Penguin molt revisited: dive performance following the molt of adult king penguins (Aptenodytes patagonicus) and conflicting physiological demands at sea.

The annual molt, when penguins replace their entire plumage within a short period while fasting ashore, is the energetically most challenging period of their annual cycle. During molt, king penguins (Aptenodytes patagonicus) lose most of their insulating subcutaneous fat and half of their pectoral muscle mass. Such massive changes might impact their physiological capacity during deep foraging dives of their post-molt trip, when they need to recover body reserves for the upcoming breeding season. In immature king penguins, the dive/foraging performance following molt is significantly altered, suggesting physiological constraints as a consequence of the severe changes during molt. To investigate whether adult king penguins are similarly affected by their molt fast, we implanted data-loggers into six adult birds before their annual molt cycle and recorded dive behavior and subcutaneous temperature as an index of peripheral perfusion. The results suggest that physiological dive capacity following molt is less affected in adults. For example, unlike in immatures, recovery time at the surface for dives to a given depth did not differ between pre- and post-molt trips in adults. Nevertheless, important foraging parameters, such as the time spent at foraging depth, were significantly reduced following molt, suggesting some carry-over effects, which probably explain the longer period spent at sea. Subcutaneous temperature changes, reflecting differences in perfusion, illustrate the interplay between conflicting physiological demands (fat deposition/mobilization and feather synthesis versus heat loss/thermoregulation). Despite all these challenges, adult birds managed to rebuild their subcutaneous fat layer during post-molt, the most important energy source for their upcoming courtship/incubation fast.