Blood cortisol and faecal cortisol metabolite concentrations following an ACTH challenge in unanaesthetized brown bears (Ursus arctos)

Abstract

Faecal cortisol metabolites (FCMs) are increasingly used to index physiological stress in wildlife. Cortisol and other stress hormones act to mobilize glucose, providing energy for the organism to respond to environmental perturbations. Cortisol, the predominant glucocorticoid (GC) in most mammals, is metabolized by the liver and excreted as FCMs. For FCMs to serve as a meaningful physiological index of stress in brown bears (Ursus arctos), we sought to quantify the relationship between blood cortisol and FCM concentrations. Consequently, we conducted an adrenocorticotropic hormone (ACTH) challenge on nine unanaesthetized captive brown bears at the Washington State University Bear Research, Education, and Conservation Center. We collected 10 ml of blood at 0, 3, 6, 24, 48 and 72 h post-injection to measure changes in blood cortisol concentrations. Faecal samples were collected between 7:00 am and 8:00 pm from 24 h prior to injection through 72 h post ACTH challenge. We found that FCM concentration was positively correlated with blood cortisol concentrations and that peak blood cortisol concentrations occurred between 3 and 6 h following an ACTH challenge, whereas FCMs peaked between 10 and 27 h after injection.

Keywords: Elisa assay; endocrinology; enzyme-linked immunosorbent assay; glucocorticoids; hormone; hormone challenge; stress.

Figure 1

Time course of serum cortisol concentration (mean ± SEM) for nine brown bears (U. arctos) following injection of 5 μg/kg (i.v.) of Cortrosyn. The dotted line represents the population-level baseline concentration (24.96 ng/ml).

Figure 2

(a) Mean serum cortisol concentration (±SEM) for nine brown bears (U. arctos) by sex (4 males, 5 females) (b) Mean serum cortisol concentration (±SEM) of nine brown bears by age group (4 young, 5 old). Dotted line represents population-level baseline concentration (24.96 ng/ml). Bears were injected intravenously with 5 μg/kg of Cortrosyn.

Figure 3

Daily mean faecal cortisol metabolite concentration (±SEM) for nine brown bears (U. arctos). The dotted line represents population-level baseline concentration (21.90 ng/g). Bears were injected intravenously with 5 μg/kg of Cortrosyn.

Figure 4

(a) Daily mean FCM concentration (±SEM) by sex (4 male, 5 female) for nine brown bears (U. arctos). (b) Daily mean FCM concentration (±SEM) for nine brown bears by age (4 young, 5 old). The dotted line represents population-level baseline concentration 21.90 ng/g. Bears were injected intravenously with 5 μg/kg of Cortrosyn.

Figure 5

Correlation between blood cortisol concentrations (ng/ml) and FCM concentration of nine brown bears (U. arctos). Correlation determined by random intercept-only linear mixed model with individual bear as the grouping variable. Conditional R² = 0.446, Marginal R² = 0.380.