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1 Department of Biology, University of California, Santa Cruz 95064; and 2 Neuroendocrinology Lab, Division of Life Sciences, National Aeronautics and Space Administration Ames Research Center, Moffett Field, California 94035
Northern elephant seal (Mirounga angustirostris) pups rely on the oxidation of fat stores as their primary source of energy during their 8- to 12-wk postweaning fast; however, potential endocrine mechanisms involved with this increased fat metabolism have yet to be examined. Therefore, 15 pups were serially blood sampled in the field during the first 7 wk of their postweaning fast to examine the changes in plasma concentrations of cortisol and thyroid hormones (TH), which are involved in fat metabolism in other mammals. Cortisol increased, indicating that it contributed to an increase in lipolysis. Increased total triiodothyronine (tT3) and thyroxine (tT4) may not reflect increased thyroid gland activity, but rather alterations in hormone metabolism. tT3-to-tT4 ratio decreased, suggesting a decrease in thyroxine (T4) deiodination, whereas the negative correlation between total proteins and free T4 suggests that the increase in free hormone is attributed to a decrease in binding globulins. Changes in TH are most similar to those observed during hibernation than starvation in mammals, suggesting that the metabolic adaptations to natural fasting are more similar to hibernation despite the fact these animals remain active throughout the fasting period.
fat metabolism; food deprivation; leptin; marine mammals
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