Elephant seals lead two entirely different lives: one on land and one at sea. The adaptations for feeding at sea, such as blubber for warmth in cold waters, streamlining to reduce drag, and a source of energy to see them through long fasts, impose difficulties on land. It is costly to move their large bodies quickly for any distance on land. Their blubber layer is like a puffy coat that causes overheating on hot days on land. The loss of feet for flippers leaves them vulnerable to fast-moving and agile terrestrial carnivores, which partially explains their historical preferences to breed on islands devoid of bears, wolves, and coyotes. The evolutionary changes in their bodies must be a compromise between what is necessary for successful foraging at sea and breeding and molting on land. For example, elephant seals hold their breath while sleeping on land and during dives at sea.