Interplay among steroids, body condition and immunity in response to long-term captivity in toads

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.

Figure 1

Field and captivity variation of plasma hormone levels of Rhinella icterica toads. Differences in (A) plasma corticosterone and (B) plasma testosterone levels of calling and non-calling individuals in the field and under captivity (N = 6 and 4 (0 - field), 4 and 2 (d7), 3 and 2 (d30), 3 and 3 (d60), 5 and 1 (d90) for calling and non-calling individuals, respectively, for both variables). Letters above the bars represent statistical differences for ANOVA, with different letters representing statistical difference within groups with P ≤ 0.05. Asterisks represent statistical differences between groups (calling or non-calling) at each specific time with P ≤ 0.05. Bars represent mean ± standard error. Abbreviations: CORT: Corticosterone; T: Testosterone.

Figure 2

Field and captivity variation of immune response of Rhinella icterica toads. Differences in (A) Bacterial killing ability, (B) Phagocytosis (%) and (C) Phagocytosis efficiency (%) for individuals in field and captivity (N = 10 (0 - field), 6 (d7), 6 (d30), 6 (d60), 6 (d90) for all variables). Letters above the bars represent statistical differences for ANOVA, with different letters representing statistical difference with P ≤ 0.05. Bars represent mean ± standard error. Abbreviations: BKA: Bacterial killing ability.

Figure 3

Correlations between plasma hormone levels and immune response in Rhinella icterica toads in captivity. (A) Correlation between CORT and BKA; (B and C) Correlations between T and phagocytic response. Abbreviations as follow: CORT: Corticosterone; BKA: Bacterial killing ability; T: Testosterone. (N = 22). Data were pooled to include all captivity durations (7, 30, 60, and 90 days in captivity).

Figure 4

Selected structural equation modeling for body index, steroids and immune variables in R. icterica and R. schneideri in captivity. (A and B) Path diagrams of the two causal selected models for R. icterica. (C–E) Path diagrams of the three causal selected models for R. schneideri. Path coefficients shown are all standardized values in sequence with higher AIC and dAIC < 2.0: (A) Model 1 χ² = 2.841, df = 5, P = 0.724, AIC = 610.9; (B) Model 7 χ² = 2.782, df = 4, P = 0.595, AIC = 612.95; (C) Model 3 χ² = 1.618, df = 3, P = 0.655, AIC = −97.1; (D) Model 4 χ² = 1.618, df = 3, P = 0.655, AIC = −97.1; (E) Model 5 χ² = 2.243, df = 3, P = 0.524, AIC = −96.4. Numbers within arrow indicate the completely standardized solution coefficient values. Positive numbers represent positive relations and negative numbers represent negative relations. One-arrow represents a regression result and two-arrow represent a correlation result. Abbreviations as follow: CORT: Plasma corticosterone levels; BI: Body index; T: Plasma testosterone levels; BKA: Bacterial killing ability; PE: Phagocytic efficiency. (N = 19 and 20, for R. icterica and R. schneideri, respectively).