How to Molt While Fasting in the Cold: The Metabolic and Hormonal Adaptations of Emperor and King Penguins

Abstract

Like other penguin species, molting Emperor Aptenodytes forsteri and King A. patagonica penguins remain ashore and fast for several weeks during which their entire plumage is simultaneously renewed. Molt therefore involves the use of endogenous lipid reserves and body protein to meet the nutrient requirements not only for new feather synthesis, but also for the increase in energy expenditure associated with the decrease in thermal insulation. This energetic and metabolic stress results in a 45% decrease in body mass and in a 50% loss of body protein. There is no evidence that metabolic hormones such as glucagon, insulin and corticosterone play a major role in the control of metabolic events specifically related to molt. In contrast, the marked increase in circulating thyroxine at the onset of new feather synthesis in adult Emperor and King penguins, and in Emperor Penguin chicks, suggests that this hormone triggers molt by stimulating the development of the new plumage. On the other hand, triiodothyronine could intervene in the control of energy metabolism in situations of acute cold exposure, as during the loss of old feathers in the antarctic adult Emperor Penguins.