habitat fragmentation has been shown to impact amphibian declines. Studies from a variety of vertebrates indicate that stressed animals exhibit an acute increase in circulating plasma glucocorticoid (GC) levels and consequent immunomodulation. To further explore the relationship between GCs and immunity, we subjected three species of newly captured Brazilian toads, Rhinella ornata, R. icterica and R. schneideri to restraint with or without movement restriction (maintenance in a moistened cloth bag vs. maintenance in a bin) for 24 h. We compared various parameters from baseline (field conditions) with values after restraint, including those associated with stress (corticosterone [CORT] plasma levels), and the neutrophil/lymphocyte ratio [N:L ratio]), potential reproduction (testosterone [T] plasma levels), and innate immunity (bacterial killing ability [BKA]). General responses to the restraint challenge (baseline vs. restraint) included increased CORT levels and N:L ratio, and decreased T levels and BKA. Additionally, CORT levels and N:L ratio tended to increase more from restraint with movement restriction than to restraint without movement restriction, indicating toads showed increased stress response to the more intense stressor. All variables showed interspecific variation at baseline conditions: R. ornata had higher CORT levels when compared to the other two species, while R. icterica had the highest BKA values. After restraint (with or without movement restriction), R. ornata displayed higher values for T and N:L ratio, and showed higher CORT values after restraint without movement restriction; however, the CORT values were similar among species after restraint with movement restriction. In terms of immunity, in response to restraint, BKA was different among species only after restraint with movement restriction, with R. schneideri showing the lowest BKA values. Our results show that restraint increases common markers of the stress response, and could reduce potential reproduction and innate immune responses in toads from all studied species. Our results also showed variation at the interspecific level, with the amplitude of change in the studied variables being consistent and more pronounced following restraint with movement restriction for the three-studied species.