Warming up to a new coat: molting king penguins exhibit hyperthermia and increased peripheral heat loss

Penguins are among the most specialized thermoregulators on the planet; however, the same adaptations that maximize heat retention underwater likely hinder heat dissipation on land, possibly creating dangerous thermoregulatory challenges when encountering warming terrestrial habitats. Penguins are subject to strictly terrestrial phases, such as molting, when metabolic heat production, insulation, and energetic constraints are heightened. We assessed thermoregulation in molting captive king penguins (<i>Aptenodytes patagonicus</i>) using simultaneous measurements of core and surface temperatures to test two hypotheses. Under the thermal challenge hypothesis, an initial rise in heat dissipation effort (i.e., increased peripheral vasomotion) followed by a rise in core temperature would indicate failure to prevent hyperthermia. Under the warm-up hypothesis, an initial rise of core temperature concomitant or followed by an increase in peripheral vasomotion would indicate regulated hyperthermia, possibly to accelerate feather development. Core and surface temperatures increased drastically but concomitantly during molt, providing tentative support to the warm-up hypothesis. Molting penguins did not pant, suggesting that peripheral heat dissipation was sufficient to regulate molting-induced hyperthermia. Core and subcutaneous temperatures in wild individuals resembled patterns measured in captivity, despite lower heat load and additional options for behavioral thermoregulation. These results indicate that hyperthermia is prevalent in molting king penguins, and documenting the timing of temperature changes provides novel insights for the molting physiology of penguins. Because molting-induced hyperthermia may contribute to heat load, we caution that molting may increase the susceptibility of wild penguins to heat stress, especially as regions near the poles warm at a disproportionately rapid rate.<b>NEW & NOTEWORTHY</b> Penguins may experience heat stress while molting, which causes increased metabolic heat generation and insulation. We assessed thermoregulation in molting captive king penguins (<i>Aptenodytes patagonicus</i>) using simultaneous measurements of core and surface temperatures. By measuring temperature throughout the entirety of the molt, we found that hyperthermia and increased peripheral heat dissipation are prevalent in molting king penguins. We caution that molting-induced hyperthermia may contribute to the susceptibility of penguins to heat stress in the wild.