Comparison of Hormone Levels from Permanently Captive and Rehabilitated Harbor Seals (Phoca vitulina)
IAAAM Archive
Danielle R. O'Neil1; S. Atkinson2
1School of Fisheries and Ocean Sciences, University of Alaska, Fairbanks, Fairbanks, AK, USA; 2Alaska SeaLife Center, Seward, AK, USA


This project seeks to compare young, rehabilitated harbor seals to adult captive harbor seals using two metabolic hormones and morphometrics. Baseline information on circulating metabolic hormones and standards of hormone concentrations in seals is one approach to further investigate harbor seal immunocompetence. Neonatal marine mammals that are admitted to rehabilitation facilities are more inclined to suffer from immunosuppression due to abandonment, malnourishment and anthropogenic stress. Analysis of hormone concentrations, immune function, and body condition via morphometrics provide an indication of an animal's physiology. Immunocompetence in an individual animal or species is a complex series of metabolic and physiological parameters. Hormones, such as cortisol (an adrenal hormone) and thyroxine (a thyroid hormone), are good indicators of metabolic state and potential physiological stressors. Spikes or depressions of these metabolic hormones can aid in determining the well-being of an animal. Cortisol and TT4 abnormalities, over time have been known to cause immunosuppression in other pinniped species. Stressors include nutritional stress via inadequate food quantity, feeding changes (weaning pressure), maternal abandonment, disease processes, noise and human or other species contact. Do these anthropogenic stressors alter metabolic hormone levels in pups more than in captive seals that have theoretically habituated to the stress?

This study examined cortisol and TT4 levels in neonatal harbor seals in two rehabilitation facilities, the Alaska SeaLife Center and the Marine Mammal Center in California. Sera from permanently captive harbor seals housed at the Alaska SeaLife Center were also assayed for comparison to the rehabilitation seals. Preliminary results indicate that TT4 concentrations ranged from 0.7 to 11.4ng/ml in animals age two days to 14 weeks. Cortisol concentrations ranged from 2.3 to 76.4ng/ml. In captive animals, TT4 ranges spanned from 0.8ng/ml to 6.3ng/ml, with highest levels seen in mid to late summer. Captive cortisol levels ranged from 2.4ng/ml to 41.3ng/ml (Table 1).

Locale changes, introduction of other rehab animals and alternating feeding regimes, may result in hormonal changes. We are currently correlating these parameters to weaning and changes in patterns of the two hormones in rehab seals. Spikes in cortisol and thyroxine were expected during weaning. High levels of cortisol at weaning suggest a change in metabolic rate due to diet adjustment in 66 percent (19 in 29) of the rehab pups. TT4 was elevated in 59 percent (17 in 29) of the rehab pups.

The importance of establishing a baseline in rehabilitation settings is to determine the health status of animals entering the facility. As shown in Table 1, captive and post wean seals' TT4 and cortisol concentrations do not seem to differ from each other. Most pre-weaned marine mammals have elevated TT4, which increases their metabolic rate to compensate for life outside the womb. The pre-weaning rehab pups did not have elevated TT4, suggesting that rehab procedures aid to stabilize metabolic rate, or the animals are possibly euthyroid. The lower cortisol concentrations post-weaning indicate that rehab pups have overcome or avoided chronic stress, possibly due to habituation or adaptation to the rehab facility. Ultimately, a completed document containing "normal" and "disease state" thyroxine and cortisol levels in neonates would be extremely important in facilitating expedient medical regimes.

Table 1. Cortisol and TT4 levels in pre- and post-wean pups and adult captive.


TT4 (ng/ml)

Cortisol (ng/ml)

Weight (kg)

Length (cm)



Mean SE

Mean SE

Mean SE

Mean SE

Pre-wean rehab pups


3.1 0.31

16.9 1.76

10.9 0.34

75.5 1.26

Post-wean rehab pups


2.9 0.22

11.1 0.87

19.5 0.54

84.9 1.14

Adult captives


3.0 0.10

12.2 0.50

63.4 0.9

137.6 1.01


We would like to thank the following individuals and organizations for their help with this project: The Exxon Valdez Oil Spill Trustees Council, Dr. Frances M.D. Gulland, Dr. Marty Haulena, Denise Greig, the TMMC medical staff, Dr. Natalie Noll, Dr. Pam Tuomi, the entire ASLC animal rehabilitation department and the ASLC mammal staff.

Speaker Information
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Shannon Atkinson

Danielle R. O'Neil

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