Abstract

The importance of cortisol and thyroid hormone concentrations in relation to the health status of cetaceans is unknown. Endocrinology data are key elements in assessing the health of captive dolphins. Comparative baseline values for endocrinology data in healthy and stressed or ill individuals are needed for captive dolphins of various species. This study established cortisol and thyroid hormone baseline values from seven rough-toothed dolphins (Steno bredanensis) and compared concentrations between stranded and healthy animals. Serum cortisol and thyroid hormones, including total and free thyroxine and tri-iodothyronine, were analyzed using radioimmunoassay techniques for 100 samples from individuals housed at Mote Marine Laboratory and Aquarium and Dolphin Quest French Polynesia.

The hormone concentrations were analyzed for the entire sample set and also subdivided into healthy versus stranded individuals. The means and ranges as well as trends over time are reported to examine the changes in the individuals that were rehabilitated. Of five rehabilitated dolphins that were serially sampled for 62 to 100 days, two were released back to the wild, one continued to thrive in captivity, and two died.

Cortisol concentrations averaged 18.6 ng/ml (SD=25.4) and ranged between 0 and 130.4 ng/ml when including both stranded and healthy individuals. However, in healthy individuals or in the stranded dolphins after 50 days of rehabilitation, cortisol concentrations were less than 6 ng/ml. This suggests that cortisol values between 0 and 6 ng/ml are baseline values for individuals acclimated to captivity in this dolphin species. In all of the stranded animals, cortisol concentrations were initially higher and declined as rehabilitation continued. These higher initial cortisol concentrations provide reference values for individuals experiencing medical challenges and/or environmental adjustments in this species. These results indicate that cortisol likely reflects a stress response in S. bredanensis, at least in relation to physiological demands associated with ill health or dramatic environmental change.

Thyroid hormone concentrations showed a large range of values, averaging 84.1 ng/ml (SD=32.1) for total T4, 10.2 pg/ml (SD=4.3) for free T4, 1071.7 pg/ml (SD=425.1) for total T3, and 1.06 pg/ml (SD=0.56) for free T3 in S. bredanensis. Generally, thyroid hormone concentrations reflected the outcome of treatment for each individual, increasing over time in the successfully rehabilitated animals, and declining in those that died. These values for healthy and ill individuals will have diagnostic use and contribute toward a better understanding of the endocrinology of this cetacean.