Effects of prolonged fasting on plasma cortisol and TH in postweaned northern elephant seal pups

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 p ostweaning 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 postwean ing fast to examine the changes in plasma concentrations of cortisol and th yroid hormones (TH), which are involved in fat metabolism in other mammals. Cortisol increased, indicating that it contributed to an increase in lipol ysis. Increased total triiodothyronine ((TH) and thyroxine (tT(4)) may not reflect increased thyroid gland activity, but rather alterations in hormone metabolism. tT(3)-to-tT(4) ratio decreased, suggesting a decrease in thyro xine (T-4) deiodination, whereas the negative correlation between total pro teins and free T-4 suggests that the increase in free hormone is attributed to a decrease in binding globulins. Changes in TH are most similar to thos e observed during hibernation than starvation in mammals, suggesting that t he metabolic adaptations to natural fasting are more similar to hibernation despite the fact these animals remain active throughout the fasting period .