Seals are actually able to reduce the temperature of their brains to decrease the amount of oxygen needed to sustain metabolism during dives. In a recent study, the brains of harp (Pagophilus groenlandicus; shown above) and hooded (Cystophora cristata) seals were found to cool by as much as 2.5°C during dives in 4°C water. This translates into a reduction in brain oxygen consumption of 25%. Temperatures within the aorta were also found to decrease during diving. Studying the vasculature of the front flippers, they discovered that blood could be shunted to large superficial veins allowing heat to escape to the environment as opposed to routing the blood through arterio-venous heat exchangers used to conserve heat. The amount the blood is cooled is controlled by water temperatures and the ratio of blood flow to the carotid artery compared to the brachial artery. What a neat adaptation to low oxygen environments!
A. Schytte Blix, L. Walløe, E.B. Messelt,L.P. Folkow. Selective brain cooling and its vascular basis in diving seals. J Exp Biol 213: 2610-2616, 2010.