Glucocorticoid hormone levels increase in the blood stream in response to a large range of social and physical stressors, affecting variables associated to fitness, such as reproductive output and immunocompetence. Although quantification of plasma levels of glucocorticoids has been used as an indicator of environmental stress in different clades of vertebrates, the existence of interspecific differences in magnitude of stress response, their functional bases and ecological correlates remain poorly investigated. In the present study, we tested the following hypotheses: (1) phyllogenetically close species of Bufonids differ in innate immunocompetence, baseline activity of the hypothalamic-pituitary-interrenal (HPI) axis, and response of the HPI axis to stressors; and (2) species showing higher baseline activity of the HPI axis are characterized by lower innate immunocompetence. To test these hypotheses, we compared plasma bactericidal ability against Escherichia coli, baseline field levels of corticosterone (CORT) during reproductive season, and levels of CORT under stressful conditions (captive maintenance plus physical restraint) of three species of toads from the genus Rhinella from Botucatu (SP/Brazil): R. icterica, R. schneiden, and R. ornata. We found interspecific variation in bactericidal ability and CORT levels, with R. ornata showing higher baseline and post-stress CORT levels, and R. schneiden showing higher plasma bactericidal ability. Bactericidal ability and baseline CORT levels are inversely related among species, suggesting that lower innate immunocompetence might be associated to immunosupressive effects of elevated CORT levels. Possible associations between these results and interspecific differences in reproductive behavior and sensitivity to environmental changes are discussed.
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