Abstract
Stressful experiences can promote harmful effects on physiology and fitness. However, stress-mediated hormonal and immune changes are complex and may be highly dependent on body condition. Here, we investigated captivity-associated stress effects, over 7, 30, 60, and 90 days on plasma corticosterone (CORT) and testosterone (T) levels, body index, and innate immunity (bacterial killing ability and phagocytosis of peritoneal cells) in toads (Rhinella icterica). Toads in captivity exhibited elevated CORT and decreased T and immunity, without changes in body index. The inter-relationships between these variables were additionally contrasted with those obtained previously for R. schneideri, a related species that exhibited extreme loss of body mass under the same captive conditions. While T and phagocytosis were positively associated in both species, the relationship between CORT and bacterial killing ability was dependent on body index alterations. While CORT and bacterial killing ability were positively associated for toads that maintained body index, CORT was negatively associated with body index in toads that lost body mass over time in captivity. In these same toads, body index was positively associated with bacterial killing ability. These results demonstrate that steroids-immunity inter-relationships arising from prolonged exposure to a stressor in toads are highly dependent on body condition.
Highlights
Stress events and their intensity may be assessed by measuring glucocorticoid (GC) hormone levels in plasma and other fluids in most vertebrates[1]
Given that previous studies suggested that long-term captivity is a chronic stressor for this species[29], we tested the following hypotheses: 1) toads in captivity exhibit increased CORT and decreased T, body index and immune response when compared to wild ones; 2) these effects are exacerbated throughout the time toads are kept in captivity; 3) CORT and immune response are negatively correlated over time in captivity; 4) T, body index and immune response are positively correlated over time in captivity
We tested the following additional hypothesis: 8) when body index decreases in response to long-term captivity, immune responses are directly associated with variation in body index and indirectly related with CORT and T; and 9) if body index does not vary over time in captivity, immune responses are directly associated with plasma CORT and T levels
Summary
Stress events and their intensity may be assessed by measuring glucocorticoid (GC) hormone levels in plasma and other fluids in most vertebrates[1]. The immunomodulatory role of GCs is well explored and described, especially for mammals, where bimodal effects depend on intensity and duration of exposure to stressors[4] In this context, immunostimulatory parameters (e.g. cellular function and inflammatory responses) are frequently higher in response to acute elevation of GC levels, while suppressive immune effects (e.g. decreased immune cell proliferation and proinflammatory cytokine production) are more commonly observed under chronically elevated GC conditions[4,16]. Few studies have examined how captivity itself affects stress physiology and immune response, in amphibians[8,28,29] In this context, the first aim of the present study was to investigate the effects of captivity duration, 7, 30, 60, and 90 days, on plasma corticosterone (CORT) and testosterone (T) levels, body index, and innate immune responses, measured as bacterial killing ability and phagocytosis of peritoneal cells, in male toads of Rhinella icterica. We tested the following additional hypothesis: 8) when body index decreases in response to long-term captivity, immune responses are directly associated with variation in body index and indirectly related with CORT and T; and 9) if body index does not vary over time in captivity, immune responses are directly associated with plasma CORT and T levels
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