The metabolic cost of subcutaneous and abdominal rewarming in king penguins after long-term immersion in cold water

Marine endotherms in the polar regions face a formidable thermal challenge when swimming in cold water. Hence, they use morphological (fat, blubber) adjustment and peripheral vasoconstriction to reduce demands for heat production in water. The animals then regain normothermia when resting ashore. In the king penguin (Aptenodytes patagonicus) metabolic rate is lower in fed than in fasted individuals during subsequent rewarming on land. This has been suggested to be a consequence of diversion of blood flow to the splanchnic region in fed birds, which reduces peripheral temperatures. However, peripheral temperatures during recovery have never been investigated in birds with different nutritional status. The aim of this study was, therefore, to measure subcutaneous and abdominal temperatures during the rewarming phase on land in fasted and fed king penguins, and investigate to which extent any different rewarming were reflected in recovery metabolic rate (MR_R) after long term immersion in cold water. We hypothesized that fed individuals would have a slower increase of subcutaneous temperatures compared to fasted penguins, and a correspondingly lower MR_R. Subcutaneous tissues reached normothermia after 24.15 (back) and 21.36 min (flank), which was twice as fast as in the abdomen (46.82 min). However, recovery time was not affected by nutritional condition. MR_R during global rewarming (4.56 ± 0.42 W kg⁻¹) was twice as high as resting metabolic rate (RMR; 2.16 ± 0.59 W kg⁻¹). However, MR_R was not dependent on feeding status and was significantly elevated above RMR only until subcutaneous temperature had recovered. Contrary to our prediction, fed individuals did not reduce the subcutaneous circulation compared to fasted penguins and did not show any changes in MR_R during subsequent recovery. It seems likely that lower metabolic rate in fed king penguins on land reported in other studies might not have been caused primarily by increased circulation to the visceral organs.