The endocrine control of reproduction and molt in male and female emperor (Aptenodytes forsteri) and adelie (Pygoscelis adeliae) penguins. II. Annual changes in plasma levels of thyroxine and triiodothyronine.

Changes in plasma thyroxine (T4) and triiodothyronine (T3) levels were studied during a breeding season and in more detail during the postbreeding molt in male and female emperor (Aptenodytes forsteri) and adelie (Pygoscelis adeliae) penguins under natural conditions in the Antarctic. During the 4-month natural fast that accompanies courtship and incubation in male emperors, plasma T4 and T3 levels were maintained around 11 and 0.6 ng/ml, respectively. In courting, fasting female emperors plasma T4 levels were maintained around 10 ng/ml for more than 1 month; plasma T3 levels were around 0.8 ng/ml but were markedly depressed (0.1 ng/ml) at the time of copulation although they increased again (2.2 ng/ml) at oviposition. During the 5-month period of chick rearing, plasma T3 (males and females) and T4 (females) were maintained at the same levels as during courtship and incubation, but plasma T4 levels in male emperors were slightly lower (7 ng/ml). Similar plasma T4 and T3 levels were observed in breeding adelie penguins. These results do not provide any convincing evidence for thyroid-gonadal interrelations in breeding penguins, but demonstrate their capacity to maintain plasma thyroid hormone levels during very prolonged natural fasts. During the heavy postnuptial molt when the birds were fasting, in both species and sexes, marked but separate peaks in plasma T4 and T3 levels occurred concurrently with the initial growth of the new feathers, and with the subsequent shedding of the old plumage, respectively. Peak plasma T4 levels were observed at the time of the emergence of the new feathers out of the skin, and peak plasma T3 levels were roughly concurrent with the maximum daily body weight loss. This is the first strong evidence that increases in plasma T4 and T3 levels are correlated with different stages of molt in a wild seabird. Increased plasma T4 but not T3 levels at the time of feather papilla eruption suggest that T4 is concerned with feather growth, but is not exclusive of a role of T3. Increased plasma T3 but not T4 levels during the reduction in thermal insulation in molting penguins suggest that this hormone rather than T4 might be active in energy metabolism in penguins.