Penguins face a major thermal transition when returning to land in a hypothermic state after a foraging trip. Uninsulated appendages (flippers and feet) could provide flexible heat exchange during subsequent rewarming. Here, we tested the hypothesis that peripheral vasodilation could be delayed during this recovery stage. To this end, we designed an experiment to examine patterns of surface rewarming in fully hypothermic (the cloaca and peripheral regions (here; flippers, feet and the breast) < 37 °C) and partially hypothermic (cloaca at normothermia ≥ 37 °C, but periphery at hypothermia) king penguins (Aptenodytes patagonicus) when they rewarmed in the laboratory. Both groups rewarmed during the 21 min observation period, but the temperature changes were larger in fully than in partially hypothermic birds. Moreover, we observed a 5 min delay of peripheral temperature in fully compared to partially hypothermic birds, suggesting that this process was impacted by low internal temperature. To investigate whether our laboratory data were applicable to field conditions, we also recorded surface temperatures of free-ranging penguins after they came ashore to the colony. Initial surface temperatures were lower in these birds compared to in those that rewarmed in the laboratory, and changed less over a comparable period of time on land. This could be explained both by environmental conditions and possible handling-induced thermogenesis in the laboratory. Nevertheless, this study demonstrated that appendage vasodilation is flexibly used during rewarming and that recovery may be influenced by both internal temperature and environmental conditions when penguins transition from sea to land.
Keywords: Bird; Heterothermy; Thermal imaging; Thermal windows; Thermoregulation; Vasoconstriction; Vasodilation; Vasomotor response.