Serum Cortisol and Thyroid Hormone Concentrations in Stranded and Healthy Rough-Toothed Dolphins (Steno bredanensis)
The importance of cortisol and thyroid hormone concentrations in relation
to health status of cetaceans is not known. Endocrinology data are key elements in health
assessments 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 one hundred 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 62-100 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 species of dolphin. 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.