Energy and protein requirements for molt in the king penguin Aptenodytes patagonicus.

Adult king penguins annually fast ashore for 1 mo for molting. By the end of molt, they have lost 44% of their prefasting body mass. About 18% of new feather synthesis occurs at sea, thus reducing both nutrient requirement and fasting duration. Plumage synthesis continues during the first 3 wk of fasting. Loss of old feathers occurs between day 12 and day 21 of the molt, and it is associated with a peak in daily body mass loss. The dry mass of epidermal structure synthesized during molt is 395 g. Body composition analysis indicates that fat oxidation accounts for 85% of total energy expenditure. The proportion for protein is 15%, a value twofold higher than during the breeding (nonmolting) fast. The mean energy expenditure is also 21% higher during the molting fast (3.04 W/kg). Compared with other birds, the energetic cost of feather synthesis is the lowest in king penguins (85 kJ/g) and consequently the energetic efficiency is the highest (25%). Changes in tissue composition during molt show that integument is the main lipid source (72% of the lipid loss) and thus the main source of energy (61% of the total energy expenditure). The integument and the pectoral muscles play a major role in molting protein metabolism, providing 20 and 57%, respectively, of the total protein needs for feather synthesis and/or energy expenditure. This result emphasizes the role of integument as a protein source, because the large premolting muscle hypertrophy is not sufficient to account for the totality of the protein cost of molt.