Countercurrent heat exchange in dolphin flippers promotes conservation of body heat.

Aquatic birds and mammals, such as penguins, seals, and whales, can be endothermic in an aquatic environment for two major reasons: First, they are all air breathers and do not expose a large respiratory surface to the surrounding water. Second, many endothermic aquatic animals, including penguins, seals, and whales, are well insulated from the heat-sapping external environment by a thick layer of fat, while others, such as the sea otter, are insulated by a layer of fur that traps air. The parts of these animals that are not well insulated, principally appendages, are outfitted with countercurrent heat exchangers, vascular structures that reduce the rate of heat loss to the surrounding aquatic environment.