Abstract

Relatively little work has been done to characterize the stress response of the bottlenose dolphin (Tursiops truncatus) outside of a single study of the adrenal response to adrenocorticotropic hormone (ACTH) stimulation.¹ We investigated the endocrine response to exogenous ACTH administration as well as an acute stress test in the bottlenose dolphin to better understand the serum pharmacokinetics of cortisol, its relationship to other endocrine products, and its dynamics across different matrices (e.g., blubber, feces).

Five dolphins participated in the ACTH stimulation test. Animals voluntarily beached and then received an intramuscular injection of ACTH (dosage range 0.02–0.22 U/kg). Two of the same dolphins repeated the procedure with IM injections of sterile saline as a sham test. Blood samples were collected from the peduncle just prior to injection and 15 and 30 minutes post-injection. The dolphin was then placed back in the water. Additional voluntary blood samples were collected 60 and 120 minutes post-injection and the next two mornings following the test. In the second study, five dolphins performed an out-of-water stress test. Dolphins voluntarily beached for a period of two hours during which blood samples were collected from the peduncle at 15-minute intervals. Blubber biopsies were collected just after beaching and at 60-minute intervals. For both tests, fecal samples were collected the day prior to, the day of, and for two days following the tests. Blood samples were processed for cortisol, aldosterone, free and total T3 and T4, and ACTH while blubber and fecal samples were processed for cortisol metabolites. Administration of ACTH was followed by an increase in cortisol and aldosterone during the time beached, but declined dramatically once animals were returned to the water (i.e., near baseline within two hours). Similar results were observed in sham-tested and stress-test dolphins, suggesting that dolphins mounted a stress response as a result of the handling procedure and either did not respond to exogenous ACTH or did not receive a sufficient dose to produce a noticeable effect. The biological half-life of cortisol was calculated as ~ 62 minutes based on stress-test recovery and that of aldosterone as ~ 29 minutes. Cortisol and aldosterone were strongly correlated in all dolphins, and both were significantly and positively related to ACTH, which appeared to be a stronger secretagogue for aldosterone than cortisol. Significant deposition of cortisol in the blubber occurred with stress-induced increases in serum cortisol, but the rate at which deposition occurred was variable within and across animals, possibly due to differences in the degree and timing of peripheral vasoconstriction/dilation. Fecal cortisol followed patterns of serum cortisol and was notably increased within a couple of hours of the stress test.

These findings demonstrate similarities and differences in the stress response between dolphins and terrestrial mammals and better enable the interpretation of cortisol measures from samples that can be obtained without handling artifact (e.g., dart biopsies, feces), an issue that is important to animal monitoring for welfare and conservation purposes in managed-care facilities and in the wild.

Acknowledgements

The authors wish to thank Drs. Eric Jensen and Jason Thornton of the U.S. Navy Marine Mammal Program, Drs. Lara Cotte and Jenny Meegan, veterinary technicians Sacha Stevenson and Mark Baird, and all of the participating animal trainers of the National Marine Mammal Foundation. The authors thank the Office of Naval Research for financial support of this project and Dr. Daniel Crocker of Sonoma State University for assistance with laboratory processing.

* Presenting author

Literature Cited

1.  Thomson CA, Geraci JR. Cortisol, aldosterone, and leukocytes in the stress response of bottlenose dolphins, Tursiops truncatus. Can J Fish Aquat Sci. 1986;43:1010–1016.