Abstract

The biologic response to stress is a dynamic factor of everyday life that enables an animal to adapt to environmental or psychological demands.⁵ A stimulus, or stressor, initiates a series of physiologic reactions involving specific biochemical mediators of the autonomic nervous system, neuroendocrine axis, and immune system.² The effects of these reactions elicit both positive and negative metabolic changes ranging from increased energy production and immunomodulation, to lethargy, increased morbidity, and immune dysfunction.^2,4 The quantification of the role of stress and its effects on an animal's well-being has been especially challenging to measure in cetaceans.⁶ Several studies have documented levels of cortisol, aldosterone, and leukocyte distribution following perturbations such as public interaction, handling, and adrenocorticotropic hormone (ACTH) stimulation in captive and wild cetaceans.^1,3,7,8,9 However in cetaceans, cortisol concentrations vary little except in extreme stress or distress making interpretation of some results difficult.⁶ This study was designed to augment those data and to examine, for the first time, the diagnostic value of using plasma ACTH levels, in addition to other hematologic and physiologic changes, in detecting stress response-related changes in long-term captive belugas.

This study tests our hypotheses that stress-related parameters (plasma ACTH, cortisol, aldosterone): 1) follow a diurnal rhythm pattern; 2) become elevated with out-of-water physical examination; 3) are not elevated during or following periods of wading human contact; and 4) that plasma ACTH is a more sensitive indicator of stress response-related changes in longterm captive belugas than cortisol. Three adult belugas, all 19-20 years of age, were sampled by utilizing trained behavioral fluke presentation for blood collection. This habituated behavior was used to obtain baseline levels for the analytes of interest. Subsequent blood collections were obtained at predetermined times throughout the day, during scheduled physical examinations, and before and after wading-contact sessions to compare to baseline control levels.

Our results demonstrated that plasma ACTH has a diurnal rhythm with the highest values observed in early morning. Cortisol levels were higher in morning and evening. Aldosterone levels did not manifest a discernible pattern. Baseline analyte levels were as follows: plasma ACTH concentrations ranged from 0.3-24.3 pg/ml, serum cortisol from 0.66-4.17 μl/dl, and serum aldosterone from 10-43.6 pg/ml. All stress-related hormones were significantly elevated during physical examination. Plasma ACTH levels were most increased with a 5-10 fold elevation during physical examination, whereas cortisol and aldosterone showed only a 2-4 fold elevation. Stress response analytes from animals involved in wading-contact sessions did not differ significantly from baseline levels. Correlative hematological and serum chemistry parameters are currently being analyzed.

To our knowledge, this is the first report of baseline levels of plasma ACTH in belugas and the first to document changes following a routine perturbation. Moreover, this study suggests that plasma ACTH, although very labile, is a sensitive indicator of the stress response that correlates well with the level of perturbation experienced. The results also support our impression that well-managed wading contact programs are not stressful to the animals involved.

References

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