Just like humans, bottlenose dolphins must deal with stressful situations throughout their lives. Dramatic habitat changes, prey scarcity, fishery interactions, acoustic disturbances, and harmful algal blooms are just a few of the things that can elicit challenging conditions for these animals. During these stressful conditions, animals often need to tap into their energy reserves to deal with these stressors. One way they do this is by producing the hormone cortisol; the adrenal gland secretes much more of this hormone in times of stress. If the stressor is short-lived, the cortisol boost helps maintain normal physiologic function while the animal is dealing with the ramifications of the stressor (example: abandoning a foraging area to escape painfully loud construction noises). In this way, having a healthy adrenal gland is critical to help dolphins cope in challenging situations. However, if cortisol levels remain abnormally elevated for long periods of time (many days to months), the high hormone levels can impair reproduction, immune response, growth, and development, and make the animal more nutritionally vulnerable when prey is scarce.
Measuring cortisol levels in wild dolphins can potentially help researchers identify populations that are dealing with challenging conditions. However, cortisol is primarily measured from blood samples, and obtaining blood samples from most cetacean populations is next to impossible. On the other hand, skin and blubber samples collected from dart biopsies are commonly obtained from cetaceans all over the world. As such, the objective of this study was to measure cortisol in the blubber from biopsies and to evaluate how the measurements relate to those in the blood for which reference values have been made. There are very few places on earth in which both blood and blubber samples can be collected from the same individuals, but Sarasota Bay, with its special natural laboratory situation, is one of those places.
It has been previously demonstrated that these Sarasota Bay dolphins show healthy cortisol levels that, as expected, rise in the blood following net encirclement during health assessments. We found that the same is true in the blubber, however, the cortisol levels rise much more slowly in the blubber than they do in the blood. Cortisol measurements were obtained from blood and blubber samples of 62 dolphins at various times points after encirclement with a net. Preliminary results indicate that average cortisol levels were twice as high as the estimated baseline levels within 10-15 mins in the blood and within 45-60 mins in the blubber. The levels in the blood appeared to plateau as early as 30-40 mins post-encirclement, while in the blubber they were still increasing at our last estimated time point, at 150 mins post-encirclement. This information greatly helps us understand the relationship of cortisol levels in blood and blubber samples and it indicates that measuring cortisol from dart biopsies of other populations could be a very useful way to assess adrenal health and to evaluate previous stress response activity. Along with other hormone measurements, measuring cortisol levels may represent a way to evaluate whether particular human activities are causing high-levels of detrimental chronic stress in cetacean populations. Once these relationships are established, potential mitigating actions could then be taken- especially in times when animals are more vulnerable. Funding for this project was provided by the Office of Naval Research’s Marine Mammals and Biology Program.
This article appeared on page 13 of the 2017 SDRP Annual Report, Nicks n Notches.